Vulnerabilities - Global downward trend casualties since 1900

Storm surge events very likely will become more severe due to sea-level rise. Their impact may become more catastrophic due to population growth in coastal zones as well. The impacts of storm surges caused by windstorms or cyclones in the past have been studied based on a record of major global storm surge events for the period 1900-2015 (71). According to the authors of this study, their list of 121 events can be considered to be the first compilation of global historic storm surge events with high numbers of fatalities (at least 2000 fatalities until the year 1980, and at least 500 fatalities after 1980).

The estimated total number of people killed by storm surge events over the period 1900-2015 according to this database amounts to about 1 million, while the total number of people affected is 172 million. This is on average some 8000 fatalities and 1.5 million people affected per year. The largest share of fatalities (71%) has occurred in South Asia. This is caused by a dense coastal population, frequent cyclones, and in some places low protection levels and lack of warning (71).

The occurrence of very substantial loss of life (>10 000 persons) from single events has decreased over time, especially since the 1960s. This is remarkable since world population has approximately doubled since the 1960s, and increased six-fold since 1900, with much of this increase occurring in the coastal regions (5). Storm surge mortality, the fraction of people exposed to the storm surge flood that are killed, has decreased consistently for all global regions, except South East Asia (71). According to the authors this downward trend is probably related to improvements in the prediction of storms and typhoons, and more effective warning and evacuation of the population. Over the last decades, flood protection and forecasting, quality of residential buildings, early warning and evacuation, have improved considerably in most parts of the world.

The authors stress that the current decrease of number of fatalities and event mortality could slow down or even reverse. Sea-level rise could contribute to more frequent events, higher flood depths and thus less effective protection, and factors such as land subsidence and population growth can affect potential loss of life. Continued investments in the reduction of the vulnerability of coastal regions will remain important, therefore, through forecasting, emergency and land use planning as well as physical protection (71). 

Global trends in flood mortality since 1975

Over the last five decades, floods have become less deadly. The global number of flood events has increased over time, but the average number of people killed and affected per event has decreased. This was concluded from an analysis of global data of flood fatalities for the period 1975–2022 (141). These data are part of the widely used EM-DAT International Disaster Database (https://www.emdat.be). The analysis includes 5582 flood events with one or more fatalities.

Since 1975, the global number of flood events has increased over time, albeit with a slower increase since 2000 compared to previous decades. However, floods have become less lethal. The doubling of global population since 1975, partly in flood prone areas along coasts and in river basins, has not increased the total number of annual flood fatalities over time. In fact, since 1975, the average number of people killed and affected per event has decreased. The latter reflects the impact of increased flood protection, better warning, forecasting and early warning communication and other forms of risk reduction over the last decades. The annual number of large events – with over 100 fatalities – has decreased since 2000 (141).

In their analysis, the researchers also made a distinction in low-, middle- and high-income countries (based on World Bank data). They showed that especially middle-income countries have succeeded in reducing mortality from flooding. For low-income countries mortality increased after the year 2000 whereas no trend was found for high-income groups. According to the authors of the study this may be due to an increase of the number of people living in floodplains. In low-income countries, people moving into floodplains are too poor to adequately protect themselves against flooding. Indeed, as we have shown before, more and more natural floodplains are being converted into agricultural land and built-up areas, globally, increasing exposure to flooding (141).

The study includes riverine, coastal, and flash flooding. The analysis shows that riverine and flash floods are the most frequent flood type for flood events up to 1,000 fatalities. Around 60 events of these types of flooding occur each year with 5 or more fatalities. The study also highlights the relatively high mortality from flash flooding, consistent with previous findings. Although coastal floods from storms and tropical cyclones occur less frequently, they are dominant for events over 1,000 fatalities. Particularly due to these large events, most flood fatalities – around 85% – occurred in Asia (141).

Vulnerabilities - Global upward trend population exposed to flooding since 2000

According to satellite imagery of 913 large flood events from 2000 to 2018, the total global population in locations with satellite-observed inundation (from rivers and the sea) grew by 58–86 million from 2000 to 2015 (116). This represents an increase of 20 to 24 per cent in the proportion of the global population exposed to floods, ten times higher than previous estimates (117). Climate change projections for 2030 indicate that the proportion of the population exposed to floods will increase further.

Vulnerabilities - Global assessment impact urbanization

The impact of urbanization, excluding climate change

The impact of urbanization on the extent of urban areas exposed to flood and drought hazards has been assessed, without factoring in the potential impacts from climate change. The results of this assessment are summarized below (12).

Urban areas in coastal zone

In 2000, over 10% of total global urban land was located within the low-elevation coastal zones (LECZ, defined as ‘‘the contiguous area along the coast that is less than 10 m above sea level’’) that covers only 2% of the world’s land area. Most of the urban land in the LECZ was primarily located in the developed countries in Northern America and Western Europe along with China. By 2030, however, most of the urban land within the LECZ will be found in the developing countries. From 2000 to 2030, globally the amount of urban land within the low-elevation coastal zones is projected to increase by 230%; for Western and Eastern Europe this increase is projected to be 100% and 7%, respectively, resulting in 13% (Western Europe) and 2% (Eastern Europe) of the urban area being located in LECZ in 2030, respectively (12).

Urban areas exposed to high-frequency floods (coastal and river)

With respect to high-frequency flood zones, including exposure to both coastal and river floods, in 2000 about 30% of the global urban land was located in these zones; by 2030, this will reach 40%. For Western Europe these numbers are 34% (2000) and 34% (2030), and for Eastern Europe 9% (2000) and 10% (2030) (12).

A broad shift is projected in the urban exposure from the developed world to the developing world from 2000 to 2030. The emerging coastal metropolitan regions in Africa and Asia will be larger than those in the developed countries and will have larger areas exposed to flooding. By 2030, India, Southern Asia, and South-eastern Asia are expected to have almost three-quarters of the urban land under high-frequency flood risk (12).

Urban areas in dry-lands

The urban extent in dry lands will also increase strongly. Across all regions and all dry-lands, most urban expansion is expected to occur in semi-arid regions of China. The most urban expansion in hyper-arid regions is expected in Northern Africa. For Western Europe the (projected) percent urban extent in dry-lands are 5% (2000) and 6% (2030), and for Eastern Europe 11% in both years.

Urban areas exposed to both floods and droughts

Overall, without factoring in the potential impacts from climate change, the extent of urban areas exposed to flood and drought hazards will increase, respectively, 2.7 and almost 2 times by 2030. Globally, urban land exposed to both floods and droughts is expected to increase over 250%. In particular, Southern Asia, India, and South America used to have the most urban land in 2000 in areas exposed to both frequent floods and to recurrent droughts. By 2030, Mid-Latitudinal Africa is expected to join these three regions in having the largest urban extents exposed to both floods and droughts. The largest increase in the amount of urban land exposed to both floods and droughts is expected in Southern Asia (12).

The impact of urbanization, including climate change

Global economic exposure to both river and coastal flooding has been estimated for the period 1970–2050, using two different methods for damage assessment (1):

1. The method based on population density and GDP per capita resulted in an estimate of total global exposure to river and coastal flooding of 46 trillion USD in 2010 and a projected increase to 158 trillion USD by 2050.
2. The method based on land use within areas subject to 1/100 year flood events resulted in an estimate of total flood exposure of 27 trillion USD in 2010 and projected increase to 80 trillion USD by 2050.

The largest absolute exposure changes between 1970 and 2050 are simulated in North America and Asia. In relative terms the largest increases are projected for North Africa and Sub-Saharan Africa (1). The results are based on a large number of assumptions and should only be interpreted as a first estimate of indicative values.

Present and future flood losses in the 136 largest coastal cities have been quantified (5). Average global flood losses in 2005 have been estimated to be approximately US$6 billion per year, increasing to US$52 billion by 2050 with projected socio-economic change alone. With climate change and subsidence, present protection will need to be upgraded to avoid unacceptable losses of US$1 trillion or more per year. Even if adaptation investments maintain constant flood probability, subsidence and sea-level rise will increase global flood losses to US$60–63 billion per year in 2050. To maintain present flood risk, adaptation will need to reduce flood probabilities below present values. The increase of the magnitude of losses when floods do occur makes it critical to also prepare for larger disasters than we experience today. The cities that were identified to be most vulnerable to these trends are distributed all over the world, with a concentration in the Mediterranean Basin, the Gulf of Mexico and East Asia. There are no European cities in the Top 20 of cities ranked by risk in 2005 (expressed as the highest economic average annual losses). Also, there are no European cities in the Top 20 for cities with the highest economic average annual losses in 2050, assuming an optimistic sea level rise of 20 cm between 2005 and 2050 and a policy where the current flood probability is maintained. Even if adaptation investments maintain constant flood probability, subsidence and sea-level rise will increase flood losses, however. Thus, the 20 cities where economic average annual losses increase most (in relative terms in 2050 compared with 2005) in the case of optimistic sea-level rise and maintenance of current flood probability contains 5 European cities: Marseille, Napoli, Athens, Istanbul and Izmir (5).

Vulnerabilities - Global assessment impact sea-level rise

This section is based on (40), (41) and (119).

Sea-level rise is just one of the factors that determine future flood risk

Coastal flooding is often the result of extremely high water level events due to the combined contributions of large waves, storm surge, high tides, and mean sea-level anomalies. Waves, storm surges, and tides in turn are influenced by the morphology of the coastal zone. The impact of sea-level rise on the risk of coastal flooding must be assessed as part of all these contributing factors. It is difficult, therefore, to predict the effect of sea-level rise on episodic flooding events due to the unpredictable nature of coastal storms, nonlinear interactions of physical processes (e.g., tidal currents and waves), and variations in coastal geomorphology.

Sea-level rise will increase the frequency of occurrence of a current high-water level event, such as a once-in-a-hundred-years flood for instance. To what extent a certain amount of sea-level rise increases this frequency may be completely different at different locations. The impact of sea-level rise on magnifying (multiplying) the frequency of occurrence of a specific flood level depends on the combination of the factors mentioned above, and these factors are different from one place to another. 

Theory: Different coastlines, different impacts sea-level rise

At many coastlines low frequency floods will occur far more often 

Along many coastlines upper bounds exist on tide, storm surge, and maximum wave heights due to limiting processes (e.g., wave breaking and physical limits in wind speed, fetch, and duration prevent unbounded wave heights). At these coastlines the water level that occurs, say, once in 100 years is only slightly lower than the one that occurs once in 500 years. And the water level difference between events that occur once in 500 years and once in 1000 years is even less. The increase of water level with an increase of the return period of an event levels off when events become less likely due to the upper bounds of the factors that contribute to the extreme water level events. Along these coastlines just a little bit of sea-level rise already leads to a relatively large shift of the return period of a certain flood level, and these flood levels will occur far more often even if sea-level rises only a few centimetres.

Where tropical storms occur, impact of sea-level rise is less important

Things are different at coastlines that are under the influence of tropical storms: along these coastlines the low-frequency extreme events are determined by these rare tropical storms. The frequency of occurrence of, say, a once in 100 years flood level is determined by the likelihood of these tropical storms. Along these coastlines a little bit of sea-level rise leads to a much smaller shift of the frequency of occurrence of an extremely high flood level.

