Air temperature changes until now - Mainland Portugal

Mean annual air temperature varies from a minimum of 6ºC in the central interior highlands (Serra da Estrela) to a maximum of 17ºC along the southern coastline. Mean monthly air temperature varies regularly throughout the year, with the maximum values being registered in August and minimum in January. In the summer, mean maximum temperature varies between 20ºC and 25ºC in the western littoral and is above 30ºC in the interior central region and Alentejo. Mean minimum temperature varies between below 0ºC in Serra da Estrela and in the interior mountainous regions and 9ºC to 10ºC for the southern coastline (1).

The annual number of days with a minimum temperature below 0°C (frost days) can be higher than 50 in the northern centre. The number of days with minimum temperature above 20ºC (tropical nights) is more than 20 in some interior regions in the south and centre. On the other hand, the spatial distribution of the average number of days with temperature values above 30ºC (hot days) is maximum in the interior of Alentejo (south) with more than 90 days (1).

Changes in surface air temperature extremes over mainland Portugal since the early 1940s were investigated on the basis of daily maximum and minimum temperatures available from time series from 23 weather stations. Two periods have been discriminated (6, see also 8):

• 1941–1975: The maximum and minimum temperature decreased by 0.17°C per decade and 0.19°C per decade, respectively;
• 1976–2006: The maximum and minimum temperature increased by 0.49°C per decade and 0.54°C per decade, respectively.

Besides, in the 1976–2006 period many stations revealed statistically significant positive trends in the annual number of tropical nights, summer days, warm spells, warm nights and warm days. At the seasonal level, statistically significant increments of extreme heat events for spring and summer, and a decrease of cold extremes in winter were quantified (6).

In Portugal and Spain, the August 2018 heat wave was the warmest since that of 2003. Recent climate change has exacerbated this event making it at least 1°C warmer than similar events since 1950 (16).

Urban heat island Lisbon

The annual-mean urban heat island effect over Lisbon in the period 1971 – 2005 was found to be positive during nighttime (+3 °C, city warmer than rural), with a maximum during winter (+4 °C) and a minimum during summer (+2 °C). During daytime, the annual-mean urban heat island effect was found to be slightly negative, with an annual-mean < −1 °C (city cooler than rural). During daytime this effec changed signal between +1 °C during winter and −1 °C during summer (17). These results are consistent with previous studies reporting a negative daytime urban heat island effect over relatively dry regions, where urban surfaces are characterized by lower surface albedo and higher surface roughness compared to the rural surroundings (18). 

Air temperature changes until now - Azores

The Azores comprises nine islands. The lowest island (Graciosa) has a maximum elevation of 402 m, whereas Pico has the highest point of the Azores and of Portugal (2351 m). The Azores archipelago is located in the subtropical area of the Northern hemisphere anticyclones (1).

The most determining factor that influences the weather conditions is the Azores anticyclone. In S. Miguel the annual mean temperature varies between 9ºC and 17ºC. In Terceira island the annual mean temperature varies between 10ºC and 17ºC. Since the 70’s the temperature has been raising in Angra do Heroísmo, Terceira Island, Azores (1).

Precipitation changes until now - Mainland Portugal

The precipitation regime in Portugal can be explained by two different seasonal atmospheric mechanisms. In summer, northerly or northeasterly winds bring warm and dry air into Portugal, which is either of continental or maritime origins. During winter, the large-scale circulation is mainly driven by the position and intensity of the Icelandic low, and Portugal is affected by westerly winds that carry moist air and produce rainfall events mainly in northern Portugal (2).

Mean annual precipitation over mainland Portugal is of about 900 mm, though with considerable spatial variability; the coastal north has the highest precipitation levels (>2500 mm) while the lowest (< 500 mm/year) are observed in the southern coast and in the eastern part of the territory. On average, about 42% of the annual precipitation falls during the 3 month winter season (December to February), and the lowest precipitation values, corresponding to only 6% of the annual total precipitation, occur during the summer (June to August). During the transition seasons Spring (March to May) and Autumn (September to November) the amount of precipitation is highly variable (1,9).

The annual average number of days with precipitation equal or above 30 mm is higher in the northwest region (Alto Minho), with more than 30 days/year. The last 30 years have been particularly dry in mainland Portugal. 2005 was the driest of the last 78 years, followed by 2007 and 2004. Precipitation shows high inter-annual variability, with well known vulnerability to climate variability, namely to droughts in the south and floods in the north (1).

