Many experts have stressed that this does not mean that from now on global warming has exceeded this critical level. The year 2024 was a special one with the El Niño phenomenon causing extra global warming. According to these experts, global warming will drop below 1.5°C in the coming years because we will enter a new, cooler phase in the El Niño/La Niña cycle. This cycle leads to an alternation of higher and lower average global temperatures. In a recent paper, the famous climate researcher James Hansen and several colleagues argue that global warming will not drop below 1.5°C. In their extensive article they conclude that we have already reached the +1.5 °C threshold of the Paris Climate Agreement (267).

They analysed changes in temperature and possible causal factors in the period 2020-2024, when global warming accelerated. In their own words: “like a detective who dusts a doorknob and lifts a fingerprint with clear adhesive tape, we extract fingerprints of the mechanisms that caused the acceleration of global warming”. Their research led them to conclude that the recent acceleration was due to a strong reduction of ship aerosols since 2020, because of new legislation. Less aerosols in the atmosphere means more solar radiation reaches the earth. It’s the opposite effect of volcanic eruptions like the Pinatubo eruption in 1991 that blasted so many aerosols into the atmosphere that global average temperature was lowered by 0.3°C (267).

The effect of the strong reduction of ship aerosols is an additional global warming of 0.2 °C, and this effect on global warming will increase in the distant future. About 40% of this effect is achieved in 10 years, 60% in 100 years, and 90 percent in 1,000 years. This long timescale of the temperature response is a consequence of the ocean’s great thermal inertia. Over continents, the early response is larger because the response is not held down tightly by the ocean’s great thermal inertia. According to Hansen and his colleagues, this ship aerosols effect will prevent global temperature from falling much below +1.5 °C relative to pre-industrial time. In addition, we will reach the +2 °C threshold of the Paris Climate Agreement by 2045 (267).

The authors stress that their conclusion on the major role of decreasing ship emissions on recent accelerated global warming is at odds with that of the IPCC. The IPCC underestimates the effect of aerosols on global warming, they conclude (267).

Average warming

The global (land and ocean) average temperature in the decade 2006-2015 was 0.83°C to 0.89°C higher than pre‑industrial levels (1850–1899 average). This makes it the warmest decade on record. 15 of the 16 warmest years on record have occurred since 2000, and 2015 was the warmest year on record - around 1°C warmer than the pre-industrial period. Over the decade 2006-2015, the rate of change in global average surface temperature was between 0.10 and 0.24 °C per decade (124). 

Many have argued that the responsibility of countries for the consequences of climate change should be linked to their contribution to the stocks of greenhouse gases in the atmosphere. Recently, an interesting approach has been published to quantify this responsibility, and provide guidance for determining just approaches to liability for damages related to climate change (226). In this approach, high-income countries have climate debts, and low-income countries have climate credits.

The approach focuses on the stocks of CO2 in the atmosphere, and measures a country’s responsibility to its contribution to global cumulative historical emissions since 1850. A correction was made for differences in population size: a country with twice the population size of another country is allowed to emit twice as much CO2. The CO2 emissions are based on consumption data. This way, countries are also held responsible for emissions abroad due to the production of goods they have imported.

The approach is a so-called ‘fair shares approach’. National fair shares were defined with reference to 350 ppm atmospheric CO2. This level is the safe planetary boundary (227), the point climate change can be said to have begun to be a problem (climate breakdown). This point was reached in 1990. Only CO2 was included in the present analysis, because the next most significant gas (methane) is so short lived that it cannot be meaningfully included in calculations of long-term stocks. By calculating the total CO2 emitted from 1850 to 1990, the budget for cumulative historical emissions within the planetary boundary was derived: 830 gigatonnes of CO2. This budget was distributed among countries according to each country’s population as a share of the global population, with populations averaged from 1850 until today. The extent to which nations have exceeded or overshot their fair share of this given safe global emissions budget is a measure of their national responsibility for climate damages. Countries that have exceeded their fair share would then be said to owe a climate debt to countries that have remained within their fair share.

The results show that the USA has contributed 40% of total overshoot emissions. The USA is therefore responsible for 40% of climate breakdown. The USA and the 28 countries of the EU together are responsible for 69%. The G8 countries (the USA, EU­28, Russia, Japan, and Canada) are together responsible for 85%. The majority of the world’s countries are in climate credit. 34% of the total global credit is awarded to India. According to this method, China bears no responsibility yet for climate breakdown, but China will soon overshoot its fair share and become a contributor to climate breakdown.

According to the IPCC the projected change in annual global mean surface air temperature (°C) with respect to the present day (1986–2005) is between 1.0 (RCP2.6) and 2.0 °C (RCP8.5) by the mid-21st century and between 1.0 (RCP2.6) and 3.7 °C (RCP8.5) by the end of the 21st century. These projections are based on tens of models. See the table below (104).

In most of the world a large part of the observed temperature changes in the past can be attributed to anthropogenic forcing. It is estimated that hot summers in many regions of the world are now ten times more likely to occur than would have been the case if greenhouse gas and aerosol concentrations had stayed at preindustrial levels (114). Human impact is largest in the Mediterranean and the Sahara region. These results agree with previous studies (115).

The study on the likelihood of hot summers for different parts of the world suggests that the Mediterranean, Sahara, large parts of Asia and the Western US and Canada will be among the first regions for which hot summers will become the norm (i.e. occur on average every other year), and that this will occur within the next 1 - 2 decades. After 2035 very few summers in the Mediterranean will be colder than the heat wave summer of 2003. These early dates are consistent with the strong summer temperature responses to climate change in these regions as documented in the IPCC 4th assessment report (AR4) (114).

1.5 °C global warming 

Even at 1.5 °C global warming, a significant increase in heat wave magnitude is expected over Africa, South America, and Southeast Asia. Deadly heat-related climatic conditions are expected to increase over most of the tropical developing countries. Heat waves will become not only more intense, but also more frequent (165).

Typical European summers at a global warming level of 1.5°C are those of 2003 and 2017. The 2003 summer temperatures in Europe will be commonplace around the 2040s (173).  

2 °C global warming 

At 2 °C global warming, most of the tropical countries will face severe heat waves at least once every five years. The increase from 1.5 °C to 2 °C global warming will double the frequency of extreme heat waves over most of the globe, including most of the tropical countries, the continental United States and the Mediterranean countries. Exceptional heat waves, with magnitude similar or higher than that of Russia 2010, are expected to occur especially over regions particularly vulnerable to climate change, such as Algeria, the Horn of Africa and the Arabian Gulf (165). These regions have been identified as hotspots for critical future human habitability because of extreme temperatures (166). In fact, exceptional heat waves will occur in some areas where these heat waves do not occurunder 1.5 °C warming. In particular, 10% of the land over East Africa and Southeast Asia will be affected by exceptional heat waves at least once every 20 years (165).

Summer temperatures in Europe in 2010 are comparable to the average summer in a 2°C warmer world. Under 2°C of global warming, every other European summer month would be warmer than the warmest summer month on record in current climate conditions (174). 

Impact on population 

• Severe heat waves (like Balkans 2007): At 1.5 °C global warming, 13.8% of the world population will be exposed to severe heat waves at least once every 5 years, and around half of the world population at least once every 20 years. These fractions strongly increase to 36.9% (every 5 years) and 70.9% (every 20 years), respectively, when global warming rises from 1.5 °C to 2 °C (165). 
• Extreme heat waves (like France 2003): The number of people exposed to extreme heat waves every 20 years will increase from 9.0% to 28.2% when global warming increases from 1.5 °C to 2 °C, corresponding to a difference of around 1.4 billion people.
• Exceptional heat waves (like Russia 2010): At 2 °C global warming, 8.3% of the world population may be hit by exceptional heat waves at least once every 50 years. This corresponds to around 452 million people more than in a 1.5 °C world. These people are mainly located in developing countries such as the Horn of Africa, the area of the Gulf of Guinea, Indonesia and the coastal regions of South-America from Venezuela to Brazil. 

Compared with 2 °C warming, limiting global warming to 1.5 °C will result in around 1.7 billion fewer people frequently (every 5 years) exposed to severe heat waves, and 420 million and 65 million fewer people frequently exposed to extreme heat waves and exceptional heat waves, respectively. 

This study shows that implementing ambitious mitigation strategies to limit warming below 2 °C or even to 1.5 °C will drastically reduce exposure to the most severe impact of temperature related extreme events in terms of intensity and frequency of extreme heat waves. What’s more, it will drastically reduce the probability of occurrence of exceptional heat waves, with magnitude similar of higher than that occurred in Russia 2010 (165).

Summer monthly mean temperatures at 2°C of global warming are projected to become around 1°C higher over Europe than at 1.5°C of warming (175). On a daily basis, the differences in extreme temperatures will probably be even larger. At 2°C global warming extreme maximum daily temperatures can be reached that are up to 1.5°C higher than maximum temperatures at 1.5°C global warming, according to a recent model study on summer temperature variability at these levels of global warming (174). This is due to the fact that the variability in summer temperatures is higher at 2°C global warming compared with 1.5°C. As a result, the 10% most extreme summer maximum temperatures in a 2°C warmer world cannot be reached when global warming is restricted at 1.5°C. 10% may not seem that much, but these events would correspond to the most extreme and severe heat waves, the ones with the most critical consequences (174).  