The US as an example: different coastlines, different impacts sea-level rise

An example of a coastline with upper bounds on tide, storm surge, and maximum wave heights is the West coast of the United States. Along this coastline sea level rise will strongly increase the frequency of current low frequency events. The East coast of the United States is an example of a coastline under the influence of tropical storms. Along this coastline the frequency of current low frequency events will increase much less with sea-level rise (41).

Latest research: a few cm of sea-level rise may double coastal flooding frequency

The effect of projected future sea-level rise on changes in the frequency of extreme flood levels was studied on a global scale. In this study the current conditions on waves, tide and storm-surge were included; they were not changed for future time slices (40).

A dire future for the top 20 cities

These recent results on increased flooding potential under sea-level rise suggest a dire future for the top 20 cities (by GDP) vulnerable to coastal flooding (42), and for many wave-exposed cities such as Mumbai, Kochi, Grande Vitoria, and Abidjan which may be significantly affected by only 5 cm of sea-level rise. Less than 10 cm of sea-level rise doubles the flooding potential over much of the Indian Ocean, the south Atlantic, and the tropical Pacific. Only 10 cm of sea-level rise doubles the flooding potential in high-latitude regions such as the North American west coast (including the major population centres Vancouver, Seattle, San Francisco, and Los Angeles), and the European Atlantic coast (40).

Strong increase flooding potential Tropics

Just like the US West coast and the European Atlantic coast, the Tropics are also not influenced by extreme water levels (due to hurricanes, e.g.). These regions, therefore, will also experience greater increases in flooding frequency due to sea-level rise. Small amounts of sea-level rise, e.g. 5-10 cm, may more than double the frequency of extreme water-level events in the Tropics as early as 2030. A current once-every-50-years flood event will occur every 5 years at 10 cm sea-level rise (40). This is an especially critical finding as numerous low-lying island nations in the Tropics are particularly vulnerable to flooding from storms today, and a significant increase in flooding frequency with climate change will further challenge the very existence and sustainability of these coastal communities across the globe (43).

Far less impact on coastlines influenced by tropical storms

The impact of sea-level rise on flooding potential increase is much less for coastlines that are influenced by tropical storm tracks, such as the mid-latitudes of the north-western Pacific below Japan, the mid-latitudes of the north-western Atlantic (the U.S. east coast, Gulf of Mexico, and Caribbean Sea), and the southwest tropical Pacific encompassing Fiji and New Caledonia. The rare occurrence of extreme events (hurricanes, e.g.), and not sea-level rise, will remain the dominant hazard on these coastlines. The number of current high frequency flood levels, however, will strongly increase. In these coastal areas nuisance flooding, therefore, will strongly increase and will continue to be a major (increasing) problem (40).

Extreme sea levels at 2 °C global warming in 2100

Regional differences

Flood protection is generally designed to protect societies against extreme sea levels that can occur with a certain probability, say – for instance – once a century. The return period of these extreme events will change much more rapidly for a certain sea level rise at coastlines where the contribution of storm surges, tides and waves is relatively small. A few decimeters of sea level rise may shift the likelihood of a current once-a-century extreme sea level to a once-a-decade or even an annual event at a Mediterranean coast while the same sea level rise hardly changes the likelihood of a once-a-century event in Northern Europe. As a result, the implications for flood protection may be much larger for countries bordering the Mediterranean Sea compared with Northern Europe (119).

A global study

Scientists have estimated the impact of future sea level rise on the return period of extreme sea levels globally. More specifically, they looked for the level of global warming that would shift the likelihood of a present-day once-in-a-century event to an annual occurrence by 2100. They focused on the contribution of sea level rise only, and assumed that waves and storm surges would not change. They did so for over 7,000 locations (119).

A 100-fold increase

By the end of the century the present-day once-a-century extreme sea level would have become an annual event in close to half of the locations considered, even if global warming would not exceed the 1.5 to 2 °C limits of the Paris Agreement. The uncertainty range in this estimate is large, however. On the pessimistic end of this range, this dramatic shift would be a reality for practically all sites. On the optimistic end, this dramatic shift could be avoided for about 70–80% of the locations (119).

Most dramatic for tropics and Mediterranean

The shift would be most dramatic for coastlines in the tropics and parts of the Mediterranean and the Arabian Peninsula. Here, these 100-fold changes in frequency of extreme sea level events will take place even if global warming in 2100 is limited to 1.5 or 2°C. In coastal regions where tidal amplitudes are larger, at the highest latitudes of both hemispheres, even higher levels of warming will not produce such frequency increases (119).

These findings are important for local flood risk policy and necessary future improvements of flood protection. Even if the Paris Agreement goals will be achieved, extreme sea level events will be experienced at unprecedented frequencies in many parts of the world’s coasts.

Vulnerabilities - Global assessment impact land subsidence

The land in many parts of the world is subsiding, mainly because of groundwater depletion. Especially in densely populated coastal zones and deltas, subsidence contributes more to relative sea-level rise than absolute sea-level rise (118). During the past century, land subsidence due to groundwater depletion occurred at 200 locations in 34 countries (111). Land subsidence increases coastal and fluvial flood risk. Coastal flood risk is determined by relative sea-level rise: the sum of absolute sea-level rise and land subsidence.

For 8% of the global land surface there is a 50% probability that the land is threatened by land subsidence. Potential subsidence areas are concentrated in and near densely urban and irrigated areas with high water stress and high groundwater demand. These areas include 22% of the world’s major cities (111).

For 1.6% of the global land surface the probability of land subsidence is high or very high. This area includes 19% of the global population and 12% of global GDP. Part of this area is flood-prone; potential subsidence threatens 484 million inhabitants living in flood-prone areas, 75% of whom live in fluvial areas and 25% of whom live near the coast (111).

Land subsidence is a major contributor to flood risk. 50% of the global population exposed to flooding hazards lives in areas where the land is subsiding (112). Subsidence threatens 15 of the 20 major coastal cities ranked with the highest flood risk worldwide (113).

During the next decades, global population and economic growth will continue to increase groundwater demand and accompanying groundwater depletion and, when exacerbated by droughts, will probably increase land subsidence occurrence and related damages or impacts. By 2040, the part of the global population threatened by land subsidence is predicted to rise by 30%, affecting 1.6 billion inhabitants, 635 million of whom will be living in flood-prone areas (111).

Coastal cities

In several coastal cities, land subsidence is much faster than the IPCC reported in its latest assessment. This is concluded from a detailed study of vertical land movement of 48 coastal cities representing 20% of the global urban population. In this study, the velocities of local land subsidence were quantified from satellite radar observations over the period 2014 to 2020. For each coastal city, subsidence rates were quantified relative to a nearby reference point and these rates, therefore, represent relative land subsidence within the city. Since both the reference point and the rest of the city are subject to the same large-scale land movements of tectonics and glacial isostatic adjustment, these processes are not included in the relative data (129).

The data on relative subsidence rates allow for the identification of specific areas and neighbourhoods in cities that are undergoing rapid subsidence and thus facing accelerated relative sea level rise and greater exposure to coastal hazards. This local subsidence is often caused by pumping up groundwater, or by oil and gas extraction and compaction of sediments underlaying the city (129).

Whereas the IPCC estimated that the maximum rate of subsidence in these coastal cities is 5.2 mm per year, these radar observations show that these cities are subsiding up to a rate of 16.2 mm per year, being the median value of subsidence rates across the city. Locally, subsidence can be much faster, even up to a few centimetres per year for some of these cities. The authors of this study conclude that the higher level of spatial detail in their data and the fact that their analysis extends further inland, reveals that the spatial variability of land subsidence in cities is much larger than the IPCC data suggest, including relatively high subsidence rates locally (129).

Apparently, local land subsidence strongly varies spatially in response to the spatial distribution of both human activities related to groundwater use and subsurface geology. In parts of these coastal cities, local land subsidence due to, in particular, human activities dominates over absolute sea level rise and land movement because of glacial isostatic adjustment. This highlights the importance of integrating these subsidence observations in future assessments of relative sea level rise (129).

Vulnerabilities - Global vulnerabiliy underestimated?

According to a study published in 2019, the elevation of densely populated coastal zones appears to be much lower than has been assumed so far. The authors of this study conclude that global impacts of sea-level rise will likely be far greater than studies so far have shown (95). The results of this study are summarized below. Similar results are presented by (128).

Estimates of coastal zone elevation

In large parts of the world, high-quality (airborne) land elevation data are unavailable or expensive. As an alternative, satellite-based data can be used. These data are available for parts of the globe that are home to 99.7% of the world population. The quality of these data suffers from the fact that satellites measure the elevation of upper surfaces and not bare earth terrain. The satellites measure the height of trees and buildings, and not the land surface. Satellites-based elevation data, therefore, have a positive bias when used to represent terrain elevations, especially in densely vegetated and in densely populated areas (96). Mean bias can be several metres for land areas with an elevation in between 1 and 20 m (97). The elevation of densely populated coastal zones appears to be much lower than has been assumed in many studies so far.

This problem has been addressed recently by using neural network techniques to correct the data. As a result, the bias in elevation was reduced from a couple of metres to less than a few decimetres. For coastal cities such as Miami, New York City, and Boston, for instance, vertical bias was reduced from 4.71 m to less than 0.06 m. Needles to say that a global assessment of the number of people exposed to coastal flooding based on these data paints a completely different picture compared to studies in the past (95).

Estimates of number of people exposed

With these new data on elevation, vulnerabilities have been updated. In this update, coastal defences are not considered since location data of the defences are not available at the global scale. Changes in population growth and migration are also not considered; the results indicate threats relative to present development patterns (95).

Current vulnerabilities, updated

The new data show that, for the present day, 110 million people live on land below the current high tide line, globally, much higher than the previous estimate of 28 million. 250 million people now live on land below annual flood levels, again much higher than the previous estimate of 65 million (5).

Future vulnerabilities, updated

The new elevation data triple the estimates of global vulnerability to sea-level rise and coastal flooding. The projected rise of the current high tide line under a high-end scenario of climate change (RCP 8.5) will increase the number of people living on land below this line from 110 million today to 190 million (range: 150–250 million) in 2100. Under this high-end scenario, the number of people living on land below annual flood levels is projected to increase from 250 million people now to 340 million people for mid-century, and 630 million people for 2100 (95).

If we would succeed in keeping global warming below 2°C, and the Antarctic ice sheet would not become unstable, the number of people living on land below annual flood levels would still be 360 million in 2100 (95).

It was previously estimated that currently about 640 million people live in the low elevation coastal zone, defined as areas below 10 m (98). With the new elevation data, this estimate rises to just over one billion people.

Asia most vulnerable

The new elevation data paint a particularly grim picture for several Asian countries. Bangladesh, India, Indonesia, and the Philippines see a 5-fold to 10-fold change in estimated current populations below the projected high tide line after applying these new data (95).

More than 70% of the total number of people worldwide currently living on implicated land are in eight Asian countries: China, Bangladesh, India, Vietnam, Indonesia, Thailand, the Philippines, and Japan. Chronic coastal flooding or permanent inundation threatens areas occupied by more than 10% of the current populations of nations including Bangladesh, Vietnam, and many Small Island Developing States by 2100 (95).

Vulnerabilities - Global assessment impact storm surge

Extreme sea levels are rising

Sea level rise itself does not lead to flooding events, it’s the extreme sea levels that we need to keep an eye on. Sea level rise, of course, is the major factor that changes the likelihood and height of these extreme sea levels (131). But more factors play a role. In addition to sea level rise, tides, storm surges and waves also contribute to these extreme sea levels. And scientific studies indicate that these extreme sea levels will probably become far more likely in this century. Even if we succeed in limiting global warming to 1.5°C above pre-industrial levels, the goal outlined in the Paris Agreement, the extreme sea levels that now occur once every 100 years on average will have become an annual event by 2080 for about half of the world's coastline (132). Meanwhile, many scientists report that achieving the goal of 1.5°C global warming is already beyond our reach.