Due to the large variability that characterizes the precipitation regime in Portugal, it is difficult to detect temporal trends. The precipitation regime seems to become more homogenous over the Portuguese southern territory (2). A tendency towards drier climatic conditions in the south of Portugal and an increase in the length of the greatest dry spell have been observed over the last decades. Also, extreme precipitation variability and climate uncertainty seem to have increased in recent times (3). In spring, statistically significant drying trends were found in the period 1941-2007 at 57 meteorological stations scattered across Portugal together with a reduction in extremes (9). In autumn, wetting trends were detected for all indices, although overall they are not significant at the 5 % level. The drying trends in spring, summer and winter, and wetting trends in autumn seem to indicate a tendency for a reduction in the duration of the rainy season (9).

Also in the period 1950-2003, statistically significant drying trends were found in spring, mainly in northern and central Portugal (15), while weak wetting trends were detected in autumn (15). Extreme precipitation was shown to decrease in this period in spring over central Portugal and slightly increase in autumn over northern Portugal and nearby Lisbon (15).

Precipitation changes until now - Azores

The Azores comprises nine islands. The lowest island (Graciosa) has a maximum elevation of 402 m, whereas Pico has the highest point of the Azores and of Portugal (2351 m). The Azores archipelago is located in the subtropical area of the Northern hemisphere anticyclones (1).

The most determining factor that influences the weather conditions is the Azores anticyclone. The climate in this archipelago is temperate and humid. Given the altitudinal temperature variation, the climate is rainy and cool in high altitude regions. The season between September and March is predominantly rainy, characterized by the frequent passage of depression disturbances related to the polar front. In the remaining months the anticyclone’s influence reduces precipitation (1).

In S. Miguel the average annual precipitation varies between 3,000 mm and 4,000 mm. In Terceira island the average annual precipitation varies between 1,000 mm and 3,400 mm (1).

Precipitation changes until now - Madeira

The archipelago of Madeira is located in the North Atlantic, 1,300 km from the Azores and 900 km from the European continent. It comprises Madeira and the Porto Santo islands and two groups of deserted islets. Madeira island has a total surface area of 728 km2. The highest point of the island is Pico Ruivo (1,862 m). Porto Santo Island is located roughly 40 km Northwest of Madeira and has a maximum altitude of 517 m (Pico do Facho) (1).

The climatic conditions are moderate, both during the winter and the summer, except in the highlands where there are lower temperatures. The island’s complex relief creates many micro-climates. Mean annual temperature can vary between 8ºC in higher altitude and 18ºC to 19ºC in the coastal areas. From among the weather parameters, precipitation registers the broadest variability. Annual precipitation in Madeira varies between 3,400 mm (highest points) and 600 mm (Funchal basin) (1).

There is a significant contrast between the north side and the highest spots, where abundant precipitation occurs, and the south side. In the Funchal area and at other parts of the southern coast there are less than 80 days with precipitation per year, whereas at the northern coast over 120 days/year are registered. In the highlands there are over 200 days with precipitation per year, 70 of which have high values of precipitation (more than 10 mm). Since the 70’s the mean temperature has raised in Funchal and Porto Santo at rates of about 0.6ºC/decade and 0.34ºC/decade, respectively (1).

Sea water temperature changes until now - Mainland Portugal

A generalized warming of sea water was observed along the Portuguese coast during the last decades (1980–2010), mainly in spring and summer. Mean annual sea surface temperature increased by +0.1°C per decade along the Northwestern and Southwestern coast, and by +0.2°C per decade along the Southern coast. Warming is highest for the summer: +0.2 - 0.4°C per decade along the Northwestern- and Southwestern coast, and +0.4 - 0.5°C per decade along the Southern coast (12).

Air temperature changes in the 21st century - Mainland Portugal

A systematic increase in temperature in the order of 3-7ºC is estimated for the summer season in continental Portugal in the 21st century, affecting in particular inland northern and central regions. Increased frequency and intensity of heat waves is also foreseen (1).

In Lisbon the average summer temperature may rise from 28°C to as high as 34°C, with the frequency of hot days (>35°C) increasing from the current figure of 5 to roughly 50 per year (5).