Geographically, particularly significant warming has been observed in the past 50 years over the Iberian Peninsula, in central and north‑eastern Europe and in mountainous regions (3,4). In the past 30 years, warming was strongest over Scandinavia, especially in winter, whereas the Iberian Peninsula warmed in summer (146). High-temperature extremes like hot days, tropical nights, and heat waves have become more frequent, while low-temperature extremes (e.g. cold spells, frost days) have become less frequent (2,5).

Urbanization explains only a very small part of warming in Europe: only 0.0026°C/decade of the annual-averaged pan-European temperature trend of 0.179°C/decade over the period 1960–2012 (76).

Elevation-dependent warming

There is growing evidence that the rate of warming is amplified with elevation, such that high-mountain environments experience more rapid changes in temperature than environments at lower elevations (77). Elevation-dependent warming can accelerate the rate of change in mountain ecosystems, cryospheric systems, hydrological regimes and biodiversity. More rapid changes in high-mountain climates would have consequences far beyond the immediate mountain regions, as mountains are ‘water towers’ and the major source of water for large populations in lower elevation regions (78). Besides, mountains provide habitat for many of the world’s rare and endangered species and are therefore sensitive to environmental change (77).

Elevation-dependent warming may be caused by a number of mechanisms (77). One of them is the snow–albedo feedback. The surface absorption of incoming solar radiation increases around the retreating snowline, causing enhanced warming at that elevation (79). In the Swiss Alps, the daily mean 2-m temperature of a spring day without snow cover is 0.4 °C higher than one with snow cover (80) (mean value for 1961–2012). The current snowline, which varies in elevation across different mountain ranges, is expected to retreat to higher elevations as the overall climate system warms. Other mechanisms linked to elevation-dependent warming involve clouds, water vapour and aerosols (77).

The IPCC concluded in 2019 that mountain surface air temperature observations in the European Alps show warming over recent decades at an average rate of 0.3°C per decade, with a likely range of ± 0.2°C, thereby outpacing the global warming rate 0.2 ± 0.1 °C per decade, (214). In the European Alps, warming has been found to be more pronounced in summer and spring (215)

Anthropogenic influence on record‐breaking temperatures trends

From the 1980s onwards, a change in the record evolution is observed: the number of upper temperature records increases and the number of lower temperature records decreases (96). This agrees with previous studies (97). Daily minimum and maximum record-breaking temperatures tend to occur less and more often, respectively, than in a stationary climate (96). This may point at a changing climate. However, over the second half of the twentieth century (observations over the period 1956–2005) no statistically significant changes in the annual and seasonal evolutions of the upper and lower records over Europe can be detected yet. According to climate model simulations, the upper and lower record evolutions at the end of the twentieth century are still in the range of the model’s internal variability (96). 

Summer length

A straightforward way to define the summer is the time of the year when daily near-surface air temperature exceeds a certain threshold. However, this definition would neglect the fact that people at different latitudes, for instance in Northern and Southern Europe, have different perceptions of a summer day simply because they are used to different climates. For a Scandinavian, 22°C may feel like a summer day, for somebody in Spain this may feel like summer has not arrived yet. A better way to define summer is to use relative thresholds, such as the moment when the daily mean temperature is above the 75th percentile of daily mean temperature in a certain period. When this definition is used, the average summer length in the Northern Hemisphere was 115 days in 2014. So far, global warming, reaching a 1°C global surface mean temperature increase in 2014, has led to a 15 days of summer length increase during 1961–2014 according to observations (245).

Model simulations indicate that the average summer length in the Northern Hemisphere will increase by 20% under a moderate scenario of climate change (SSP2-4.5) and by 50% under a high-end scenario of climate change by the end of this century, compared with 2014. Under the high-end scenario the summer would last for nearly half a year in 2100. These summer length increases correspond to an increase by 0.38 and 0.59 days/year under the moderate and high-end scenario of global warming, respectively (245).

Considering the entire extra-tropical continental area of the Northern Hemisphere, an average increase of 25 days in summer length has been reported for the period 1953-2012; the largest increment has taken place in Europe (228).

The characteristics of heatwaves have been studied for three regions in Europe: (1) the Iberian Peninsula, including Portugal and Spain, (2) France, and (3) the part of northwest and central Europe including northern France, Belgium, the Netherlands and Germany. In this study, a heatwave is defined as a period of at least three consecutive days with temperatures higher than the 95th percentile of maximum daily temperatures in the months July and August in at least 5% of the investigated region in the reference period 1961-1990 (259).

The study shows that since 1950, in all these regions and for the months June, July and August, the frequency of heatwaves has increased. Also, the spatial extent, duration and intensity of heatwaves have increased. These changes, however, have not occurred evenly across Europe. Changes are most dramatic for northwest and central Europe in the last two decades. Both the increase of heatwave frequency and heatwave intensity in the period 1998–2021 was much larger in these northern European countries than in France and the Iberian Peninsula (259).

These differences across Europe are due to the fact that daily maximum temperatures have not risen at the same rate. In Portugal and Spain, the warming trends for daily mean and daily maximum temperatures are similar. In France, daily maximum temperatures have risen 28% more than daily mean temperatures, and in the northwest the difference is as high as 40%. These results show that the differences in heatwaves between northern and southern Europe have become smaller: the heatwaves in the north are increasingly similar to those in the south. According to the authors of this study, this may be a result of increasingly warmer air brought in from Spain and Portugal during heatwaves in the north (259).

European heat wave changes since the 1950s

European summer temperatures have increased far more rapidly than the trends for the Northern Hemisphere since the early 1950s (157). While mean summer temperature in the Northern Hemisphere has increased by 0.21 °C per decade between 1951 and 2015, the increase was 0.46 °C per decade for Europe.

This rise in mean European summer temperatures has contributed to the intensity of recent European heat waves, like the ones in 2003, 2010, and 2015. What if climate had remained as it was in the mid-20th century, in terms of the number of hot days? This was analysed for temperature records of 14 climate stations across Europe (156).

If Europe’s climate hadn’t changed since the 1950s, the number of hot days with maximum temperature over 30 °C would have been 10-25% less. The number of hot days over 35 °C would have been 25-50% less, according to this study. This serves to illustrate that climate change is not simply a matter of gradual shifts in mean values, but also of strong increases of the frequencies of extremes. As a result heat waves are becoming more extensive and prolonged, with stronger impacts on society (156).

The heat wave of 2003

Much of Europe was affected by a heat wave during the summer of 2003 (June, July and August). It is estimated that this was the hottest summer since at least 1500 (6). Seasonal temperatures were the highest on record in Germany, Switzerland, France and Spain. Average summer (June–August) temperatures were far above the long-term mean, by up to five standard deviations, implying that this was an extremely unlikely event under current climatic conditions (7).

In addition to an increase in the number of extremely hot days, heat intensity on a hot day has increased as well. Across Europe, extremely hot days are now about 2.3 °C warmer than in 1950. This is an increase of 0.33 °C per decade, larger than the average global warming of about 0.2 °C per decade (210). Especially in Central Europe hot days are now much hotter than half a century ago: hot extremes have warmed about 50% more than the corresponding summer mean temperatures. In the rest of Europe, the trends of temperature increase for hot days and mean summer temperatures are similar (12).

The observed trends for hot extremes are remarkably consistent with climate model projections (211)

Cold days now more than 3 °C warmer than in 1950

Likewise, a day is extremely cold when its minimum daytime temperature falls below the limit of the 1% coldest days in 1950-2018. The annual number of extremely cold days and nights has decreased by a factor of 2-3 on average across Europe from 1950 to 2018: from more than 5 around 1950 to around 2 days/year in 2018. Cold extremes are now more than 3 °C warmer than in 1950: an average increase of 0.49 °C per decade. In Central and Northern Europe the warming of cold extremes is substantially higher than the corresponding winter mean warming and about 2.5 times larger than the average global warming (12).

The observed trends for cold extremes are remarkably consistent with climate model projections (cold extremes: 212).

Almost everywhere on the planet, long-term trends in the lowest minimum temperature of the year in the period 1901-2018 are strongly positive: over 50-100 year time scales, cold extremes are getting warmer in all areas of the globe with sufficient data to study these trends, including virtually all of the northern midlatitudes. In the northern midlatitudes the trend is a factor three to more than five times the global mean temperature trend, except near coasts (224). This is even faster than the winter mean temperature trend, which is only a factor two to three higher than the global mean temperature rise (225). 

Results from other studies

An analysis of a combination of historical (1960–2014) and model (2015–2021) data shows that the hottest summer days in North-West Europe are warming approximately twice as fast as mean summer days. This pattern stands out as relatively unusual across the Northern Hemisphere. Also, climate models fail to capture this difference in trends (255).