Impact of storm surges will change

The frequency and intensity of storms, and the paths these storms follow are changing. As a result, storm surges, and their contribution to extreme sea levels, are also changing.

In a recent study, scientists focused on the contribution of storm surges to these extreme sea levels (130). Global warming will affect the occurrence and strength of storms and tropical cyclones, although these changes are highly uncertain. The strength and frequency of occurrence of storms at mid-latitudes, including most of Europe, will probably not change that much, although the path these storms generally follow will move poleward causing substantial changes in some regions (133). The characteristics of tropical cyclones, however, may change dramatically. They may not become more likely (134), but they are projected to become stronger, more intense in response to warmer sea surface temperatures (135). Besides, the path these tropical cyclones follow may shift poleward in a warmer climate. As a result, mid-latitude regions may experience more post-tropical storms (136).

In their study, the scientists distinguished three periods: the past (1951–1980), the present (1985–2014) and the near-future (2021–2050). Based on model projections, they estimated the change in magnitude and frequency of storm surges in the near future. They used a high-end scenario of climate change (SSP5-8.5). This scenario is probably too dramatic for the long term. For the near future of 2021–2050, however, the impact of this scenario will not be that different from the impact of most other scenarios.

Decrease for Mediterranean, increase for the North Sea

According to the results, storm surges haven’t changed that much so far relative to the historic period of 1951–1980. It looks like in the current situation natural variability dominates over the impact of climate change. For the near future, storm surge levels are projected to change by more than 5% for approximately 25% of the coastal locations in this study. In absolute terms this is a change larger than 0.1 m. The largest and most coherent storm surge decrease occurs in the Mediterranean Sea. An increase of storm surge levels is projected for parts of the North Sea (Northwest Europe), the Yellow Sea (Eastern China), Patagonia, and northern Australia (130).

The storm surge decrease for the Mediterranean may be due to a decrease in storm frequency (137). According to the authors of this study, the increase for the coastal regions mentioned above could possibly be linked with a poleward shift of storm tracks and increased risk of post-tropical cyclones (138).

Small changes, large uncertainties

The authors conclude that changes in storm surge could contribute to changes in future extreme sea levels, although sea level rise will be the main driver of these changes. In addition, they stress that waves are an important contribution as well, especially since they are correlated with storm surges (139). However, they also stress that the projected storm surge changes are small compared to various uncertainties and natural variability.

Vulnerabilities - Global assessment impact storm surge and waves

Water levels at the coast during extreme events are the combined effect of tides, storm surges and waves (wave setup and run‐up). In flood risk assessments, the impact of waves is often left out. The contributions of storm surge and waves to extreme water levels should be combined, however. When doing so, it makes a huge difference if these contributions are considered to be independent or not. The same storms causing storm surges also generate wind waves. Because of this, the contributions of storm surges and waves are not independent statistically. The probability of their joint occurrence is higher than that expected considering the extremes of each variable separately, with a consequent increase of the likelihood of coastal flooding (90).

The first global assessment of the dependence between storm surges and wind waves and its effect on extreme coastal water levels was published recently. The assessment focused on the period 1979-2014. With respect to the contribution of waves, only wave setup (and no wave run-up) was considered (90).

The study shows that in more than half of the world’s coastal regions, storm surges tend to be accompanied by large wind waves, thus increasing the potential coastal flooding. The probability of facing a 1 in 100‐year event is more than doubled in 30% of the global coastlines when accounting for the dependence between storm surges and waves. In other words, in these locations, an event expected at most once in 100 years without considering dependence between storm surges and waves, is in fact a 1 in 50‐year event because of this dependence. Likewise, a fivefold (tenfold) difference in the 50‐year return period is found in nearly 20% (8%) of the global coastlines for calculations in which this dependence is taking into account or left out (90).

The importance of considering this joint occurrence of storm surge and waves is illustrated for the North Sea. An extreme water level of 3.2 m resulting from the combined action of surges and waves is expected once every 50 years. Without dependence of storm surge and waves, this water level would be a 1 in 532‐year event (90). 

Vulnerabilities - EU assessment for a constant protection level

In the European Union and the United Kingdom, 100,000 people are already exposed to floods every year, and the average annual flood damages are estimated to be EUR 1.4 billion (140).

Without flood defences, almost 6 % of the European population would be living in the 100 year flood area (coastal and river floods) and the corresponding economic loss could be €236 billion (data for 2010). Estimated flood protection reduces economic damage substantially by 67 to 99 % and the number of people flooded is reduced by 37 to 99 % for the 100 year event (11).

With no adaptation, coastal flooding in the 2080s is projected to affect an additional 775,000 (B2 climate change scenario) and 5.5 million (A2 climate change scenario) people per year in the countries of the EU (9). Direct costs from sea level rise in the countries of the EU without adaptation could reach 17 billion Euros per year by 2100 (10). Without the current flood defences, Amsterdam, Hamburg and Copenhagen would be among Europe's most vulnerable coastal cities, with a worst case expected annual damage in 2100 of over 1.1 billion Euros for each of these cities (91). 

Vulnerabilities - Global assessment rich versus poor countries

The world needs to adapt to sea level rise, changes in storm surge and extreme waves, higher peak river discharges and flash flooding, but a large part of the global population lacks the financial means to do so. Researchers looked at this and quantified the number of people in high-, middle- and low-income countries that would be exposed to a 1-in-100-year flood event without adequate flood protection. They looked at all kinds of flooding: from heavy downpour (pluvial), from rivers (fluvial), and the sea (coastal) (123).

1.61 billion people vulnerable to flooding

According to their estimates, globally, 1.81 billion people live in locations that get flooded by at least 15 cm of water in the event of a 1-in-100-year flood. In other words, without adequate flood protection, nearly one in four of the world’s population has a 1% chance of getting wet feet, or worse, every year. Nine out of ten of them – 1.61 billion people – live in low- and middle-income countries. These figures highlight the significant risks faced by developing countries. The two most populous countries, India and China, have the highest absolute exposure headcounts with 390 million and 395 million, respectively, and account for about one-third of all people exposed to flood risk globally (123).

‘Imaginary’ for high-income countries

One out of ten of the 1.81 billion people that would be exposed to flooding without adequate flood protection – 193 million people – live in high-income countries. For these people, the results in this study are ‘imaginary’: they do have adequate flood protection and coloring a 1-in-100-year flood level on a world map for these countries has no real meaning. Take The Netherlands, for instance. This country has the highest percentage of a country’s population living in areas that would face at least 15 cm of flooding in the event of a 1-in-100-year flood without considering flood protection systems (123). Naturally, the researchers also stress that this number has no real meaning in terms of ‘flood risk’. The Netherlands has the world’s most comprehensive flood protection systems. It is extremely unlikely that a 1-in-100-year storm surge at the coast or peak river discharge would result in flooding somewhere in the country.

A reality for the poor

Forget about the ‘imaginary’ numbers for high-income countries. That’s not the core message of this study. For nine out of ten ‘flood exposed’ people this exposure is not imaginary at all. A 1-in-100-year flood event presents a real risk for them. The contrast between the vulnerability of people living in low-lying areas in high-income countries versus those living in low- and middle-income countries becomes even more dire when focusing on the economic risk of flooding. Again, under the assumption of no flood protection, $9.8 trillion of economic activity is exposed to flooding in a 1-in-100-year flood event. 84% of this is located in high- and upper middle-income countries. As a result, in the words of the researchers, ‘planners would bias their attention toward areas with high-value assets and large resources. But in so doing, they risk overlooking areas with high socio-economic vulnerability, where flood risk mitigation measures are most urgently needed to protect lives and livelihoods’. Low- and lower-middle-income countries account for 52% of exposed people, but only 16% of exposed economic activity (123).

The study shows that of the 1.81 billion people exposed to flooding worldwide, around 780 million live in high-risk flood zones while living on less than $5.50 a day. This means that four out of every ten people exposed to flood risk globally are living in poverty. At least 170 million are living in extreme poverty – on less than $1.90 per day – 88% of which in Sub-Saharan Africa and South Asia (123).

Core message

The researchers conclude that, under current conditions, more people than previously known are exposed to flood risks. What’s more, climate change and urbanization are expected to further aggravate these risks in coming years (124). The world needs to adapt but, when prioritizing investments in flood protection, chances are that those investments will be concentrated in high-income countries and economic hubs, simply because the economic value of assets and activities that need to be protected are relatively high there. As a result, low-income countries would remain disproportionately exposed to flood risks and continue to be more vulnerable to disastrous long-term impacts (123).

The impact of extreme sea levels versus socioeconomic developments

Flood hazard depends on the probability of extreme sea levels, and these are the combination of changes in sea level, tides, storm surges and waves. Flood risk results from the combination of this hazard and potential consequences of coastal flooding. Future changes in the latter depend on changes in demographic growth, migration to coastal zones and economic development. In addition to sea-level rise, the other components that determine extreme sea levels are also changing due to climate change (76). Europe has a coastline of over 100,000 km, with coastal zones that are densely populated and pivotal for its economy. Hence, also socioeconomic developments control flood risk. Socioeconomic drivers, such as demographic growth, migration to coastal zones and economic development, are mainly responsible for the increasing flood losses to date (77).

The change of Europe’s coastal flood risk in future decades, based on changes in all these contributing factors, has been assessed. High-resolution projections of all these extreme sea-level components, based on a moderate (RCP4.5) and high-end scenario of climate change (RCP8.5), were combined with projections of gross domestic production (GDP), population and exposed assets. Flood damage not only depends on inundation depth but also on the local value of investments; therefore, region-specific depth-damage relations were linked to land use (78). Updated information on the level of coastal protection was used (79). No upgrade of these protection standards was assumed in the calculations.

Expected annual damage. Flood risk can be expressed in economic damage and in number of casualties. For economic damage, the combination of flood probability and damage consequences results in the quantity ‘expected annual damage’.

Currently, under the present climate conditions (reference year 2010), Europe’s expected annual damage from coastal flooding is€1.25 billion. About half of this number is attributed to the UK (31%), France (10%) and Italy (9%). By 2050, this damage is projected to increase to €12.5 - 39 billion a 10- to 30-fold increase. In the second half of this century, the projected damage even increases to €93 - 961 billion, a staggering increase of 75 to 770 times. The UK, France and Norway show the highest absolute increase towards the end of the century among all scenarios.

Expected annual damage from coastal flooding is currently around 0.01% of GDP for Europe, compared to nearly 0.04% (approximately €6 billion per year) for river flooding (80). This share is projected to grow in the coming decades to range between 0.29 and 0.86% of GDP by the end of this century for the scenarios considered, which is far larger than the share of future river flood risk to GDP in high-income countries (81). This implies that if flood protection standards are not upgraded along Europe’s coasts and rivers, the total flood risk in Europe will increasingly be dominated by coastal flood hazards from the mid-century onwards. At a country scale, relative economic impacts can be even more pronounced (up to 5% of a country’s GDP by 2100), with the highest burden relative to its economy projected for Cyprus, Norway, Ireland and Denmark.