Changes in the maximum and minimum temperature distributions and associated changes in the likelihood of extreme events have been studied for the future (2071–2100) compared with 1961–1990 (based on simulation data for IPCC B2 and A2 scenarios from the PRUDENCE project). Projected maximum temperature was found to increase by 3.2°C (4.7°C) for the B2 (A2) scenario in summer and by 3.4°C in both scenarios for spring. For minimum temperature, the results were similar, with increases for summer (spring) ranging from 2.7°C (2.5°C) in the B2 scenario to 4.1°C (2.9°C) in the A2 scenario (6).

Similar changes for 2071-2100 with respect to 1971-2000 were projected with high-resolution regional climate models, under a moderate (RCP4.5 scenario) and high-end scenario (RCP8.5) of climate change (14). According to these projections:

• The median values of maximum and minimum temperature increase by 3.5 - 4°C and 3 - 3.5°C, respectively, by the end of the century under the high-end scenario of climate change. The variability (the spread around the median) of maximum temperature will also increase; for minimum temperature variability is not projected to change significantly. In general, by the end of the century, projected maximum temperatures increase more than projected minimum temperatures.
• Maximum temperature rise is largest in summer and autumn with maximum increments of 8°C in some areas inland whereas in winter and spring it is between 2 and 4°C.
• Projected rise in maximum temperature shows a large west-east gradient highlighting the lower warming in the coastal areas and the opposite near the border with Spain.
• Up to mid-twenty-first century the number of tropical nights is very similar across Portugal; from then onwards the differences between the coastal areas and inland become more substantial. The number of nights increases significantly: from an average of 7 now to a projection of 60 tropical nights (under the high-end scenario) by the end of the century. Cold days almost disappear.
• Under the high-end scenario of climate change, the yearly average number of heat waves increases by seven to ninefold by 2100 near the Spanish border and the most frequent length rises from 5 to 22 days throughout the twenty-first century. 5% of the longest events will last for more than one month. The amplitude is overwhelming larger, reaching values that have not been observed in the historical period 1971-2000. More than half of the heat waves will be stronger than the extreme heat wave of 2003 by the end of the century. The future heat waves will also enclose larger areas: approximately 100 events in the 2071-2100 period (more than 3 per year) will cover the whole country. For the moderate scenario the increase in the number of heat waves is milder, but even in this scenario, an average of 5 heat waves are projected for the end of the twenty-first century. In this study heat waves are defined as periods of more than 5 consecutive days with maximum temperature above the historical period (1971-2000) 90th percentile.

Even under a high mitigation scenario (RCP2.6), the number of heatwaves will more than double in number by the end of the century (2071–2100), relative to the historical record (1971–2000) (19).

Air temperature changes in the 21st century - Azores

On the islands, the temperature increase is estimated to be more moderate than in mainland Portugal, in the order of 1-2ºC in Azores (1).

Air temperature changes in the 21st century - Madeira

On the islands, the temperature increase is estimated to be more moderate than in mainland Portugal, in the order of 2-3ºC in Madeira (1).

Heat wave and cold spell changes in the 21st century - Mainland Portugal

In Spain and Portugal the number of heat waves, their duration and intensity will increase in the course of this century. At the same time, less cold spells will occur, and they will become less intense. This was shown in a study where the impacts of a high-end scenario of climate change (the so-called RCP8.5 scenario) were assessed for two future periods, namely mid-term (2046 - 2065) and long-term (2081 - 2100), and compared with the recent-past reference climate (1986 - 2005) (13). See also (14): summarized under 'Air temperature changes in the 21st century - Mainland Portugal' (see above).

The definition of heat waves and cold spells

To what extent heat waves or cold waves may change not only depends on the scenario of climate change we choose or the time slice in the future, it also depends on the definition of a heat wave or a cold spell. Many different definitions are being used, that may lead to different conclusions. In this study (13), a heat wave is defined as the period of, at least, three consecutive days in which the maximum temperature during a day is equal or above a certain threshold temperature. Each day of the year has its own threshold that is calculated from a database over the period 1986-2005. The threshold for a certain day is the 10% highest temperature in a period of 31 days centred around that day for a period of 20 years (the recent-past climate of 1986-2005). For example, the threshold for 1 July is the temperature that is exceeded on 10 % of all days during 15 June – 15 July over the 20-year period of 1986-2005.

Likewise, a cold spell is defined as the period of, at least, three consecutive days in which the minimum temperature during a day is equal or below a certain threshold temperature, where this threshold is defined as the 10% lowest day temperatures during the 31 days around a certain day over the period 1986-2005. 