Another study has been carried out over the period 1880 to 2005. It was concluded that both the mean and the variance of daily maximum temperature have increased significantly (8). The highest trends in daily maximum temperature variability have been found in central Western Europe. The researchers suggest that their conclusions contribute to growing evidence that Western Europe's climate has become more extreme and confirm a previously hypothesized increase in the variance of daily summer temperatures since the 19th century. The study adds evidence that heat waves, such as the devastating 2003 event in Western Europe are a likely sign of global warming. According to these researchers current estimates of changes in June-August extreme daily temperatures (e.g. 9) have hitherto been conservative in their estimate of changes in extremes over the last 126 years. Over recent decades the annual number of warm extremes has increased significantly as a result of an asymmetric pronounced warming of the upper tails of the temperature distribution (10,62).

Range minimum - maximum temperature

Cold extremes are warming more rapidly than hot extremes, accprding to observations during the period 1950-2018 at weather stations all across Europe. Where hot extremes have increased by 0.33 °C per decade, the trend for cold extremes is 0.49 °C per decade (12). A 50% difference! 

Many impact studies assume that climate change results in changing daily minimum and maximum temperatures by the same amount and in the same direction. However, on local scales natural variability still plays a dominant role and can overshadow the global warming signal (86). For Europe this variability was quantified for the period 1950-2013 by quantifying trends in seasonal daily minimum temperature (defined as the 5th percentile of the daily temperature distribution for a season) and seasonal daily maximum temperature (95th percentile) (86).

The results show that especially the hot extremes show significant increasing trends over central and eastern Europe, and significant decreasing trends mainly over Norway, south-eastern Europe, and Turkey. The cold extremes, on the other hand, experienced warming trends over Scandinavia and eastern Europe and cooling trends over south-eastern Europe, mainly over Turkey. The extreme temperature range, thus, changes differently for different parts of Europe: it widens, for instance for Madrid (warm extremes increase and cold extremes decrease) whereas it reduces for Trondheim (Norway) (a large upward trend in the cold extremes and a decrease of warm extremes) (86). 

There is compelling evidence for statistically significant trends, during the last century, for increasing frequency and total precipitation of rainfall extreme events over North and Western Europe (extreme events being defined as the 90th and 95th percentiles of daily rainfall in the October to March season) (85,113). Extreme daily precipitation started to increase in southern Europe before the year 2000 (178).

Similar results were shown for an analysis of extreme precipitation trends in Europe during December 1950 to February 2008 (145). In this study an extreme precipitation event was defined as a daily amount of precipitation exceeding the 95th percentile of daily rainfall in a certain month at a certain location over the period 1961-1990. Thus, the threshold value equivalent to the 95th percentile was calculated separately for each month and weather station across Europe, allowing for the temporal and seasonal variability in precipitation. Overall, the analysis showed significant upward trends of the number of extreme precipitation events in Europe in all seasons except summer. The results also point at distinct differences between the north and south of Europe, however: an increase of the number of extreme precipitation events in the northern part of the continent and a decrease in the south. The prominent seasonality and strong regional variations detected in extreme precipitation trends are caused by the complicated impacts of both local (e.g. topography) and macro- and mesoscale (atmospheric circulation) factors that trigger precipitation formation and cause variability (145). 

The number of extreme precipitation events has increased over most of the European land area, linked to warming and increases of atmospheric water vapour. For Europe as a whole, also the intensity of extreme precipitation such as heavy rain has increased in the past 30 years, even for areas with a decrease in mean precipitation, such as Central Europe and the Mediterranean (17). The proportion of Europe that has experienced extreme and/or moderate meteorological drought conditions did not change significantly during the 20th century (1).There is evidence for Europe and the United States that the relative increase in precipitation extremes is larger than the increase in mean precipitation, and this is manifested as an increasing contribution of heavy events to total precipitation (18).

Over approximately two-thirds of the Northern Hemisphere land area, greenhouse gases have contributed to the observed intensifica­tion of annual maxima of daily and five-daily precipitation amounts during the second half of the twentieth century (51).

The change in return period of rare precipitation events that occur on average once in 5, 10 and 20 years in the 1950s and 1960s have been analyzed using extreme value theory (63). This was done for 1-day and 5-day precipitation amounts over the time period 1951–2010 in Northern and Southern Europe in all four seasons. 1-day events are indicative of extreme showers which are important for flash floods on local scales. 5-day events are indicative of wet periods which may result in high water levels in larger scale river basins. Over the period 1951–2010, Northern Europe shows a wetting trend in winter, spring and autumn, and a drying trend in summer. Southern Europe shows a drying trend in winter, spring and summer, and a wetting trend in autumn. In Northern Europe, the picture for the changes in extreme precipitation is approximately the same as that for the trend in total precipitation amount. In Southern Europe the 20-year 1-day and 5-day events stay about the same in winter, but become slightly wetter in other seasons, although the regional trend in total precipitation amount in winter and summer indicates drying. Despite considerable decadal variability, the results of this study indicate that 1-day and 5-day precipitation events that occurred on average once in 5, 10 and 20 years in the 1950s and 1960s generally became more common during the period 1951–2010. For all regions, seasons and return periods, the median reduction in return period between 1951-1970 and 1991-2010 is about 21% with variations between a decrease of about 2% and 58% (63).

Trends in annual and seasonal precipitation volumes

Annual precipitation since 1960 shows an increasing trend of up to 70 mm per decade in northeastern and northwestern Europe, and a decrease of up to 90 mm per decade in some parts of southern Europe. At mid-latitudes no significant changes in annual precipitation have been observed. Mean summer precipitation has significantly decreased by up 
to 20 mm per decade in most of southern Europe, while significant increases of up to 18 mm per decade have been recorded in parts of northern Europe (146). 


Annual precipitation trends in the 20th century showed an increase in Northern Europe (10–40%) and a decrease in some parts of Southern Europe (up to 20%). Mean winter precipitation has increased in most of Western and Northern Europe (20 to 40%), whereas Southern Europe and parts of Central Europe were characterized by drier winters (1). Drying has been observed in the Mediterranean and Eastern Europe and no clear trends have been observed in Western Europe (15).

Mean winter (December–February) precipitation is increasing 20–40% in most of Western and Northern Europe, because western circulation was stronger in winter. Conversely, Southern Europe and parts of Central Europe were characterized by a drier winter. Trends in spring and autumn were not significant (16). The changing precipitation regimes in Europe since 1950, reflected in fewer winters with wet conditions in southern Europe but more in northern Europe, coincide 
with changes in storm track activity over the central/eastern North Atlantic toward the northern
 British Isles (159). 

More extremely strong fronts over Europe

High-impact, small-scale weather events, such as hailstorms, wind extremes, or intense precipitation, are frequently associated with the passage of large-scale fronts (134). These fronts are related to strong transitions in temperature and humidity across the front. The stronger temperature and humidity transitions across the fronts, the stronger the fronts are. Extreme weather is more likely linked to strong than weak fronts (133).

Scientists found an increase in the number of extremely strong, large fronts (with a minimum length of 500 km) over Europe over the period 1979-2014 (133). The increase is most pronounced after the year 2000, and refers to the summer (June-August) and autumn (September-November) seasons. The scientists argue that increasing atmospheric humidity primarily drives this trend in the number of strong and extremely strong fronts. Their findings are consistent with the fact that most extreme fronts are identified during the summer, when atmospheric humidity is higher than in the other seasons. Besides, their findings also agree with the observed increase in humidity over Europe in the considered period (135).

These extremely strong fronts can occur at various places over Europe: mountainous regions such as the western Alps, the Dinaric Alps, the Scandinavian mountains in south-western Norway, and the Cantabrian mountains in north-western Spain, as well as the west coast of Ireland and also many parts of France, northern Germany, Denmark, and the Baltic Sea (133).

The upward trend in the number of strong fronts is potentially one driving agent behind an increase in the number of extreme precipitation events over Europe (136). Indeed, several studies suggest an increase in the likelihood of extreme summer precipitation events for Europe (137).

Data from satellite monitoring from 1966 to 2005 show that monthly snow-cover extent in the northern hemisphere is decreasing by 1.3% per decade. Snow-cover trends in the mountain regions of Europe vary considerably with region and altitude. Recent declines in snow cover have been documented in the mountains of Switzerland (19), Slovakia (20), and in the Spanish ski-resorts in the Sierra Nevada and the Pyrenees (21), but no change was observed in Bulgaria over the period 1931–2000 (22). Declines, when observed, were largest at lower elevations. Lowland areas of central Europe are characterized by recent reductions in annual snow-cover duration of about 1 day/year (23). At Abisko in subarctic Sweden, increases in snow depth have been recorded since 1913 (24), and trends towards greater maximum snow depth but shorter snow season have been noted in Finland (25).

Mean and maximum winter snow depths

Mean and maximum winter snow depths are decreasing over Europe except for the coldest regions. This decrease is widespread and has accelerated since the 1980s in parallel with stronger global warming. This was concluded from daily snow depth observations over the period 1951-2017 in a large number of European countries (180). In this study, mean winter snow depth is defined as the average daily snow depth during the winter months December, January and February. Maximum winter snow depth is defined as the 95th percentile of daily snow depth values in this winter period.

The decrease in mean snow depth is stronger than the decrease in maximum snow depth, and this contrast has become stronger in the last few decades. Averaged over Europe, the trends in maximum snow depth are about one tenth weaker than the mean snow depth trends. The average of decreasing trends in mean snow depth for the studied weather stations is −12%/decade. The average of decreasing trends in maximum snow depth is −11%/decade.