Expected annual number of people exposed. For the number of casualties, the combination of flood probability and number of people exposed to flooding results in the quantity ‘expected annual number of people exposed’.

The current expected annual number of people exposed to coastal flooding equals 102,000, with the UK (28%), Italy (12%) and Croatia (12%) representing more than half of this number. By 2050 this number is projected to rise to around 533,000 – 742,000, further climbing to 1.52 - 3.65 million people by the end of the century.

Worst-case scenario. There is a wide uncertainty around these central estimates due to, for instance, different ice-sheet scenarios and the spread in outcomes of different models. In fact, a worst-case scenario cannot be excluded where total European coastal flood damage could annually exceed €2.5 trillion and six million people could be exposed to coastal flooding each year by the end of the century.

Extreme sea levels main driver. The results of this study show that the intensification of extreme sea levels with global warming is the main driver of the strong rise in Europe’s coastal flood risk. This does not agree with recent findings for river flooding (81) and previous coastal impact studies that suggest socioeconomic drivers dominate future flood risk (82).

In time, the footprint of socioeconomic change on future coastal flood risk becomes less important. This is particularly the case for Estonia, Greece, Lithuania and Latvia, where the negative socioeconomic outlook strengthens the contribution of coastal hazards. Minor contributions from socioeconomic growth are also projected for Bulgaria, Germany, Denmark, Malta, the Netherlands and Norway. On the other hand, socioeconomic growth is mainly responsible for the rise in economic losses in Portugal, Romania and Slovenia, but also in Finland, Poland and Sweden. For the latter three countries, this is due to the ongoing land uplift at the Baltic Sea that moderates the contribution of sea-level rise and results in a weaker physical footprint in future damages.

Adaptation. In this study, flood protection standards were not upgraded in time. Adaptation, however, is already ongoing in several European countries, and others will follow. Hence, future coastal flood risk will probably be lower than the estimates presented above. It is to be expected that flood protection standards will be upgraded. If European countries wish to keep coastal flood risk constant relative to the size of their economy by 2050, protection measures will need to resist extreme sea levels that, on average over Europe, are about half a metre higher. This further climbs to one metre or more, depending on the scenario, towards the end of the century. According to this study, the required increase in protection level may reach two metres or more in some low-lying coastal stretches, for example along the coasts of Belgium, Germany, Portugal and the Netherlands.

Northwestern Europe: trends in compound flooding

Compound floods in delta areas are the co-occurrence of high coastal water levels and high river discharge. Correlations between high water levels at the coast and in rivers have been reported in the scientific literature both globally (105), and regionally across Europe (106). There are two mechanisms that may cause such events. First, high coastal water levels may affect river flows and water levels by backwater effects or by reversing the seaward flow of rivers, particularly in low-lying areas. Second, high coastal water levels and high river discharge may stem from a common meteorological driver. Severe storm periods may be associated with high winds leading to storm surges, and at the same time with high precipitation followed by inland flooding (104).

Compound flooding may lead to significant impacts and much more disastrous consequences than each of these extremes individually. The occurrence of compound floods from extreme coastal water levels and peak river discharge was studied for northwestern Europe, along the coastlines from France to Sweden. Datasets were used for locations at the North Sea and the Baltic Sea covering most of the previous century (104).

In the definition of compound floods, water level extremes at the coast and in rivers do not have to occur at exactly the same spot and the same time. The impact of a storm may be noticed in river discharge a couple of days later than in storm surge levels at the coast because it will take some time for rainfall to reach the river. Also, the river outlet may be far from the area where the storm was felt. In this study a flood event was called a compound event when high water levels at the coast and in the river were no more than 500 km apart and occurred no more than 7 days apart (104).

There appears to be a distinct difference between mid-latitude and high latitudes regions in northwestern Europe: compound flooding occurs more often in mid-latitude regions, along the west facing coasts of the United Kingdom, Germany, the Netherlands, and Sweden. No clear explanation was given for this. There may be a link with weather oscillations that have a stronger impact on certain parts of northwestern Europe. This may explain the presence of decadal and multidecadal variability in the magnitude and frequency of compound floods. There have been compound flood-rich and flood-poor periods, like the 1960s and the 1990s, respectively (104).

This variability makes it difficult to find long-term trends related to climate change or other human interventions. Although no attempt was made to relate trends in compound floods to climate change, the authors stress that global warming could increase the severity and frequency of compound extremes (107) by changes in wind speed and in rainfall intensity of severe storms.

Global mean sea-level rise - Observations

Global mean sea level rise results from the contribution of a number of components: an increase in the mass of the ocean, mainly as a result of changes in the mass of glaciers, Greenland and Antarctica ice sheets, and liquid water storage on land (reservoirs behind dams and groundwater), and an increase of the volume of ocean water because it warms up.

IPCC results (2019)

In 2019, IPCC has presented updates on observed global mean sea level (GMSL) (92):

The sum of ice sheet and glacier contributions over the period 2006 – 2015 is the dominant source of sea level rise, exceeding the effect of thermal expansion of ocean water (92).

Rate of sea-level rise is accelerating, as predicted

Since 1993 global mean sea level is being measured by satellites. These satellite altimeter data are far more accurate in monitoring global mean sea level rise than tide gauges since the latter, naturally, only cover a relatively small part of the earth’s ocean. Unlike tide-gauge data, satellites sample the open ocean and allow for precise quantitative statements regarding global sea level.

The IPCC concluded in 2019 that global mean sea level (GMSL) is rising, with acceleration in recent decades due to increasing rates of ice loss from the Greenland and Antarctic ice sheets, as well as continued glacier mass loss and ocean thermal expansion. The IPCC concluded that mass loss from the Antarctic ice sheet over the period 2007 -  2016 tripled relative to 1997 - 2006. For Greenland, mass loss doubled over the same period (92). The IPCC concluded that GMSL from tide gauges and altimetry observations increased from 1.4 mm/year over the period 1901 - 1990 to 2.1 mm/year over the period 1970 - 2015 to 3.2 mm/year over the period 1993 - 2015 to 3.6 mm/year over the period 2005 - 2015 (93).

Now that satellite data span a time period of over 25 years, several studies are carried out to quantify the acceleration of sea-level rise. Here we summarize some recent results:

• A preliminary satellite-based estimate of the climate-change–driven acceleration of sea level rise was obtained by (67), based on 25 years time series of satellite altimeter data. They found an average climate-change–driven rate of sea level rise of 2.9 mm/year. Current acceleration of sea level rise would lead to 65 ± 12 cm global mean sea level rise by 2100 compared with 2005, they concluded. This roughly agrees with the projections of the IPCC made in 2013 under a high-end scenario of climate change (the so-called RCP8.5 scenario) (72). Thus, the observed acceleration would more than double the amount of sea level rise by 2100 compared with the current rate of sea level rise continuing unchanged (67). Again, this is no surprise; the acceleration has always been part of the future projections of the impacts of climate change. The estimated 65 cm global mean sea level rise by 2100 may turn out to be a conservative lower bound on future sea level change. After all, this projection is based on the assumption that sea level changes similarly in the future as the observations over the available time series of 25 years. However, sea level may rise more rapidly in the coming decades, for example due to rapid changes in ice sheet dynamics (67).
• The same satellite data have been studied by (68), be it over a shorter time period of 2005-2015, but they combined these data with observations of changing salinity and temperature in the oceans, and of land water and land ice contributions to global mean sea level rise. Thus, they were able to ‘scrutinize and understand the sources of the sea level acceleration in the last decade’. According to their results, the acceleration during the last decade is about 3 times faster than its value during 1993-2014. The acceleration comes from three factors, they conclude: a declining amount of water that is stored on land (~41%), land ice melting (~15%), and thermal expansion of the water of the oceans that are warming up (~44%). The first factor refers to, among other, fast amounts of groundwater that have been extracted and in the end reach the oceans. The contribution of the thermal expansion of the oceans to the acceleration of sea level rise is about three times as large as the contribution of melting land ice.
• The conclusion that the rate of global sea level rise is increasing agrees with the results of previous studies. In one of these studies a significant acceleration of sea level rise was identified from 2.67 ± 0.19 mm/year in 1993-2004 to 3.49 ± 0.14 mm/year in 2004-2015. The 0.8 mm/year increase between these two periods dominantly results from increased land ice loss from Greenland (+ 0.5 mm/year) (39).  In another, a gradual increase of the rate of sea level rise over the period 1993-2014 was quantified from 2.2 ± 0.3 mm/year in 1993 to 3.3 ± 0.3 mm/year in 2014 (70).
• According to calculations that include data from American and European satellites, the average acceleration of sea level rise between 1991 and 2019 was 0.1 mm/year2 (or to be more precise, 0.095 mm/year2). This means that the rate of sea level rise is now 1 mm per decade faster than 10 years ago. If, for example, the oceans rose by 2 mm in 2000, by 2010 they would have risen by 3 mm, and around 2020 by almost 4 mm (100).

Current sea level rise of 3.5 mm per year agrees very well with the sum of all components mentioned above and reported in other scientific studies. Next to these components, the contributions of other components are still more or les blind spots in scientific literature: expansion of the ocean due to the warming of deep ocean water > 2 km, and water flowing into the ocean from permafrost melting, for instance. Apparently, the latter components still hardly contribute to current sea level rise (39). 

The results by (39) contradict the results of a previous study that points at lower (13) sea level rise based on data over almost the same period: 2.6 - 2.9 ± 0.4 mm/year. According to both studies sea-level rise is accelerating, due to an accelerating contribution from the Greenland and West Antarctic ice sheets over this period (15,16). However, this acceleration in sea-level rise over the last 25 years according to (39) is 0.8 mm/year, much higher than the Intergovernmental Panel on Climate Change projections (15,17) of about +0.07 mm/year.  

Sea-level rise varies from year to year, however, due to short-term natural climate variability (especially the effect of El Niño–Southern Oscillation) (7,14): the global mean sea level was reported to have dropped 5 mm due to the 2010/2011 La Niña and have recovered in 1 year (14).

Global mean sea-level rise - Projections

For 2046-2065 and  2081-2100 compared to 1986-2005, projected global mean sea level rise (metres) is in the range (72):

The IPCC has updated the projections for the low-end and high-end scenarios in 2019 (92):

Sea level does not and will not rise uniformly. Thermal expansion, ocean dynamics and land ice loss contributions will generate regional departures of about ± 30% around the GMSL rise. Local anthropogenic subsidence and change in wave height and period are important contributors to future changes in relative sea level (RSL) at the coast (93).

For 2200 onwards compared to 1986-2005, projected global mean sea level rise (metres) is in the range (8):

Global mean sea level rise will cause the frequency of extreme sea level events at most locations to increase. Local sea levels that historically occurred once per century (historical centennial events) are projected to occur at least annually at most locations by 2100 under a wide range of climate change scenarios (92).

The IPCC concluded in 2019 that sea-level rise under a low-end scenario of climate change (RCP2.6) will be limited to around 1m in 2300 while under a high-end scenario (RCP8.5), a multi-metre sea-level rise is projected (92).

Global median sea-level rise - Projections 2100

Drivers of sea level rise

Drivers that contribute to sea level rise are thermal expansion of ocean water, melting glaciers, mass loss of the Greenland and Antarctic ice sheets, and changes in water volumes stored on land. Recently it was shown that global sea level rise may strongly accelerate from hydro fracturing and ice cliff instability of the Antarctic ice sheet (62), causing significantly more sea level rise by the end of this century than presented in the IPCC’s Fifth Assessment Report. Studies that have included these new insights project sea level rise at the end of this century up to one meter higher than the IPCC estimates (63).