This way, the daily climatological threshold series follows closely the seasonal cycle. What’s’ more, the threshold also varies with location in Spain and Portugal because the thresholds for heat waves and cold spells are calculated for a lot of locations across the Iberian Peninsula. This tailor-made definition allows for an assessment of regional differences in heat wave and cold spell changes.

Up to 40 times more heat wave days in Spain and Portugal

Both halfway and at the end of this century, the number of heat waves and heat wave days are expected to increase in all studied locations across Spain and Portugal. Relative to the reference for 1986-2005, the number of heat waves is projected to increase between 2 and 4 fold. The number of heat wave days is expected to increase even more, and reach between 4 and 40 times the value for 1986-2005. The latter is due to the fact that not only the number of heat waves but average heat wave duration is also expected to increase significantly at all locations. Besides, heat waves will become more intense (hotter) and especially night-time temperatures will increase strongly, making it hard for citizens to recover from the heat of the day. Projected changes are smallest in the north (near the cities of Gijón and Corunha) and highest in Barcelona. Under a high-end scenario of climate change, many future summer days at the end of this century will be heat wave days, particularly in Barcelona, Cáceres and Sevilha (13).

No more cold spells at the end of this century?

Very few cold spells were detectable in the results of this study for mid-term future, and none at the end of this century, except for Barcelona. The cold spells that still may occur, are of smaller duration and intensity than those of today (13). Cold days almost disappear by the end of the century (14).

Precipitation changes in the 21st century - Mainland Portugal

Precipitation in Portugal is projected to decrease throughout this century, and more clustered into extreme events, accentuating the vulnerability of Portuguese water cycle to global warming (10,15). These changes may have dramatic impacts on a very wide spectrum and vital sectors of Portuguese economy, like agriculture, forestry, water supply and energy production. The results of five regional climate models (RCMs) were used to characterize the precipitation changes over Portugal in response to an intermediate scenario of climate change (the so-called SRES A1B emissions scenario). This was done for 2071–2100 compared with 1961–2000. According to these results, yearly precipitation will decrease over Portugal, between 15 %, in the north, and over 30 % in the south. Mean seasonal precipitation is expected to decrease substantially in all seasons, excluding winter. This reduction is statistically significant; it spans from less than 20 % in the north to 40 % in the south in the intermediate seasons, and is above 50 % in the largest portion of mainland in summer. Besides, rain intensity (extreme precipitation volumes) will be much higher than the present climate (10). 

As a result of a reduction of the rainy season, different scenarios forecast a reduction in annual rainfall in the continent in the 21st century by 20-40% of current levels. The majority of the models predict a moderate increase in rainfall in the north in the winter season for the period 2070-2099 relative to the baseline period of 1961-1990. Model projections are more variable for the centre and south in the winter season within this same period. A reduction in rainfall is projected particularly in the spring and autumn (1,7,11). Recent studies, based on climate models and past observed records, predict a future increase in droughts in the south of Europe as a result of increased evapotranspiration and a relatively slow decrease of rainfall amounts and precipitation frequency (4).

Precipitation changes in the 21st century - Azores

In the Azores, changes are predicted in the annual rainfall cycle in the 21st century but without substantial impact on total precipitation (1).

Precipitation changes in the 21st century - Madeira

A significant reduction (about 30%) in annual precipitation is projected for Madeira for the period 2070-2099 relative to the baseline period of 1961-1990 (1).

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 Portugal.

1. Portuguese Environment Agency with the cooperation of Ecoprogresso – Environment and Development Consultants, SA (2009)
2. Durão et al. (2009)
3. Costa and Soares (2009)
4. Kostopoulou and Jones (2005); Vicente-Serrano and Cuadrat-Prats (2007), both in: Costa and Soares (2009)
5. Verschoor (2009)
6. Ramos et al. (2011)
7. Stigter et al. (2012)
8. Espírito Santo et al. (2014a)
9. Espírito Santo et al. (2014b)
10. Soares et al. (2015)
11. Guerreiro et al. (2016)
12. Baptista et al. (2018)
13. Cardoso Pereira et al. (2017)
14. Cardoso et al. (2019)
15. Santos et al. (2019)
16. Barriopedro et al. (2020)
17. Nogueira et al. (2020)
18. Zhao et al. (2014); Shastri et al. (2017); Krayenhoff et al. (2018); Manoli et al. (2019), all in: Nogueira et al. (2020)
19. Cardoso et al. (2023)