The steep decrease in mean snow depth can be related to a combination of decreases in mean snowfall, less days with snowfall, and stronger snowmelt which (combined) lead to shorter periods with snow cover. The results agree with the expectation that in all but the coldest areas in Europe the fraction of below-zero temperatures will decrease, thereby decreasing the fraction of winter precipitation that falls as snow and increasing the amount of snowmelt (181).

For the coldest areas and high latitudes in Europe, an increase in mean and maximum snow depths is observed. Here the increase in precipitation, probably associated with the warming climate, dominates any snowmelt signal in the mean snow depth. For these areas, where temperatures in winter are still below the freezing point, future projections indicate that increases in total precipitation outweigh the effects of warmer temperatures, thereby increasing snow depth (182).  

At the Hungarian section of the river Danube, the date of the first ice appearance has shifted 19-29 days later over the 1876-2011 period, while the date of the final ice disappearance has shifted 18–23 days earlier (153). In Lake Kallavesi (eastern Finland), the freezing date has shifted to 15 days later in the period 1833-2011, while the break-up date has shifted to 12 days earlier in the period 1822-2011 (154). The ice break-up date in the Lake St. Moritz (Swiss Alps) has shifted to 15–20 days earlier since 1832 (155). 

Evaluating high tide levels along the North Sea in the past century showed clear changes in mean levels (related to sea-level rise) but no storm-related variations (27). Similarly, in the northern Adriatic Sea the trends for high sea levels and the subsequent occurrence of storm surges cannot be associated with any trends in storminess (28).

Storm tracks associated with extratropical cyclones have moved polewards over the past 25 years (before 2012) (52). A northward shift in mean storm track position since about 1950 is consistent in studies on wind climate in Europe over the last decades (111). This northeast shift together with the trend pattern of decreasing cyclone activity for southern mid- latitudes and increasing trends north of 55 - 60°N after around 1950 seems consistent with scenario simulations to 2100 under increasing greenhouse gas concentrations (112).

The centennial retreat of European glaciers is attributed primarily to increased summer temperatures. However, changes in winter precipitation, reduced glacier albedo due to the lack of summer snowfall and various other feedback processes, such as the increasing debris cover on the glacier, can influence the behaviour of glaciers, in particular on regional and decadal scales (146). 

Data on the longest and most continuous series for six glaciers in the European Alps (In Austria, Switzerland and France, over the period 1962-2013) show a clear and regionally consistent acceleration of mass loss over recent decades over the entire European Alps (140).

The area of glaciers in the European Alps has shrunk from 4244 km2 at the end of the Little Ice Age (year 1850) to 1806 km2 in 2015, a loss of 57%. In this period volume was reduced from about 280 km3 to 100 km3, a loss of 64%. On average, glacier surfaces lowered by −43.6 m from 1980 to 2015, which is about −0.26 metre per year. From 2000 to 2015, glacier surfaces lowered 3 times as fast, −0.82 metre per year. Many glaciers now have only remnants of their former coverage left. At least 1938 glaciers melted away completely, causing deglaciation of entire catchments (268).

Globally: update 2000-2023

In 2000, glaciers, excluding the ice sheets of Greenland and Antarctica, covered a global area of approximately 706,000 km2 with an estimated total volume of 158,170 ± 41,030 km3. This volume is equivalent to a potential sea level rise of 324 ± 84 mm (270). 

Recently, many experts from around the globe joined efforts to update the global glacier mass loss changes. They combined their data and expertise and ensured that their data are mutually comparable. Their exercise revealed that from 2000 to 2023, glaciers worldwide lost about 5% of their ice. This is a loss of 273 ± 16 gigatonnes in mass annually. This loss increased by 36 ± 10% from the first (2000–2011) to the second (2012–2023) half of the period. The period 2019–2023 includes the 4 years with the largest annual ice loss of more than 400 Gt per year. The largest loss was recorded in 2023: 548 ± 120 Gt per year (271).

Glacier mass loss was about 18% larger than the loss from the Greenland Ice Sheet and more than twice that from the Antarctic Ice Sheet (272).

All world regions experienced glacier mass loss from 2000 to 2023. Glacier loss differs from one world region to another. Since 2000, glaciers have lost between 2% and 39% of their ice regionally. The largest regional contributions to global glacier mass loss are from Alaska (22%), the Canadian Arctic (20%), peripheral glaciers in Greenland (13%), and the Southern Andes (10%) (271).

The glaciers of Central Europe, including the Alps and the Pyrenees, showed the largest relative ice loss of all regions: a loss of almost 39% from 2000 to 2023. A slowdown of mass loss was found in Iceland and Scandinavia, because of regional cooling and an increase in winter precipitation (273).

The observed global ice loss from 2000 to 2023 contributed 18 ± 1 mm to global sea level rise in this period. In the first two decades of this century, melting glaciers contributed 20% to the observed global mean sea level rise. The contribution by changes in ocean temperature and salinity was larger (33%) and the contribution of the Greenland Ice Sheet was smaller (17%). Smaller contributions originate from changes in land water storage and the Antarctic Ice Sheet (271).

Scenarios that are often used for climate projections are the relatively low RCP2.6, the moderate RCP4.5 and the high-end RCP8.5 scenario. Climate models project further increases in global average temperature over the 21st century. For the period 2081-2100 (relative to 1986-2005), increases of between 0.3°C and 1.7°C for the lowest emissions scenario (RCP2.6 (Representative Concentration pathway)), and between 2.6°C and 4.8°C for the highest emissions scenario (RCP8.5) are estimated (124).

The EU and UNFCCC target of limiting global average temperature increase to less than 2°C above pre-industrial levels is projected to be exceeded between 2042 and 2050 by the three highest of the four RCPs (128). The lowest, RCP2.6, implies strong reduction in greenhouse gas emissions over this century and negligible or even negative emissions at the end of the century (129). 

The moderate SRES A1B scenario leads to a global mean temperature increase in 2100 in the likely range of 2.8-4.2°C, which approximates to RCP6 and lies clearly between RCP4.5 and RCP8.5 (74). The RCP4.5 has a target radiative forcing of 4.5 W/m2 and requires, for example, a decline in overall energy and fossil fuel use and a substantial increase in renewable energy forms (116).

Scenarios for previous IPCC reports

Future climate change projections are based on scenarios of potential socio-economic futures that determine the level of greenhouse gas emissions to the atmosphere. Researchers generally use the scenarios that have been published by the IPCC in 2000, the so-called IPCC SRES scenarios (49).

Each socio-economic scenario provides a description of possible future developments. IPCC distinguishes between so-called A and B families; these are widely applied and cover a broad range of possible population growth and economic development. The A scenarios represent a vision of the future where economic development is the priority, whereas the B scenarios represent a future where environmental sustainability plays a central role. Family ‘1’ scenarios are more globalised than ‘2’ scenarios.

A1 describes a future world of very rapid economic growth and a population that increases from 5.3 billion in 1990 to peak in 2050 at 8.7 billion and then declines to 7.1 billion in 2100. Rapid introduction of new and efficient technologies is assumed, as is convergence among regions, including large reductions in regional differences in GDP. The A1 family has 3 subgroups:

• A1F1: high use of fossil fuels
• A1T: high use of non-fossil energy sources
• A1B: an intermediate case

The B1 family describes a convergent, more equitable world, and has the same population scenario as the A1 family; however, there is introduction of clean and resource-efficient technologies, and global solutions are found for economic, social and environmental sustainability. Additional climate initiatives are not assumed.

The A2 and B2 families have higher, continuously increasing population scenarios (to 15.1 and 10.4 billion in 2100, respectively).

These projections are based on both a moderate and a high-end scenario of climate change. The moderate scenario (SSP2-4.5) leads to a global mean temperature increase of about 2.5°C when compared to the pre-industrial era. According to experts, this is a realistic scenario. The high-end scenario (SSP5-8.5) leads to a global mean temperature increase of about 5°C when compared to the pre-industrial era. This scenario assumes that current emissions of greenhouse gasses will continue unabated this century; experts stress that this is not realistic. In these projections, three 20-year time periods were considered: the reference period 1995–2014, 2046–2065 (medium-range future), and 2081–2100 (long-range future).

Moderate scenario of climate change (244): 

• By the middle of this current century, the warming in northern and southern Europe can reach 1–2.5°C under the moderate scenario of climate change. By the end of the century, warming in these parts of Europe can reach 3–4°C. Warming does not seem to surpass 1°C in the British Isles and surrounding areas even for the long-term future.
• The number of hot days, with a mean temperature > 30°C, is projected to increase in the southernmost regions of Spain and Turkey. Compared with the reference period 1995–2014, this increase is around 15–30 days per year in the mid-term future and 35–45 days per year for the long-term future. The rest of Europe does not show an increase in the number of hot days.
• The number of very hot days, with a maximum temperature > 40°C, will probably not change much in Europe until the end of the century, except for some localized areas in Spain and southern Turkey.
• Changes in the number of tropical nights are relevant for human health, since warm nights do not allow a complete human body recovery from the diurnal heat stress. The number of tropical nights will increase in the Mediterranean area by 20–40 nights per year mid-century, reaching 40–60 nights per year by the end of the century in the south-central part of Portugal and Spain, and in Italy, Greece, and some areas of southern Turkey.
• The number of frost days, with a minimum temperature < 0°C, is projected to decrease in practically all of Europe already by mid-century. The decrease is strongest in northern Europe. By the end of this century, this decrease reaches 40–60 days per year in Scandinavia and even above 100 days per year in the Norwegian Sea and Iceland, under a moderate scenario of climate change.