Scenarios of socio-economic development and of global warming

How fast global sea level will rise depends on the size of the processes that contribute to sea level rise. These processes in turn depend on the combination of socio-economic developments around the globe and the impact of greenhouses gases on global warming. Socio-economic developments are described in scenarios, storylines that include elements like population and economic growth, and an ambition to mitigate climate change and adapt to the consequences. Different storylines reflect different impacts on the environment. The impact of greenhouses gases on global warming is described in so-called Radiative Forcing Targets: the energy intensity that warms the atmosphere, expressed in W/m2. A high-end scenario of global warming agrees with a lot of Watts per square metre. A low-end scenario with far less Watts per square metre, and hence a strong effort to mitigate the impact of greenhouses gases on global warming. High- and low-end scenarios of global warming are 6.0 W/m2 and 2.6 W/m2, respectively (64).

Five scenarios of socio-economic development, the so-called Shared Socioeconomic Pathways (SSPs), have been designed to comprehensively capture varying levels of socioeconomic challenges to mitigation and adaptation (65):

• SSP1, a sustainable pathway: low challenges to mitigation and adaptation
• SSP2, a ‘middle of the road’ trajectory: medium challenges to mitigation and adaptation
• SSP3, a future world of regional rivalry: high challenges to both mitigation and adaptation
• SSP4, a future marked by inequality: low challenges to mitigation and high challenges to adaptation
• SSP5, on-going fossil-fuel development and high energy demand: high challenges to mitigation and low challenges to adaptation.

These Shared Socioeconomic Pathways and Radiative Forcing Targets can be combined to estimate future sea level rise. This has been done in a recent study, in which also the latest findings on hydro fracturing and ice cliff instability of the Antarctic ice sheet were included (61):

• Sea level rise without mitigation: If we would not take measures to mitigate global warming, the aforementioned socio-economic scenarios would lead to sea level rise in 2100 (relative to 1986 - 2005) that varies from 89 cm for the SSP1 scenario up to 132 cm for the SSP5 scenario. These are median values: the likely ranges (including 66% of the model results) are 57 - 130 cm for SSP1, and 95 - 189 cm for SSP5. Projections for the other scenarios are in between these values (61).
• Sea level rise with mitigation: The scenarios of global warming (radiative forcing targets) reflect different levels of success to mitigate climate change. It appears that the level of mitigation that is being reached dominates the 2100 sea level response. For the low-end scenario of global warming, and thus the highest mitigation target that is being reached (2.6 W/m2), projected median sea level rise is 52 cm (likely range 34 - 75 cm). For the high-end scenario of global warming, and least successful mitigation (6.0 W/m2), projected median sea level rise is 91 cm (likely range 61 - 132 cm). Thus, according to this study, sea level rise in 2100 could be limited to around 50 cm (median value) if the most ambitious climate mitigation measures would be implemented in time (61).

The 132 cm sea level rise in the SSP5 scenario without mitigation is a few decimetres lower than recent results on comparable scenarios (66). However, this study also shows that sea level could rise up to 2 m without mitigation. This highlights that ambitious climate policies are needed to avoid the most severe impacts from rising sea levels around the globe. As such, these higher estimates point to a growing risk of potentially catastrophic sea level rise by the end of the 21st century under unchecked climate change. The study also shows, however, that strong mitigation efforts could prevent the onset of the rapid dynamics that cause the additional sea level contribution from the Antarctic ice sheets. Still, even when mitigation policy proofs highly successful, sea level rise will continue well beyond the 21st century (61). 

Global high-end sea level rise - Projections 2100

In 1990 the Intergovernmental Panel
 on Climate Change (IPCC) published its First Assessment Report. They projected the global mean sea level to rise by up to 1.15 m in 2100 (high estimate). In the following assessments the projected upper limit of sea level rise was adjusted downward step by step. In the most recent Fifth Assessment Report, however, the upper limit of projected sea level rise was adjusted upward again. In fact, studies published after the release of the Fifth Assessment Report show a further upward adjustment of sea level rise projections. Recent estimates suggest that global mean sea level rise could exceed 2 m by 2100. These projections are higher than previous ones and are based on the latest understanding of how the Antarctic and Greenland Ice Sheets have behaved in the past and how sensitive they are to future climate change (31,89).

For global mean sea level rise projections the estimated contribution of ice sheets is particularly important, but highly uncertain. This is due to the fact that our understanding of the way Greenland and Antarctic ice sheets move is limited. Older studies seem to have underestimated ice flow. The ice thins as it flows toward the coast until it crosses the grounding line to form floating ice shelves. The latter, still attached to the main ice sheet, are restrained from flowing faster by friction at their sides or by local seafloor highs. According to recent studies lateral and basal friction may be less than previously thought, and the speed of ice flow into the ocean faster. When ice shelves break off entirely they leave cliffs that calve icebergs directly into the ocean (32).

Recent projections of sea level rise may capture the ice sheet contribution better than in the past, leading to upward adjustment of sea level rise projections. Large uncertainties remain, however. Coastal defense measures must be flexible in the face of rising sea level estimates (31).

The impact of rapid mass loss from the Antarctic ice sheet on global mean sea level rise under a high-end scenario of global warming (the so-called RCP8.5 scenario) has been explored (35). The study was based on the latest IPCC report (33), took the same processes into account but included the results of a recent study on the impact of Antarctic ice mass loss (34). These recent results are a lot higher and more uncertain than the ones considered by the IPCC (33). This is mostly because the IPCC did not try to determine an upper end of Antarctic mass loss since the simulations to do it were not available and their estimates are meant to be a best guess instead of a high-end scenario. The outcomes of this study are a new high-end projection for global sea level rise in 2100 of 184 cm (median value) up to 292 cm (the extreme so-called 95% quantile of the range of sea level rise estimates) (35), much higher than the recent IPCC estimate of 73 cm (median value) for the same scenario of climate change.

Expert judgement study

Experts who took part in an expert judgement study find it plausible that sea level could exceed 2 m by 2100 under the business as usual scenario, more than twice the upper value put forward by the IPCC in 2014. They quantified the uncertainties of the physical processes that determine the mass balance of the ice sheets of the Greenland, West Antarctic, and East Antarctic ice sheets under two scenarios of global warming: a scenario of +2 °C stabilization in 2100 above preindustrial conditions, and a scenario of +5 °C stabilization in 2100. The first scenario reflects the possible consequences of the goal of the Paris agreement to keep global temperatures below +2 °C. The second scenario reflects business as usual from now to 2100 (89).

The experts’ estimate of the median value of the contribution of these ice sheets to sea level rise in 2100 is 26 cm and 51 cm for the +2 °C and +5 °C scenario, respectively. The upper ranges of their estimates, expressed as a 5 per cent probability, are much higher however: 81 cm for +2 °C, and 178 cm for +5 °C in 2100. This is just the contribution of the ice sheets. When thermal expansion and glacier contributions are included, it is plausible that sea level rise could exceed 2 m by 2100 under the business as usual scenario, the experts conclude. This is more than twice the upper value put forward by the IPCC in 2014 (89).

Beyond 2100, uncertainty and projected sea level rise increase rapidly. The 5 per cent probability ice sheet contribution by 2200, for the +5 °C scenario, is 7.5 m as a result of instabilities coming into play in both West and East Antarctica (89). 

Global median sea-level rise - Projections after 2100

Sea level rise projections generally focus on the second half of this century, but we all know that sea level will continue to rise for centuries or millennia into the future. Recently a study was published where the authors combined information on long-term projections of sea level rise, coastal elevation, and population density to assess coastal flood risk at the global scale from multi-century sea level rise. They did so for different levels of global warming, ranging from 1.5 °C to 4 °C (120).

They showed that 4 °C global warming would lead to 8.9 m of global mean sea level rise somewhere between 200 and 2000 years from now. 1.5 °C global warming would lead to ‘only’ 2.9 m of global mean sea level rise. These numbers are median estimates: a central value in a range of estimates where 50% of the model results are higher and lower than this median value, respectively (120).

It is of course impossible to estimate the population living in low-elevation coastal zones, globally, hundreds of years from now. The authors, therefore, took the current (2010) population living near the coast and calculated the extra number of people that would be exposed to coastal flooding at higher sea levels. The built environment, they argue, is largely immovable and the current situation is a good proxy for the future (120).

Currently, 2.5%–3.0% of the global population (170–200 million) lives in coastal zones that is projected to fall below the high tide line in 2100 if mean sea level were to rise by 0.48–0.73 m. Without adequate flood protection, this part of the global population may be considered vulnerable to coastal flooding by 2100. The authors estimated that 2 °C global warming, the proposed upper limit of the Paris Climate Agreement, would lead to a median 4.7 m of global mean sea level rise on the long run and threaten land now home to roughly 10% of the global population. A pessimistic – upper limit – estimate of 10.8 m of global mean sea level rise following 4 °C global warming could affect land now home to up to one billion people, or 15% of the current global population (120).

East, Southeast, and South Asia face the greatest overall exposure to sea level rise both this century and later. Of all nations with a total population of at least 25 million, Asian countries make up nine of the top ten most at-risk nations. Land home to over half the populations of Bangladesh and Vietnam may become exposed to coastal flooding even if warming is limited to 2 °C. Many smaller nations, particularly islands, will become extremely vulnerable to coastal flooding. With 2 °C global warming, more than 80% of the population of the Cocos Islands, Maldives, Marshall Islands, Kiribati, Cayman Islands, Tokelau, Tuvalu, and the Bahamas on the long run will be living in land threatened by flooding. With 4 °C global warming, this percentage will be over 90% (120).

On the long run, with 4 °C global warming leading to a median projected 8.9 m of global mean sea level rise, at least 50 major cities with a population of at least one million, mostly in Asia, would need to defend against globally unprecedented levels of exposure, if feasible. About half of these cities are also threatened at 2 °C global warming. The vulnerable cities in Asia include megacities with a population over 10 million such as Haora, Shanghai, Hanoi, and Dhaka (120).

A study that looks hundreds of years into the future must be based on assumptions that simplify reality. Taking the current population as a constant for the multi-century scenarios is one of them. Also, the analysis assumes that global emissions do not become negative while in the long run greenhouse gases may be extracted from the atmosphere on a massive scale, reducing long-term sea level rise. On the other hand, no unstoppable collapse of major ice sheets has been included in the analysis while Antarctic ice sheet breakdown may lead to higher multi-century sea levels than projected in this study. Finally, the impact of present or future artificial coastal defenses was not considered (120).

Plausible high-end estimate

What is a credible high-end estimate for sea level rise in – say – 2100 or 2300? Practitioners (and decisionmakers) working on adaptation planning need answers to these questions. When designing flood protection, for instance, they want to be sure that they are taking the right measures in time. On the other hand, they don’t want to do more than is needed. Science is clear about the range of global sea level rise that the world most likely will face in 2100. There is a lot of debate, however, about the unlikely, high-end estimate of sea level rise that the world might face by then, and this high-end estimate generally guides adaptation planning. A group of experts on sea level rise and practitioners, therefore, joined forces, studied all the available information on sea level rise projections, and quantified high-end – plausible worst case – estimates practitioners can work with (125).

