Photo: Aruba, one of the Small Island Developing States. Credit: European Union, Copernicus Sentinel-2 imagery.

Small Island Developing States (SIDS) are a group of 57 island nations and territories that are widely considered to be highly vulnerable to the effects of climate change. Although these states are very vulnerable to flooding, only a few studies have assessed flood risk across all these states. What’s more, these studies only focused on coastal flooding and therefore have likely underestimated the total exposure to flooding. A new study, published in Environmental Research Letters, also considers inland flooding from rivers and extremely heavy rainfall. According to the authors of this study, it is the first estimate of population exposure to both coastal and inland flooding for all these states under current and future climate change.

100-year floods

The authors calculated current and future population exposed to once-in-100-year coastal or inland floods. Their results for the current situation are based on flood models. Their future projections are based on estimates of the change in projected sea level, river flows, and precipitation compared to the present day. They present results for three scenarios of climate change: a low-end, moderate and high-end scenario.

Assumptions

As usual, assumptions in these types of studies lead to over- or underestimations. An overestimation of the vulnerability of the population of these states to flooding results from the fact that the authors did not include river or coastal flood defences in their calculations. This overestimation will not dominate the results, however, as the authors focused on once-in-100-year floods, and the level of flood protection against these floods is probably relatively low for these states. An underestimation results from the fact that for coastal flood risk only changes in sea level associated with sea level rise were included, and changes in storm surge and tides were neglected. Also, the authors did not consider future population change.

Present-day exposure

The results show that 19.5% of the population of the Small Island Developing States – about 8.5 million people – are exposed to once-in-100-year floods in the current situation, of which 81% is due to inland flooding.

In 45 of these of 57 states more than 10% of the population is exposed to these 100-year floods. In six states this percentage is more than 40%. According to the authors, these numbers are plausible in the context of historical flood events.

Overall, coastal flooding only contributes 19% to the total population exposed to inland and coastal flooding for all these states. However, in the top five most exposed states, coastal flooding is the main driver of present-day population exposure because here population density is especially high in low-lying areas near the coast. For example, 100%, 76% and 55% of the population in the Bahamas, Suriname and Guyana are in the low elevation coastal zone (<10 m elevation).

Future exposure

Present-day population exposure to the 100-year floods is 19.5%. This would increase to 21% under the low-end scenario of climate change by 2100, and to 22% and 23% under the moderate and high-end scenario, respectively. States that are most exposed today remain so regardless of the climate scenario. The authors stress the importance of this finding: For small island developing states where the need for better flood protection is greatest now, this will also be the case in the future

Source: Archer et al., 2024. Population exposure to flooding in Small Island Developing States under climate change. Environmental Research Letters 19, 124020.