High-end scenario of climate change (244): 

• Under the high-end scenario of climate change, the warming in northern and southern Europe by the middle of this century is like the one projected under the moderate scenario towards the end of the century: 3–4°C. Towards the end of the century, warming in northern and southern Europe can reach 5–6°C.
• The number of hot days is projected to increase in especially southern Europe. This increase is around 35–45 days per year in the mid-term future. By the end of the century, this increase can reach up to 70–80 days per year for Portugal, Spain, Turkey, and some localized areas in Greece, and up to 40–60 days per year in other parts of southern Europe.
• The number of very hot days will probably strongly increase by the end of the century in southern Europe: up to 40–50 days per year in the southern parts of Portugal, Spain, and Turkey, and up to 5–10 days per year in the central-southern parts of Europe.
• For the number of tropical nights, the mid-century projection under the high-end scenario is like the end-of-century projection under the moderate scenario. A high increase in the number of tropical nights is projected for the end of the century in all Europe (except for Scandinavia and the British Isles). This increase is in the order of 50–80 nights per year and can reach 80–100 nights per year in Portugal and Spain.
• Under the high-end scenario of climate change, the number of frost days will reduce by more than 100 days per year in Scandinavia and north-eastern Russia by the end of the century.

Other projections for Europe overall

The annual average land temperature across Europe is projected to continue increasing faster than global average temperature (146).

The first results of the high-resolution (horizontal resolution of 12.5 km) future climate simulations (from the EURO-CORDEX program) address changes in annual mean temperature and total precipitation across Europe for three scenarios (A1B, RCP4.5 and RCP8.5) and the time periods 2021–2050 and 2071–2100 (compared with 1971–2000). For these simulations a large number of regional and global climate models have been used (72). According to these results, under a high-end climate change scenario (RCP8.5), large parts of Northern Scandinavia, Eastern Europe and the Alpine ridge might be exposed to a warming of more than 4.5°C in 2071–2100; this could be avoided by the moderate scenario (RCP4.5).

By the end of this century (2071-2100 relative to 1971-2000), annual average land temperature over Europe is projected to increase in the range of 1°C to 4.5°C under the RCP4.5 scenario, and 2.5°C to 5.5°C under the RCP8.5 scenario. This is more than the global average (130, 146, see also (29) for similar results for the period 2080-2100 compared with the 1961-1990 average. The warming is projected to be greatest over Eastern Europe, Scandinavia and the Arctic in winter (December to February), and over Southwestern and Mediterranean Europe in summer (June to August) (30,32,130). The temperature rise in parts of France and the Iberian Peninsula may exceed 6°C, while the Arctic could become on average 6°C and possibly 8°C warmer than the 1961–1990 average (31).

In summer, the warming in large parts of Central, Southern and Eastern Europe is shown to be especially connected to higher temperatures on warm days, more than to a general warming at all temperature intervals. Much of the warming in winter is connected to higher temperatures on cold days (32). The yearly maximum temperature is expected to increase much more in southern and central Europe than in northern Europe (36,232).

The summer climatic zones shift northward by at least 400-500 km by the end of the twenty first century. The mean number of days/year exceeding 30°C at the model grid point nearest to Paris increases from 9 days under current climate to 50 days under future climatic conditions (38).

It is estimated that countries in Central Europe may experience the same number of hot days as currently occur in Southern Europe and that Mediterranean droughts may start earlier in the year and last longer. Most affected are probably the southern Iberian Peninsula, the Alps, the eastern Adriatic seaboard, and southern Greece (35).

Climate model simulations project summer season to expand on average by about 10 days in a world that is 1.5 °C warmer than the reference situation at the end of the 19th century. The summer is projected to start 4–5 days earlier and end 5–6 days later. In a world that is 2 °C warmer, summer is projected to last about 17 days longer, with a 8–9 days earlier start and a 9–10 days later end (240). These results agree with previous studies (241).

Central Europe

Future vulnerability of European urban areas to climatological extreme events has been assessed for the period 2021-2050 compared to the reference period 1971-2000 (139). This was done for highly populated urban areas in Central Europe (the east of France, Germany, Austria, Czech Republic, Slovenia, north of Italy). The assessment is based on high-resolution regional climate model simulations and a scenario of moderate climate change (the so-called IPCC A1B scenario). On average for Central Europe the results show an increase of the number of heat waves (17%), as well as the number of single hot days (12%) and tropical nights (30%), and a decrease of the number of frost days (30%). For both hot and frost days the absolute decrease is largest in the northeast whereas the relative changes are largest in the northwest (139).

The results show that the number of hot days (daily maximum temperatures > 30°C) will increase particularly in the western part of central Europe, affecting cities like Paris, Stuttgart and Hamburg. At some locations, the number of hot days is to increase by over 70%. The number of tropical nights (daily minimum temperature > 20°C) shows a similar pattern since ‘hot days’ regions cannot significantly cool down and high temperatures remain throughout the night. The relative increase of the number of tropical nights shows a clear trend from west to east, ranging from roughly 70% for Hamburg and 40% for Paris, down to less than 10% in the vicinity of Vienna. Also the increase in the number of heat waves (daily maximum temperatures > 30°C on at least 5 days in a row) is higher in the western part (with cities like Paris and Zurich) than in the eastern part. In general, a decrease of the number of frost days is projected, by up to 45% at the coastline of the Atlantic Ocean. Remarkably there is no clear projected change in the number of frost days in the alpine area. The increase of the number of days that buildings have to be air-conditioned in the summer dominates over the decrease that buildings need to be heated in the winter (139).

The Alps

The IPCC concluded in 2019 that until 2050, regardless of the climate scenario, surface air temperature is projected to continue increasing (very high confidence) at an average rate of 0.3°C per decade, with a likely range of ± 0.2°C per decade, locally even more in some regions, generally outpacing global warming rates (0.2 ± 0.1 °C per decade (high confidence) (214).  

Northern Europe

Averaged over northern Europe as a whole, the annual median warming is between 2.0°C (RCP4.5) and 2.9°C (RCP8.5) by the middle of the 21st century and between 2.7°C (RCP4.5) and 5.0°C (RCP8.5) by the end of the 21st century. Projected warming over northern Europe is stronger during winter than during summer (see table below) (106). 

Areas already experiencing strong heat waves (e.g. the Mediterranean region) could experience even more intense heat waves in the future (232). But other areas (e.g. France, Germany, and the Balkans) could see increases of heat wave intensity that could have more serious impacts because these areas are not currently as well adapted to heat waves (37).

The mean duration of heat waves increases by a factor of between one and eight over most of Europe. Much higher increases of at least a factor of seven are predicted for the mean intensity, the mean number of heat waves and the frequency of heat-wave days, with greatest changes (more than tenfold increases) in the south of France and Spain (38).

The 2003 heat wave can be used within certain limits as an analog to what may occur with more regularity in the future. The physical processes that characterized the 2003 heat wave, such as soil moisture depletion and the positive feedback on summer temperatures, and the lack of convective rainfall in many parts of the continent that generally occur from June–September, are projected to occur with greater frequency in the future (50). Droughts and high temperatures are linked: low soil moisture conditions can induce higher heat wave temperatures in Europe (176).

European capitals ranked by exposure to heat waves

75% of the population of the European Union lives in urban areas, and this percentage is expected to grow to 82% by 2050 (195). Needless to stress the importance of assessing climate change impacts on the urban environment, so cities can adapt in time to climate extremes. Future heat-related mortality in particular is a major concern. Heat waves are one of the deadliest weather-related hazards (196). The death toll of the 2003 heat wave in Europe, for instance, was 40,000 people. More extreme heat waves, with high death tolls, have struck Europe since then, like the one of 2010 impacting Moscow, or the recent 2017 heat wave in western and central Europe.

In order to raise awareness that it is time to act and adapt cities to temperature extremes, future impacts of temperature extremes in European capital cities have been assessed, with emphasis on heat waves. 8 model simulations (combinations of Global Circulation Models downscaled by Regional Climate Models) were used for this, based on a high-end scenario of climate change (RCP8.5). Selected time slices are near future (2021 - 2050) and distant future (2071 - 2100), and near past (1981 - 2010) for comparison. Focus was on 31 European capitals (EU28 plus Moscow, Oslo and Zurich) (194).