Physically plausible

These estimates are based on current knowledge of all the processes that contribute to sea level rise and on projections of sea level rise that are – according to them – ‘physically plausible’. In their approach, ‘projections supported by multiple lines of evidence and eliciting broader confidence from the scientific community are of greater value as compared to projections further along the tail that feature fewer lines of evidence, and hence have lower confidence.’ They focused on 2100 and 2300 and looked at the results of two scenarios of climate change, a modest (RCP2.6/SSP1-2.6) and a strong one (RCP8.5/SSP5-8.5).

Sea level rise results from a number of contributing components: thermal expansion of the ocean, melting of glaciers, and of the Greenland and Antarctic ice sheets, and changes in the volume of water stored on land. The contribution of all these components have been assessed separately. The result are estimates of the high-end of projected global sea level rise. The results do not include processes that contribute to regional variations in sea level. Also, vertical land motion is not included; the results are absolute sea level rise (125).

Glaciers

Glacier-melt contributed 0.7 mm/year to sea level rise over the period 2010-2018 (126). The experts conclude that, by 2100, the contribution of glaciers to the high-end estimate sea level rise may be 0.15 m (modest global warming) up to 0.27 m (strong global warming). By 2300, this contribution may be 0.28 m to 0.32 m (125).

Greenland

Ice mass loss from Greenland contributed 0.7 mm/year to global sea level rise over the period 2010-2019 (127). For 2100, the contribution of Greenland to the high-end estimate varies in between 0.10 and 0.29 m. For 2300, this range is much larger: 0.39 to 2.5 m (125).

Antarctica

Ice mass from Antarctica contributed 0.4 mm/year to global sea level rise over the period 2010-2019 (127). For 2100, Antarctica is projected to contribute in between 0.39 m and 0.59 m to the high-end estimate. For 2300, the projection of the possible contribution of Antarctica is highly uncertain. This uncertainty is due to the fact that scientists do not understand well enough the dynamics of ice flow on Antarctica and how the major ice shelves may break up to estimate the possible contribution of ice loss from Antarctica to sea level rise. The Antarctic component is the most uncertain one of all components of sea level rise. Hence the large range of the contribution of this component to the high-end estimate by 2300: 1.35 m to 6 m (125).

Thermal expansion

The warming of the oceans leads to thermal expansion of ocean water and hence to sea level rise. The contribution of this component to the high-end follows directly from the thermal expansion of sea water and is estimated to be in the range 0.18 m – 0.36 m by 2100, and 0.35 m - 1.51 m by 2300. The large range for 2300 reflects the large range of projected global warming in the scenarios: +2 ̊C for the moderate and +8-10°C for the strong scenario (125).

Changes in water volume stored on land

The contribution of water lost from land depends on the way we use and manage freshwater resources on land – for instance groundwater abstraction and water stored in reservoirs behind dams – and climate change. This contribution is small and will remain small in the future. Its contribution is estimated to be 0.04 m by 2100 and 0.10 m by 2300 (127).

The sum of all these components

Now that all components have been quantified it seems relatively simple to just add them up. Unfortunately, it is more complicated than that. There will be some dependence between these contributions, and you should correct for that. The experts decided not to get to the bottom of this and just make two calculations: one assuming total independence and one assuming total dependence. Assuming total independence of these contributing components, the high-end estimate of global sea level rise is 0.72 m - 1.27 m for 2100, and 2.2 m - 8.6 m in 2300. Assuming total dependence, the range is larger: 0.86 m - 1.55 m for 2100, and 2.5 m - 10.4 m for 2300 (125).

The results for these two assumptions should be interpreted as lower and upper end estimates of projected high-end sea level rise. Since working with different ranges may be complicated in adaptation planning, the experts advice practitioners to simply average these lower and upper estimates and use the average for adaptation planning (125).

And what about the time in between 2100 and 2300?

For decisionmakers and practitioners 2300 is very far into the future. 2100, on the other hand, is too early a time horizon for many aspects of adaptation planning. High-end estimates for some time in between – say 2150 – would be nice. Unfortunately, that’s where it becomes complicated, and this is due to uncertainties in Antarctic ice mass loss. This century, a major contribution of ice shelves breaking up or an acceleration of ice flowing into sea is unlikely. That’s why the contribution of Antarctica to the high-end estimate by 2100 is relatively small. By 2300, according to the experts ‘the major ice shelves are assumed to have broken up, and sufficient time has passed to allow for accelerated Antarctic ice mass loss’. But to what extent these processes are already effective by 2150, scientists do not know. So it is not yet possible with current knowledge to give decisionmakers and practitioners figures to work with on a time scale of 100 years or so (125).

Not cast in concrete

More knowledge will become available from science on the contributions of all components to sea level rise in the coming years. The experts, therefore, recommend that their storylines are updated at regular intervals, reflecting the evolution of the body of knowledge. The upper end of 1.55 m sea level rise by 2100 is not cast in concrete. It is just the best estimate scientists currently can make (125).

High-end sea level rise projections for northern Europe

How much sea level rise is to be expected at the upper limit of current IPCC scenarios? This question has been dealt with for northern Europe, focusing on the British Isles, the Baltic Sea, and the North Sea. Probabilistic projections have been made of northern European sea level rise, including the risk and potential contribution from an Antarctic marine ice sheet collapse (18).

The upper limit of current IPCC climate change scenarios is the so-called greenhouse gasses emissions scenario RCP8.5. This scenario is consistent with a business-as-usual scenario. The central estimate for the projected global mean warming under this scenario by 2100 is roughly +5°C above the pre-industrial period. Projected regional sea level rise near a number of northern European cities under this scenario, including an Antarctic marine ice sheet collapse, is shown below.

In the Baltic region, land uplift caused by glacial isostatic adjustment (GIA) is still ongoing. This is predominantly a result of the viscoelastic response of the solid Earth to the disappearance of the Fennoscandian ice sheet at the end of the last glacial period. GIA is currently responsible for lowering sea level in the Bay of Bothnia at a rate of 1 m per century (19).

According to this study, there is a 27% chance of local sea level in London exceeding 93 cm under RCP8.5 (and a 3% chance of exceeding 1.9 m); in the past, these high values for sea level rise in the United Kingdom were considered unlikely (20). Similarly, in this study a 26% chance was estimated of exceeding the previously published high-end local sea level rise scenario constructed for the Netherlands under this RCP8.5 scenario (21). According to this study (18), the dominant uncertainty in North European sea level rise is associated with the fate of Antarctica, followed by expansion of ocean waters due to warming and uncertainties in glacial isostatic adjustment.

Table. Regional sea level rise projections over the 21st century for cities in northern Europe (RCP8.5) for 3 uncertainty percentiles. The median projection (50%) is the best guess. 

Sea-level rise in the Mediterranean region

Sea level rise in the Mediterranean Sea is expected to be in the range of 6.6-11.6 cm in the period 2021-2050 with respect 
to the reference period 1961-1990 (44). 

Extreme sea levels along Europe's coastline

Sea level rise is not the only factor that drives changes in future coastal flood risk. Astronomical tide, and episodic water level fluctuations due to climate extremes (storm surges and the set up by waves when they shoal and break at the coast) are important components as well. Storm surges are short-term increases of the water level due to the wind blowing the water to the coast or due to low atmospheric pressure causing the water level to rise. The size of storm surges depends on a number of factors including the size, track, speed and intensity of storms, near shore local water depth, and the shape of the coastline. Wave set up results from waves shoaling and breaking at the coast.

These components will also change along with sea level rise as a result of climate change. The combination of these factors should be considered, therefore, to assess future changes in extreme sea levels along Europe’s coasts and the resulting impact on coastal flood risk. Results of the first pan-European assessment of the evolution of extreme sea levels in view of climate change were published recently, considering all driving components. The results are based on projections by a large number of climate models (GCMs) for the end of this century considering two scenarios of climate change: a moderate and a high-end scenario (the so-called RCP4.5 and RCP8.5 scenarios). For this study the European coastline was divided in 10 geographical regions in order to identify spatial patterns in the data (27,36).

Relative sea level rise

Relative sea level rise is the rise of sea level with respect to land and thus combines absolute sea level rise with land subsidence and uplift. According to the study relative sea level rise is projected to increase along Europe’s coastline by around 21 and 24 cm by the 2050s under the two climate change scenarios (RCP4.5 and RCP8.5), respectively, to reach 53 and 77 cm by the end of the century. Projected sea level rise is highest along the North Sea and Atlantic coasts, followed by the Black Sea, and smallest for the Baltic Sea due to land uplift in this area (36).

Categories of ocean waves 

There are 4 categories of waves (73): 

• Storm waves: Storm waves are wind surface waves that reach unusually large amplitude due to forcing by strong winds. For example, storm to hurricane force winds ranging from 10 to 12 on the Beaufort scale have probable maximum wave heights from 12.5 to 16 m and beyond. 
• Rogue waves: Rogue waves are large-amplitude waves surprisingly appearing on the sea surface (74). They seem to appear from nowhere with a height 2 - 3 times that of the surrounding sea state, exist for a short time and then disappear. Rogue waves are also referred to as freak waves, monster waves or king waves. 
• Tsunamis: Tsunami waves are mainly generated by earthquakes, landslides or volcanic activity displacing large volumes of water. Large ships moving over appreciable depth changes are one source of small tsunamis; they have recently been observed in the Oslofjord in Norway. Another source of small tsunamis are meteotsunamis. These are waves with tsunami-like characteristics but are caused by air pressure disturbances often associated with fast-moving air such as squall lines. 
• Storm surges: A storm surge is an unexpected rise in seawater level generated by a storm. The low-pressure area near the storm’s eye reduces the weight of the air over the ocean. This creates a swell in the sea, which is pushed towards the coast by the strong winds. As the storm approaches the coast, the combined effect of the low pressure and the violent winds makes the water pile up along the shore. 

According to the IPCC in its 2019 report, significant wave heights (the average height from trough to crest of the highest one-third of waves) are projected to increase across the Baltic Sea (medium confidence) and decrease over the North Atlantic and Mediterranean Sea under RCP8.5 (high confidence). Coastal tidal amplitudes and patterns are projected to change due to sea level rise and coastal adaptation measures (very likely). Projected changes in waves arising from changes in weather patterns, and changes in tides due to sea level rise, can locally enhance or ameliorate coastal hazards (medium confidence) (92).

Extreme storm surge levels - Observations

In planning practice of measures along Europe’s coastline, storm surge extremes are generally considered to be stationary and only the rise of sea level is considered. This is not correct, according to a study in which trends in storm surge extremes along the Atlantic and North Sea coastlines of Europe have been quantified for the period 1960–2018 (121). This was done from on analysis of annual maxima water levels from 79 tide gauges. The analysis revealed that storm surge extremes have changed since 1960 and that this change is due to a combination of internal climate variability and anthropogenic climate change:

• The effect of internal climate variability is a marked north–south dipole structure characterized by an increase of storm surge extremes along the coasts of the British Isles north of 52° N and a decrease along most of the European coastlines south of that latitude from Portugal to the Netherlands. This means that the effect of internal climate variability is a trend of increasing probability of extreme events during 1960–2018 along most of the British Isles, and a trend of decreasing probability to the south.
• In addition to this natural variability effect there is also a detectable response to anthropogenic climate change of an increase of storm surge extremes in this period along the entire coastline from Portugal to Scotland. This anthropogenic fingerprint is consistent with a strengthening and eastward extension of the North Atlantic storm track, leading to increased storminess over the UK and central Europe as predicted by climate models in response to anthropogenic forcing (122).

The effect of climate change is much smaller than the effect of internal climate variability. The result of the combination of both effects is an increase of extreme storm surge levels along most of the coastline of the British Isles and a decrease along the coastline from Portugal to the Netherlands (121).

Extreme storm surge levels - Future trends

For the entire European coastline future trends in storm surge level changes between now (the period 1970 - 2000) and the end of this century have been estimated for an intermediate and high-end scenario of climate change (the so-called RCP4.5 and RCP8.5 scenarios). These estimates were based on projections made with 8 climate models (27).

From a flood risk perspective, storm surge level changes and relative sea level rise (the rise of sea level relative to the land) must be combined. The present findings indicate that at many coastal regions in Europe extreme storm surge level may increase by around 15%, and locally even up to 40%, of relative sea level rise for most of Europe’s coastline. The combined effect of relative sea level rise and storm surge increase at the end of this century is projected to exceed 1 m with respect to the actual water levels for many regions in Europe, even for an intermediate scenario of climate change (27).

Projected storm surge changes strongly vary for different parts of Europe, however. Especially there is a difference between the northern and southern half of Europe. Projected storm surge levels increase along the northern European coastline for both an intermediate and high-end scenario of climate change. Increase is highest along the eastern part of the North Sea coast, and along the west-facing coastline of the Irish Sea. Increase is also relatively high at the Baltic Sea and the Norwegian Sea. Along the European coastal areas south of 50°N, however, a moderate storm surge level increase is projected only towards the end of the century under a high-end scenario of climate change. Earlier this century, and for an intermediate climate change scenario at the end of this century, projected storm surges hardly change or even decrease along the coastline of Southern Europe (27). The combination of storm surges and wave set up shows no or minor changes along most of the southern European coastline, apart from an important decrease that is projected for the Portuguese coast and the Gulf of Cadiz, where the once-in-a-hundred-years effect of storm surges and wave set up combined may be 5-12 cm lower in 2050 compared with the current situation and 10-20 cm by 2100 (36). 

The Mediterranean Sea has been studied extensively in terms of projected storm surge dynamics and there is consensus for no changes, or even a decrease in the frequency and intensity of extreme events (27,29). This agrees with the reported historical trends (30).

Tides

The projected change of tidal elevation between now and 2100 due to a rising sea level is negligible for the entire European coastline (36).

Once-in-a-hundred-years extreme sea levels

The combined effect of sea level rise, astronomical tide, and storm surges and wave set up on the once-in-a-hundred-years extreme sea levels along Europe’s coasts is a rise by around 25 cm on average by 2050 under both scenarios, and a rise by 57 cm and 81 cm by 2100 under the moderate and high-end scenario of climate change, respectively. Strongest rise was projected for the North Sea region with increases up to 75 cm and 98 cm by 2100 under these scenarios, respectively. A similar increase is projected for the Atlantic coasts of the UK and Ireland, and considerable increases are projected for the Norwegian, the Baltic, and the Mediterranean Sea (36).

Future extreme sea levels along Europe’s coasts are mainly driven by relative sea level rise (36). Averaged over Europe, the contribution of future changes in storm surges and wave set up is less than 20% of the contribution of relative sea level rise. Storm surges and wave set up may contribute much more to extreme sea levels in some parts of Europe, however; especially for the Baltic Sea (65% and 35% by 2050 and 2100, respectively), and to a slightly lesser extent for the North Sea. Along the Portuguese coast and the Gulf of Cadiz, an opposite effect emerges: the contribution of storm surges and wave set up is projected to decrease with 30% by 2050 and 20% by 2100. According to this study, the effect of sea level rise on tides is negligible (36). This does not agree with findings in other studies that show that sea level rise can affect tides locally for RSLR < 1.0 m (37) or >2.0m (38). 

The causes of sea-level rise since 1900

Scientists have estimated that global mean sea-level rise was 1.56 ± 0.33 mm/year (90% confidence interval) over the period 1900-2018. They also concluded that sea-level rise strongly varied in this period, with higher rates of sea-level rise during the 1940s and since the 1990s, and lower rates around 1920 and 1970 (114).

Ice-mass loss - predominantly from glaciers - has caused twice as much sea-level rise since 1900 as has thermal expansion. Glaciers are the largest contributor to sea-level rise over most of the twentieth century, overtaken by the thermosteric contribution (expansion of ocean water because oceans are warming up) only after 1970 (114).

Mass loss from glaciers and the Greenland Ice Sheet explains the high rates of global sea-level rise during the 1940s. The low rates around 1970 are caused predominantly by reservoir impoundment. Between 1900 and 2003, 9,400 ± 3,100 km3 (90% confidence interval) of water has been impounded, leading to a sea-level drop of 26 ± 9 mm (90% confidence interval), with a peak in dam construction around the 1970s (115).

The acceleration in sea-level rise since the 1970s is caused by the combination of thermal expansion of the ocean and increased ice-mass loss from Greenland (114). 

The scientists could not explain the low rates in observed sea-level rise during the 1920s (114).

Recent estimates of glacier contributions to sea-level rise

IPCC results (2019)

In 2019, IPCC has presented updates on contributions by ice sheets and glaciers to projected global mean sea level (GMSL) (92):

The Greenland Ice Sheet is currently contributing more to sea-level rise than the Antarctic Ice Sheet, but Antarctica could become a larger contributor by the end of the 21st century as a consequence of rapid retreat (92). According to the IPCC, Antarctica could contribute up to 28 cm of sea-level rise by the end of the century under a high-end scenario of climate change (RCP8.5) (93). For glaciers, the long-term is of limited importance, because the sea level equivalent of all glaciers is restricted to 0.32 ± 0.08 m when taking account ice mass above present-day sea level (94).

The IPCC estimated in 2019 that the contribution of the Antarctic ice sheet in 2300 under a high-end scenario of climate change (RCP8.5) may be 2.3 - 5.4 m, considerably higher than the IPCC projections in their previous report (93).

Previous scientific studies

In the past glacier mass change was based on glaciological and local geodetic measurements. Deriving regional and global mass budgets from these measurements is complicated, because the set of measured glaciers is sparse for many regions and can be biased toward smaller land-terminating glaciers (3). Monitoring of glacier mass change on a global scale using satellite gravimetry or altimetry has only become possible with the launch of the GRACE and ICESat satellites in early 2002 and 2003, respectively. From a comparison of research results of different methods for the same time period (2003-2009) it was concluded that these local measurements are more negative than satellite-based estimates, which may call for a downward revision of the estimated total contribution of glaciers to sea level rise over the past century (2).

It has been suggested that most previous assessments have overestimated global mass losses because of the interpolation of sparse glaciological measurements that are not representative for the largest glacierized regions. This was demonstrated for the 2003–2009 period, but it has long been suspected for earlier periods as well (4). A re-examination of glacier mass wastage between 2003 and 2009 implies a sea-level contribution of 0.71 ± 0.08 mm of sea-level equivalent (SLE) per year, accounting for 29 ± 13% of the observed sea-level rise for the same period. The total land ice (all glaciers + ice sheets) contribution to global sea-level rise in this period was 61 ± 19% (2).

The results above are confirmed in a more recent study on global glacier mass changes and their contributions to sea level rise from 1961 to 2016. This study was based on repeated mapping and differencing of glacier surface elevations from in situ, air-borne and spaceborne surveys, in combination with time series of in situ point measurements on glaciers (83). Glaciers distinct from the Greenland and Antarctic ice sheets cover an area of approximately 706,000 square kilometres globally (84). Their total ice volume equals 0.4 metres of potential sea level rise (85). The quantification of global glacier mass changes reveals that glaciers contributed 27 ± 22 millimetres to global mean sea level rise from 1961 to 2016, or a contribution of 0.5 ± 0.4 mm per year when a linear rate is assumed. The present glacier mass loss is equivalent to the sea level contribution of the Greenland Ice Sheet (86), and clearly exceeds the loss from the Antarctic Ice Sheet (87). It accounts for 25% to 30% of the total observed sea level rise, which ranged between 2.6 and 2.9 ± 0.4 mm per year over the satellite altimetry era (1993 to mid-2014) (88). When annual rates are averaged over periods of five years, sea level contributions ranged between 0.2 and 0.3 mm per year until the 1980s, and then increased continuously to reach 1.0 mm per year in recent years (2011–2016) (200).

Under present ice-loss rates, most of today’s glacier volume would thus vanish in the Caucasus, Central Europe, the Low Latitudes, Western Canada and the USA, and New Zealand in the second half of this century. However, the heavily glacierized regions of the world would continue to contribute to sea level rise beyond this century, as glaciers in these regions would persist but continue to lose mass (200).

Recent estimates of the contribution of variations in the storage of water on land to sea-level rise

Over the past century, sea level rose at an average rate of 1.5 ± 0.2 mm/year, increasing to 3.2 ± 0.4 mm/year during the past two decades (23). The increase in the rate of rise is attributed to an increase in mass loss from melting glaciers and ice sheets and to the expanding volume of oceans due to ocean warming (22).

There are more contributions to sea level rise than glaciers, ice sheets and ocean warming, however. The amount of water that is stored on land is not constant. The amounts of snow, surface water, soil moisture, and groundwater storage vary in time, due to the seasons, longer-term (decadal) variations, and due to human impacts such as the storage of water in reservoirs. Thus, sea level rise varies in times as well, due to changes in the global water cycle. With respect to the global water cycle, two components can be distinguished: human-driven changes and climate-driven changes (22).

Human-drive changes in land water storage include the direct effects of groundwater extraction, irrigation, storage in reservoirs, wetland drainage, and deforestation. Climate-driven changes refer to events such as large flooding periods in the upper Missouri River basin (24), and recovery from drought in the Amazon (25) and the Zambezi and Niger basins in Africa (26).

More water stored on land, slow down sea level rise?

The contribution of human-driven has been assessed by the IPCC in their latest report: these changes contribute 0,38 mm/year to the annual averaged sea level rise. The contribution of climate-driven changes has also been assessed, over the period 2002 - 2014:  -0.71 ± 0.20 mm/year (22). The negative sign indicates that climate-driven changes have been a loss of water from the oceans over the period 2002 – 2014. Thus, over the last years, climate-driven changes in the global water cycle have increased the volume of water on land and slowed down sea level rise.

Less water stored on land, increase sea level rise?

The results of a more recent study contradict the results of previous studies presented above (like 22). According to the authors of this recent study (101) those previous studies were incorrect due to errors in the data they have used.

During the last decade, from 2005 to 2015, the rate of global mean sea level rise was about 3.5 mm/year. About 1.3 mm/year of this is due to expanding of the oceans because they are warming up (thermal expansion). The major contribution to sea level rice is ice mass loss from mountain glaciers and both major continental ice sheets, Greenland and Antarctica. Recent estimates of this contribution of ice melt inflow to the oceans can explain about 1.8 mm/year (101).

So this sum of ocean mass (ice melt) and volume (warming up) increase is 3.1 mm/year. Compared with 3.5 mm/year mean sea level rise we are missing a contribution of about 0.4 mm/year. This is a significant volume of water, similar in size to the contribution from melting Antarctic ice. Scientists recently found the missing part by analyzing satellite observations of changes in the Earth’s gravity. They showed that the amount of water stored on land in river basins has decreased and that this loss to the oceans accounts for about 0.3 mm/year (101).