Definition heat and cold waves: Heat waves are defined in many different ways. In this study (194), a heat wave is defined as a period of at least 3 consecutive days with maximum temperature above a certain threshold. No fixed value for a threshold was chosen since this would highlight heat waves in southern Europe far too much. A couple of days of 27 °C in Northern Europe, for instance, may be experienced as a heat wave by the local population, but would not be noticed if the heat wave threshold were set at 30 °C. A definition for the heat wave threshold was chosen, therefore, that allows for this threshold to vary in time and space: the threshold is the 10% highest maximum daily temperatures in a certain time of the year, as observed in a certain city over the reference period 1981-2010. This way, a lower threshold is calculated for the North than for the South of Europe.

In this study, the intensity of a heat wave day is defined as the number of degrees maximum temperature exceeds a certain threshold, calculated from observations in the past. The sum of these intensities for all days of the heat wave presents an estimate of heat wave intensity. This sum is called the Heat Wave Magnitude Index in this study (197).

In a similar way, the intensity of projected future cold waves was quantified, focused on sequences of days colder than the 10% lowest minimum daily temperatures in a certain time of the year over the reference period 1981-2010. For both cold and heat waves three labels were used to characterize these extreme evens: “severe”, “extreme”, and “very severe” events.

Changing heat waves in Europe’s capital cities: In the near future, extreme heat waves are more likely to occur in Madrid and Valletta, and (with decreasing likelihood), Lefkosia, Ljubljana, Rome, Sofia, Athens, Bucharest, Budapest, and Zurich. By then, very severe heat waves are most likely in Valletta and Madrid, and to a slightly lesser degree in Rome and Sofia. In the distant future, the probability of occurrence of very severe heat waves will be high in a large number of cities: Athens, Bratislava, Bucharest, Budapest, Lefkosia, Ljubljana, Madrid, Rome, Wien, Zagreb, Zurich, Paris, and Lisbon. By then, the probability of an extreme heat wave to occur is relatively low only in Amsterdam. Still, also for Amsterdam, extreme heat events are a serious future threat, despite the city being ranked with lowest heat wave risk in 2100 in comparison to other European capitals (194).

The Maltese capital Valletta particularly emerges from this study, being ranked Europe’s capital city with the most severe heat waves by the end of this century. These impacts will likely be even more serious due to Valletta's geographic location in the middle of Mediterranean Sea where high humidity may add to these high impacts for the local population (198). Another important finding is the projected dramatic increase in heat wave occurrence in the continental European cities of Bucharest, Budapest, Ljubljana, Prague, Paris and Zagreb.

Vanishing cold waves in Europe’s capital cities: Cold waves will still represent some threat in 2050 but they will not be the major threat in Europe’s capitals. By the end of this century, cold waves are projected to completely vanish. The increasing negative impacts of extreme heat will by far outweigh the positive consequences of vanishing cold waves (194). Specifically in urban areas, the crucial negative impacts of extreme heat include health risks, human discomfort, associated higher concentrations of pollutants, lower water quality, increase in the energy demand for cooling, and decrease in labour productivity (199).

Record-breaking summer temperatures

The probability that summers in the future (2061-2080) will be warmer than any experienced in the past roughly 100 years (2020-2014) was quantified for two scenarios: a high-end (business-as-usual) scenario of climate change (RCP 8.5) and an intermediate scenario that reflects a moderate mitigation pathway of limiting fossil fuel use (RCP 4.5). Large numbers of simulations were used for this, based on one climate model. The warmest summer that was simulated for the period 1920-2014 was defined as the historical record (164). In this study, summer characteristics for the past 100 years were modeled. This period is too short to capture the extreme heat wave of 2003. This summer was an outlier. The modeled summers, therefore, do not include extreme summers like the one observed in 2003.

The results show that, on a global basis, the probability of exceeding the historical record temperature increases from less than 10% in a present-day summer to over 80% in 2070 with projected warming under the business-as-usual scenario (RCP 8.5). This increase can be reduced to nearly half (to 41%) by moderate climate mitigation (RCP 4.5). Europe is one of the world’s regions that benefit most from mitigation by seeing risk reductions of over 50% (164).  

Future return periods of the heat waves of 2003, 2010, and 2019

According to regional multi-model experiments (11 RCMs, driven by different GCMs and forced with the A1B emission scenario), the probability of a summer experiencing mega heat waves will increase by a factor of 5 to 10 within the next 40 years. However, the magnitude of the 2010 event was so extreme that despite this increase, the likelihood of an analog over the same region remains fairly low until the second half of the 21st century (54). Under a high-end scenario of climate change (the RCP8.5 scenario), very extreme heat waves (which are much stronger than either the 2003 or the 2010 heatwave) are projected to occur as often as every two years in the second half of the 21st century (132). The projected frequency of heatwaves is strongest in southern and south-eastern Europe (132).

Model projections indicate that weekly heat spells of the magnitude of the second week of August 2003 will probably occur in 2020–2049 with a best-guess return period of ~10 years in Eastern Europe and ~15 years in Western Europe. However, a weekly 2010-like event remains very rare in the same period (best guess return periods >30-year over both regions). By the end of the 21st century, such extreme weekly heat spells are expected every ~8 years in Eastern Europe and ~4 years in Western Europe, whereas some models show regular 2003-like anomalies (about every second summer). The estimated return period involve major model uncertainties and should be carefully interpreted, given the high natural variability of such extreme events (54). Events similar to the European record hot summer of 2003 would be very likely at least 24% less frequent in a world at 1.5°C global warming compared to 2°C global warming (158).

Model projections indicate that the 2019 heat event would become usual events by 2100 with return periods of 9.2, 4.6, and 2.1 years under a low-end (RCP 2.6), moderate (RCP 4.5), and high-end (RCP 8.5) scenario of climate change, respectively (230).

Future intensity of heat waves similar to the one of 2010

The 2010 heat wave in eastern Europe and Russia ranks among the hottest events ever recorded in the region. It was likely the hottest summer in the last 500 years in this part of the world. There were over 50,000 extra heat-related deaths, and economic losses were more than US$15 billion (54).

In addition to the widespread, anticyclonic conditions that led to this heat wave, depletion of soil moisture was a crucial driver behind the extreme heat. Evaporation from the soil and transpiration by plants was very low due to the very dry soil. As a result, little of the sun’s radiation was used for evaporation and transpiration, and thus turned into a latent heat flux that cools the surface. Instead, most of the sun’s radiation was turned into surface warming (90).

The low soil moisture conditions were a necessary basis for this extreme heat wave. Still, under present climate conditions, soil moisture levels are generally high enough, and surface evaporation is strong enough to cap maximum summer temperatures (191). These constraints may weaken, however, under future warming (192), scientists warn in an article in Nature Climate Change (190). In future summers, soil moisture levels quickly decline after the start of summer, and the soil will be largely depleted of moisture long before the end of summer. Even an above-average wet spring will no longer be able to constrain summer temperatures under atmospheric conditions similar to those of the 2010 heat wave.

They simulated the atmospheric heat wave conditions of 2010, in a warmer world. They focused on 2075 under a high-end scenario of global warming (RCP 8.5). ‘Future mid-latitude heat waves analogous to the 2010 event will become even more extreme than previously thought’, they conclude, ‘with temperature extremes increasing by 8.4 °C over western Russia.’ Because of the disappearance of the evaporative cooling constraint, atmospheric conditions similar to those of 2010 will lead to much higher temperatures than the mean climate change effect.

Heat waves in Central Europe 

In the last three decades of the previous century Central Europe has experienced 22 heat waves. Future changes in the frequency of occurrence of these heat waves were studied for this part of Europe, that includes most of Germany, the Czech Republic, Slovakia, the Southwest of Poland, Northern Austria and Hungary (160). In this study a heat wave is defined by at least three consecutive hot days. A hot day occurs when on average maximum daily temperature over Central Europe exceeds the 90th percentile of the distribution of daily maximum summer temperatures for the period 1970 - 1999. This value of the 90th percentile for this historical period is also kept as the reference for quantifying future hot days and heat waves.

Compared to this historic period, the frequency of heat waves is projected to increase by a factor 2 in the near future (2020 - 2049). For the late twenty-first century (2070 - 2099), the projected frequency increase of heat waves depends on the rate of climate change: under a high-end scenario of climate change (RCP 8.5), 3-4 heat waves per summer are projected, compared to about two heat waves under a moderate scenario of climate change (RCP 4.5). These projections are based on a large number of combinations of global and regional climate models (160).

The 1994 heat wave is found to be the most distinctive during the 1970 - 1999 period. It lasted 16 days and was associated with large excess mortality in the Czech Republic (161), Poland (162) and other Central European countries. This heat wave has been ranked as the most severe in Central Europe over the whole 1950 - 2012 period (163). Such extraordinary heat waves will probably still be rather rare in the near future. At the end of this century, however, at least one event per decade similar to the 1994 heat wave is projected for Central Europe (160).  

Heat wave impacts on electricity supply

Extreme heat waves have an impact on western European electricity supply due to the increased electricity demand for cooling (83) and the power limitation of thermoelectric plants due to regulation constraints concerning downstream water temperature (40). At least the two most severe heat waves during 1979–2008 (those of 2003 and 2006) had a negative impact on western European electricity supply, with shut downs of thermoelectric plants (84).

Therefore, changes of western European heat wave characteristics have been studied for the end of the twenty first century (2070–2099), compared with 1979–2008 (82). Heat waves were defined as periods of at least 3 consecutive days of extremely high daily maximum temperature affecting at least 30 % of Western Europe. For the high temperature threshold the 98th percentile of the daily maximum temperature during March-October was chosen; for western Europe, this more or less agrees with a threshold of 30°C, commonly used to define a “hot day”.

The study is based on 19 climate change models and three scenarios of future greenhouse gasses concentrations (the so-called RCP2.6, RCP4.5 and RCP8.5 scenarios) that cover a range of moderate to high-end climate change projections. The results indicate a strong increase of the number of heat waves. Besides, the average future heat wave lasts longer, is more extended and more intense. Heat waves with similar or higher severity than observed for the 2003 heat wave remain rather rare, except for the most extreme scenario of climate change (RCP8.5) (82).

For the more moderate scenarios of climate change (RCP2.6 and RCP4.5), the median values of highest simulated severities for all model results are comparable to the 2003 heat wave; for the high-end scenario (RCP8.5), heat waves (median of all model results) with 5 times higher severity than the 2003 heat wave are simulated. For this scenario, planners would need to adapt to yet unprecedented heat wave severities. The spread in results between different models is large for all heat wave characteristics, however. Far more extreme values for heat wave characteristics have been calculated than the median values of all model results: the variability in results due to different models is larger than the variability due to different climate change scenarios (82). 

According to previous studies it is expected that precipitation will increase between 1 and 2% per decade in Northern Europe while it will decrease by less than 1% per decade in Southern Europe. At the same time seasonal differences will increase throughout Europe. Most of the continent will get wetter in winter, while during summer drying is expected in the South (-5% per decade) (38). Summer precipitation may decrease substantially (in some areas up to 70%, depending on the scenario) in Southern and Central Europe. For Northern Europe both a slight decrease (38,44) and increase (39) of summer precipitation have been reported. Relatively small precipitation changes were found for spring and autumn (44).

Models project an increase in winter precipitation in Northern Europe, whereas many parts of Europe may experience dryer summers. But there are uncertainties in the magnitude and geographical details of the changes. Geographically, projections indicate a general precipitation increase in Northern Europe and a decrease in Southern Europe. The change in annual mean between 1980–1999 and 2080–2099 (for the intermediate IPCC SRES A1B projections) varies from 5 to 20% in Northern Europe and from – 5 to – 30% in Southern Europe and the Mediterranean (1).

Extreme events

3 hourly extreme precipitation events with return periods ranging between 5 and 50 years are projected to increase over Europe in the period 2071 - 2100 compared with 1971 – 2000, according to regional climate model projections for a moderate (RCP4.5) and high-end (RCP8.5) scenario of climate change. This increase is projected for large parts of Europe for all seasons except for the summer. For the summer a bipolar pattern is projected: an increase of 3 hourly extreme precipitation in the north and a decrease in the south (223).

Extreme precipitation events in the summer become more intense in the future. This was shown for four regions across Europe (south of Norway, Denmark, Benelux and Albania) for a large number of global and regional climate models under a high-end scenario of climate change (RCP 8.5). Sub-daily rainfall extremes will be higher in the period 2081 - 2100, compared with 1986 - 2005. The absolute strongest local daily precipitation event in a 20 year period will increase by 10% - 20%/°C. Despite large natural variability, the result of an upward trend in extreme precipitation change for shorter event duration is robust across the regions and for all models (222).

This century, storms will get wetter and more erratic as the world warms, and in most of Europe (except for the most southern part) the heaviest rains will occur more often (183).

Very high daily precipitation, defined as the 95th percentile of precipitation on wet days, may increase  strongly over western, central and northern Europe at the end of the 21st century, with the smallest increase (about 20%) for a low scenario (RCP2.6) and the largest (40-70 %) for a high-end (RCP8.5) scenario of climate change (108). Extreme daily precipitation is projected to increase in southern Europe. For a high-end scenario of climate change, this increase may be more than 20% locally in the Po and Veneto basins in Italy, the Rhône in France, Northern Greece, and the basins covering Slovenia and Croatia in the Adriatic (178).

Extreme precipitation events are projected to increase by 17% in Northern and 13% in Central Europe during the 21st century, with no changes projected in Southern Europe (40). From a comparison of scenarios of European precipitation extremes based on six different regional climate models (RCMs) it was concluded that the frequency of precipitation extremes increases from the period 1961–1990 to 2071–2100 by a factor of 2 to 5 over Northern Europe in winter. The corresponding numbers for Central Europe are from no change to a frequency increase by a factor of about 2. The simulated response in winter qualitatively conforms to the observed trends in heavy precipitation over Europe, which shows an increase in winter primarily north of 45°N (41). It is premature, however, to infer the detection of an anthropogenic influence on heavy precipitation in Europe (42).

According to regional climate model calculations for the period 2071–2100, and compared with the period 1961–1990, the mean precipitation increases in Northern Europe partly due to an increased number of wet days and partly due to increased precipitation intensity. In southern Europe, both the number of wet days and total precipitation is generally projected to decrease. At the same time there are extensive areas in southern Europe that are likely to experience relatively more intense precipitation (32).

For summer, there is a much larger variation in the change of precipitation extremes between the RCMs than in winter. All models show a tendency toward larger increases or smaller decreases (depending on the model) with more and more extreme events. The larger-scale pattern shows a gradient from increases in northern Scandinavia to decreases in the Mediterranean region (41). A future simulation of summer precipitation for the future period 2031 – 2036 and a high-end scenario of climate change (RCP8.5) with a detailed weather model suggests that, compared with the period 1990 - 1995, the summers in Europe will possibly be drier, with longer dry spells, shorter wet spells and heavier precipitation (177,232).

Dry spells

In Southern Europe, the longest dry period within a year may be prolonged by one month at the end of 21st century. In Central Europe, prolongation of longest dry period is by one week, and no prolongation is projected for Northern Europe. Thus regions in Europe that are now dry are projected to become even more vulnerable (1,38).The longest yearly dry spell could increase by as much as 50%, especially over France and Central Europe (43). Studies on the impact of stringent CO2 mitigation indicate that some significant increases in drought may be inevitable even under strongest mitigation scenarios (58).

Central Europe

Future vulnerability of European urban areas to climatological extreme events has been assessed for the period 2021-2050 compared to the reference period 1971-2000 (139). This was done for highly populated urban areas in Central Europe (the east of France, Germany, Austria, Czech Republic, Slovenia, north of Italy). The assessment is based on high-resolution regional climate model simulations and a scenario of moderate climate change (the so-called IPCC A1B scenario). On average for Central Europe the results show an increase of the number of heavy precipitation events by 40 %. In Europe, heavy precipitation events are most likely in the summer (June, July and August). For Central Europe the relative increase in the number of days with heavy precipitation in these months (daily accumulated precipitation > 20 mm) is largest in northern Germany, Belgium and the Netherlands (> 30%) (139). 

Northern Europe

In their 5th Assessment Report the IPCC presented a precipitation increase averaged over northern Europe at the end of the 21st century for April through September ranging from 3 to 4% for the mid-21st century and 5 to 8% at the end of the century, under an intermediate (RCP4.5) to high-end (RCP8.5) scenario of climate change (107). For October through March the projected increase in precipitation averaged over northern Europe ranges from 8% (RCP4.5) to 11% (RCP8.5) for the mid-21st century and from 11% (RCP4.5) to 20% (RCP8.5) at the end of the 21st century (107).

Southern Europe

According to a large number of models (GCM’s), precipitation amounts of the larger Mediterranean basin will decrease in all seasons at almost all parts of the basin in 2070 - 2100 with respect to reference period of 1970 - 2000 under the A2, A1B and B1 emissions scenarios. Increased drought conditions and heavy precipitation events will very likely occur more frequently in 2100 with respect to the reference period (89). Heavy rainfall events are likely to intensify by 10-20%, in all seasons except summer (189). Similar results were found for more recent model projections (based on the RCP8.5 scenario of climate change): widespread reduction of precipitation will be particularly serious for the Iberian, Balkan, and Anatolian peninsulas (179).

A global atmospheric temperature increase of 2°C will probably be accompanied by a reduction in summer precipitation of around 10-15% in Southern France, Northwestern Spain and the Balkans, and up to 30% in Turkey and Portugal (187). Scenarios with 2-4 °C temperature increases in the 2080s for Southern Europe would imply widespread decreases in precipitation of up to 30% (especially in spring and summer months) (188). 

Intuitively one would expect an increase in the frequency of thunderstorms and lightning under global warming. After all, higher temperatures increase evaporation rates and thus atmospheric moisture and instability in the atmosphere. Thunderstorms, however, also need a trigger to develop: a mechanism to lift the moist air and – in simple words – turn it into a thunderstorm.

In a recent study, future projections in changes in lightning across Europe were presented based on a model that did include the relevant processes – in particular convection – at the local scale. A high-end scenario of climate change (RCP8.5) was used for this. The projection for 2100 was compared with the period 1998–2007, representing the current situation (247).