This loss of water from sources on land may be related to climate variability (102) or anthropogenic activity (103). In particular, river basins at high‐latitudes in the northern hemisphere, including the Lena, Mackenzie and Volga basins, have lost a lot of water to the oceans. There are also river basins that have stored more water, and thus mitigated sea level rise, such as the Amazon, Congo and Zambeze basins. The authors stress, however, that water from land resources is the most uncertain contribution to sea level change (101).

Economic impacts of climate change on coastal zones in the EU

Without adaptation the estimated costs of climate change to coastal zones in the EU in the 2060s are EUR 6 to 19 billion per year for a low-end scenario of climate change (RCP2.6), EUR 7 to 27 billion per year for a moderate scenario (RCP4.5) and EUR 15 to 65 billion per year for a high-end scenario (RCP8.5) (combined climate and socio-economic change (SSP2), current prices, no discounting). These costs rise rapidly in later years, ranging from EUR 18 to 111 billion per year for the low-end scenario (RCP2.6), EUR 40 to 249 billion per year for the moderate scenario (RCP4.5) and EUR 153 to 631 billion per year for the high-end scenario (RCP8.5) in the 2080s (60). This indicates a disproportionate increase in costs for greater warming scenarios in the second half of the century. There are major differences between Member States, with the greatest coastal zone costs projected to occur in France, the United Kingdom and the Netherlands if no additional adaptation occurs (60).

Global economic impacts of sea level rise under 1.5°C and 2°C global warming

What if we do succeed in restricting warming at 1.5°C or 2°C, following the Paris Agreement? What will be the impact on sea level rise and coastal flood risk? This was studied for projections focused on 2100. A distinction was made in two scenarios: a scenario with no additional adaptation that keeps dike heights constant at the current level, and a scenario with business-as-usual adaptation where the standard of protection is updated every five years and dike heights are raised to cope with rising sea levels and changes in population density. For the latter, an increase of global population was assumed until mid-century, reaching approximately 9 billion people before slightly declining (the so-called socioeconomic SSP2 scenario) (75).

Global sea level rise by 2100

The height of sea level rise at the end of this century depends on the moment when the 1.5°C and 2°C warming levels are reached (and kept constant until 2100). The sooner these levels are reached, the higher sea levels will rise. All in all, according to this study, median sea level rise by 2100 is up to 52 cm and 63 cm for 1.5°C warming and 2°C warming, respectively. These levels are much lower than median sea level rise under a high-end scenario of climate change (RCP8.5), which is projected to be 86 cm according to this study (75).

Annual sea flood costs by 2100

Without additional adaptation to sea level rise, projected global annual sea flood costs by 2100 are US$ 10.2 trillion per year (1.8% of GDP) at 1.5°C sea level rise and US$ 11.7 trillion per year (2.0% GDP) at 2°C sea level rise. If we fail to meet these Paris Agreement targets, and median sea level rise reaches 86 cm, global annual flood costs are projected to be US$ 14.3 trillion per year (2.5% of GDP).

With adaptation, costs could decrease to US$ 1.1 trillion per year (0.2% GDP) by 2100 for both 1.5°C and 2°C sea level projections, a reduction by a factor of 10. A similar reduction is reached under the high-end scenario of climate change: adaptation would strongly reduce total global annual flood costs from US$ 14.3 trillion per year to US$ 1.7 trillion per year. Thus, adaptation could greatly reduce flood costs, potentially by an order of magnitude and regardless of the future climate scenarios (75).

Adaptation strategies - Benefits to costs ratio of improving coastal flood defenses

The increase of extreme sea levels not only reflects sea level rise but a change in storm conditions as well. Along Europe’s coastlines extreme sea levels with a return period of 100 years are projected to rise by as much as one metre or more by the end of this century due to climate change. Needless to say that this poses significant challenges to protect coastal communities. At present, more than 200 million European citizens live within 50 km from the coastline.

No doubt the coastal countries of Europe will adapt to these changing conditions. Adaptation strategies they can choose from are: retreat, accommodate and protect. Retreat means relocating dwellings and infrastructure out of harm’s way. Accommodate includes early warning systems, emergency response, and flood proofing of structures. Protect refers to flood protection through natural (dunes) or artificial (dykes) structures, beach nourishment, and nature-based solutions.

For flood protection by improving dykes, costs and benefits have been evaluated of applying additional protection through dyke improvements along the European coastline in response to the increasing extreme sea levels. This was done for 2100 compared with the current situation, for both a moderate (RCP4.5) and high-end (RCP8.5) scenario of climate change (108).

The cost of doing nothing

At present, average annual coastal flood losses in Europe amount to €1.4 billion (in 2015 € values). Without further improvements of flood protection this number would increase to €210 billion (range: €30 billion to €845 billion) by the end of the century for the moderate scenario of climate change. Annual coastal flood losses would be up to €1268 billion (range: €161 billion to €4731 billion) for the high-end scenario. The increase of coastal flood risk will affect all coastal countries of Europe, with highest numbers for France, the UK, Italy and Denmark. For the high-end scenario this increase could amount to a considerable share of a country’s GDP, most notable in Cyprus (4.9%), Greece (3.2%), Denmark (2.5%), Ireland (1.8%) and Croatia (1.8%) (108).

Extra flood protection pays off

This scenario of ‘doing nothing’ is not realistic, however, because it pays off for countries to improve their flood protection along parts of the coastline with high value conurbations. Under the assumption that countries do invest in flood protection as long as benefits of extra flood protection exceed the costs of damage due to flooding, the expected annual damage by 2100 will only reach €9 and €24 billion under the moderate and high-end scenario of climate change, respectively. This is a 96% - 98% reduction compared to a do-nothing scenario. These investments in elevating dykes are economically efficient along 24%-32% of Europe’s coastline (108).

The Netherlands is a particular case

At country level, Belgium is the country with the highest percentage of coastline where benefits exceed costs, followed by France and Italy. The benefit to cost ratio is also very high for the Netherlands, Cyprus, Ireland and Greece. This ratio is relatively low for Malta, Bulgaria, Lithuania, Latvia and Croatia. The Netherlands is a particular case, being already very well protected (up to ~10,000 year return period) and still having the highest benefit to cost ratio in Europe. Thus, the benefits from protecting the Netherlands further are high, even though flood events are rare (108).

The results of this study agree with the IPCC report on the Ocean and the Cryosphere, which reports that coastal protection can reduce the expected annual damage in 2100 by 2 to 3 orders of magnitude (109).  

Adaptation strategies - Barriers to inland migration

Displacement estimates

Recent estimates of future human displacement driven by sea level rise vary substantially. Estimates of the number of people being at risk at about one metre of sea level rise, for instance, varies from 67 million (46) to almost 200 million people (47) or even more (48). The world’s low-elevation coastal zone is often defined as the coastal zone up to 10 metres above current sea level. What if sea level rises much faster than currently projected by the IPCC, and the people in this low-elevation coastal zone have to move? Will these climate migrants find a new home inland?

In 2000, about 630 million people inhabited the global low-elevation coastal zone (49), not including seasonal visitors and second-home owners. In Europe alone, 70% of the largest cities are vulnerable to sea level rise, most being in the low-elevation coastal zone (50). Under high rates of population growth, as many as 1.4 billion people could inhabit this zone by 2060 (51). Then and now, the most densely-populated coastal zones are the deltas in Asia.

Sea level rise scenarios of 25 to 123 cm by 2100 without adaptation are expected to see 0.2 to 4.6% of the global population impacted by coastal flooding annually, with average annual losses amounting to 0.3 to 9.3% of global GDP. Investment in adaptation reduces by 2 to 3 orders of magnitude the number of people flooded and the losses caused (99).

The number of people vulnerable to coastal flooding now and in the future may have been underestimated so far (see our section above 'Vulnerabilities - Global vulnerabiliy underestimated?' (based on (95)). 

Barriers to inland migration

The worst-case scenario of a much faster sea level rise than currently projected by the IPCC would induce large-scale inland migration. Access to hinterland resettlement may not be that easy, however. Inland migration faces several barriers, both natural and anthropogenic. The main barriers have been inventoried in a recent study (45).

According to the authors of this study, inland living space will probably be partially or wholly off-limits to newcomers. According to them these spaces are “mortgaged for transportation, carbon storage, toxic and waste dumps, urban sprawl, deserts and other wastelands, war zones, unexploded ordinance graveyards, large private enclaves held by wealthy people, and a variety of semi-permanent polluting uses”. They have estimated that the total global spatial mortgage for 2100 might be up to 105 million km2, roughly 70% of the Earth’s current terrestrial area (45).

Depletion zones

Among the barriers-to-entry in interior areas are so-called depletion zones: territories unlikely to support future human existence without unprecedented investment. These territories include degraded lands, dry-lands, and thawing permafrost landscapes. Sea level rise adds dramatically to the problem of providing a suitable living environment for millions of people by encroaching on some of the world’s most fertile landscapes - global coasts and deltas - and turning them to largely barren lands (45).

Trade-Off zones

A spatial trade-off presents itself when inland cities absorb immigrant waves moving out of the threatened coastal zone. The inland cities expand as a result, and their enlarged footprint and that of roadways and municipal waste sites that accompany them take a toll on agricultural land and open space (52). Urban growth fragments the biosphere and is difficult to reverse. And, relative to surrounding rural areas, cities become “heat islands” as they grow. The growth of cities and their extensions could therefore encroach on the net primary productivity needed for the food and other biomass needs of 9-11 billion people expected by the end of the century (52). Just how serious the squeeze on productive land will be is a function of future urban growth. By 2050, 7 out of 10 people will dwell in cities (53).

No-trespass zones

No-trespass zones are landscapes severely encumbered by legal exclusion (landownership concentration and gated cities), violence (war and conflict), and unusual risk (land mines and radioactivity). Already, fear of food scarcity due to increasingly erratic weather has spurred governments and private entities to ‘grab’ productive land in many countries, concentrating landownership in the hands of wealthy investors, developers, and speculators (54). In the United Kingdom, for instance, 70% of the land is owned by 1% of the population (55). In addition to ownership concentration itself, exclusionary land use laws pose additional friction for newcomers. As global cities boom, current practices such as exclusionary zoning and closed communities become magnets for elites and destitute groups alike. These enclaves wall off desirable living spaces for the affluent (56).

In addition, on-going wars and regional conflicts are a much-underestimated barrier to entry. Explosive remnants of war (land mines, for instance) further diminish usable replacement space while it’s lasting chemical constituents (e.g., Agent Orange) jeopardize food chains across generations, and are responsible for ‘no-man’s lands’ in 80 countries (57). The authors point at a perverse circularity here: competition for productive lands caused by the combination of war and natural disasters produces refugees whose needs for living space may fuel yet more conflict (58).

Inland adaptation

What options do governments and their allies have to counter landward impediments to climate migration? According to the authors an unprecedented transboundary effort, commitment, and collaboration will be needed. Coastal and interior landscapes must be managed consciously in anticipation of major population shifts from the former to the latter. Land use planning tools include government purchase of at-risk lands, major development restrictions, and managed retreat. The first is expensive and the second often beset by legal challenges, the authors state. Managed retreat, the gradual clearance of structures as coastlines recede inland (59,110), seems promising because, in the words of the authors, “it relaxes assumptions that coastal improvements and ownership patterns are immutable” (45). 

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Europe.

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