According to these projections, the weather regimes that are associated with thunderstorms and lightning shift to the north, increasing lightning frequency at higher latitudes in the summer, including the Nordic countries and the British Isles. Also, more thunderstorms are projected over the Alps – and other mountainous regions in the south – where fast warming of mountain valleys by solar radiation stimulates convection in the summer (247). Indeed, a significant increase in the number of summer convective-like storms has been observed over the Alps recently (250).

Lightning frequency decreases over lower terrain elsewhere and over the sea. Southern Europe experiences drier summers with less lightning. In the autumn, however, lightning frequency in the south will probably increase, partly because of higher air moisture due to very high sea- surface temperatures in the Mediterranean. Changes in lightning counts across Europe in winter and spring are limited (247).

Model simulations project widespread reductions in snow cover over the 21st century (45,146). Decreases of between 9 and 17% in the annual mean northern hemisphere snow cover by the end of 21st century are projected by individual models (45). Although winter precipitation is projected to increase in northern and central Europe, less frequent frost occurrences associated with higher temperatures are projected to reduce the number of days with snow cover. Decreases of more than 60 snow-cover days are projected to occur (for the period 2071–2100 compared with 1961–1990) around the northern Baltic Sea, on the west slopes of the Scandinavian mountains and in the Alps (46). The beginning of the snow accumulation season is projected to be later and the end earlier, and snow coverage during the snow season is projected to decrease (47).

Climate model projections for the period 1950–2100, assuming the SRES A1b emissions scenario, show that precipitation on future days, where average daily temperature is below freezing, decreases in large parts of Europe in a future warmer climate. This change occurs in addition to the number of such days decreasing strongly in the future. Observations also show that this decrease has already set in in the twentieth century. The result is in marked contrast to the climatological winter precipitation which increases in most global-warming scenarios. It has been argued that this is because these days in the future will occur for more extreme circulation types associated with, on average, drier weather conditions (64). These results were confirmed for western and central Europe for the period 2071-2100 compared with 1981-2010 (model projections based on RCP4.5 and RCP8.5 scenarios) (67).

From statistical analyses (ranking and extreme value statistics) for a large part of Europe a general and consistent tendency towards an increased frequency of windstorm-related losses over most of Western, Central and Eastern Europe was concluded for IPCC B1 and A2 scenarios, and slightly inconsistent findings for A1B scenario. From these analyses it was concluded that losses may reach unseen magnitudes at the end of the 21st century, which for some countries (e.g. Germany) may exceed 200% of the strongest event in present day climate simulations. In these analyses, it was assumed that storm damages occur only at 2% of all days; the minimum wind speed that is expected to produce any loss, therefore, is defined as the regional 98th percentile of the daily maximum wind speed (59).

These statistical analyses show 3 different tendencies for the period 2060-2100 compared with 1960-2000 (59):

1. Countries with shorter return periods of storms and higher losses for all 3 climate scenarios: Germany, Belgium, the Netherlands, Poland, Estonia, Austria, Croatia, Bosnia and Hungary;
2. Norway with longer return periods and lower losses for all 3 climate scenarios;
3. All other countries in the studied part of Europe (Czech Republic, Finland, Great Britain, Ireland, Italy, Latvia, Lithuania, Portugal, Slovakia, Slovenia, Spain, Switzerland) have typically higher losses under future climate conditions and in some cases shorter return periods. Some countries, e.g. Italy and Sweden,  actually show a tendency to longer RPs (A1B scenario).

In addition to these statistical analyses, simulations by a global climate model for the period 2060-2100 show that maximum storm losses for countries of Western Europe could increase by ~65% by the end of the 21st century, according to the IPCC A1B and A2 scenarios (59). Similar results were found in earlier studies for Central Europe (60), and some European countries (61). The significance of changes in storm magnitude strongly depends on country and scenario. For many countries, findings point towards higher loss events, significant for at least one of the tree studied IPCC climate change scenarios (B1, A1B, A2). An exception is Norway, for which weaker losses are found (59). For the Mediterranean, model simulations show a reduction in the total number of cyclones and windstorms crossing the Mediterranean region under climate change conditions (SRES A1B scenario) for the first half of the twenty-first century compared with the end of the previous century (68).

Several recent models suggest that the frequency of Atlantic tropical cyclones could decrease as the climate warms. However, these models are unable to reproduce storms of category 3 or higher intensity (48). An operational hurricane-prediction model has been applied to calculate a realistic distribution of intense hurricane activity for present-day conditions. The model projects nearly a doubling of the frequency of category 4 and 5 storms by the end of the 21st century, despite a decrease in the overall frequency of tropical cyclones. The largest increase is projected to occur in the Western Atlantic, north of 20°N (48). Changes in the storm track with increased cyclone density over western Europe in winter and reduced cyclone density on the southern flank of the storm track over western Europe in summer are projected to occur towards the end of the 21st century (109).

Medicanes: cyclones in the Mediterranean Sea

Hurricanes that form in the Mediterranean Sea are called Medicanes: a combination of both words. Medicane winds reach strength over 12 Beaufort. They are a rare phenomenon. They can develop when storms pass the warm water of the Mediterranean Sea, just like tropical cyclones develop in the Atlantic Ocean (207). In previous decades, Medicanes in the Mediterranean were mostly observed south of the Gulf of Lion‐Genoa (western Mediterranean), and (to a lesser degree) southeast of Sicily (Ionian Sea). They form predominantly from autumn to spring (206,256).

Medicanes resemble tropical cyclones (208). Sea surface temperature is a key factor that drives the formation of these hurricanes. Global warming will increase sea surface temperature of the Mediterranean Sea. This will affect the occurrence and strength of Medicanes. Changes in the characteristics of Medicanes have been studied for the near future (2016 - 2035) and the far future (2081 - 2100), compared with the situation in the recent past (1986 - 2005). A global climate model was used for this; scenarios were calculated under an intermediate scenario of climate change (RCP4.5)). Dramatic shifts were found in Medicane‐related hazards by the end of this century (206).

Medicanes will occur less frequent (see also (209)) but they are likely to become more vigorous. They will become more tropical: they will be associated more frequently with stronger winds and will lead to more intense precipitation. Flood risk posed by these storms will increase. Also, they may change the path they follow: they may become more common in the Ionian Sea and less common in the western Mediterranean Sea. The model results show that Medicanes will probably not change substantially in the next few decades. They will change significantly by the end of the century, however. In fact, the study was carried out under an intermediate scenario of climate change. When climate change turns out more strongly, the Mediterranean Sea will warm up more strongly and this will amplify the Medicanes' potential destructiveness even more (206).

The regions that are projected to lose more than 75% of their current volume on average are, for RCP4.5: Western Canada and US (85 ± 11%), Scandinavia (76 ± 25%), North Asia (88 ± 7%), Central Europe (84 ± 10%), Caucasus (89 ± 7%) and Low Latitudes (88 ± 6%). For RCP8.5 all of these regions lose on average more than 90% of their current volume by 2100 (65).

Many glaciers are projected to disappear regardless of future emissions (213).

At least half of all glaciers will be gone by 2100

There are more than 215,000 glaciers on our planet. Most of them – about 80% – have an area of less than 1 km2. Roughly half of all glaciers are projected to be lost by 2100, even if temperature increase is limited to +1.5°C (251). Glacier mass losses increase rapidly as global temperature increases beyond +1.5°C – and global warming is headed toward +2.7°C (252). This was concluded from an evaluation of the sensitivity of glaciers to global warming by 2100 relative to preindustrial levels .

Glacier mass loss: The authors of this study predict that glaciers will lose about a quarter to more than 40% of their mass by 2100, globally, relative to 2015, under a low- and high-end scenario of climate change, respectively. Scandinavia and Central Europe are among the world regions that are projected to lose 60% to 100% of their glacier mass depending on the level of global warming. They will experience near-complete deglaciation at +3°C global warming (251). Elsewhere, the glaciers that remain will continue losing mass beyond 2100 (253).

Sea level rise: Total glacier mass loss would contribute about 9 to 10 cm (± about 3 cm) to mean sea level rise by 2100 under +1.5 to +2°C global warming. Under +3°C to +4°C global warming, the contribution of glacier mass loss to sea level rise would be about 13 to 15 cm (± about 4 cm). The glaciers that are projected to disappear contribute only a small part to sea level rise – 2% to 8%, depending on the temperature change scenario – because they are generally very small (251).

From 2000 to 2019, glacier melt has contributed about 0.7 cm to sea level rise, equivalent to over 20% of the total sea level rise over this period (251).

Number of vanishing glaciers: Even if we succeed in meeting the Paris Agreement target of limiting global warming to +1.5°C, 104,000 (± 20,000) glaciers – nearly half of all globally inventoried glaciers – will disappear by 2100. At least half of those will be lost before 2050. Most of them are smaller than 1 km2 (251).

Impacts: The melting of the glaciers not only impacts sea level rise and tourism. Glaciers are also a critical water resource for about 1.9 billion people, globally, and the disappearance of glaciers will affect water availability to many. Besides, part of the meltwater of the glaciers accumulates in lakes behind walls of debris and when these walls break, glacier outburst floods may cause many casualties and a lot of damage (251).

This model study showed a huge impact of limiting warming to 1.5 °C compared with 2.0 °C warming, or more. If warming is limited to 1.5°C, sea ice cover in September will be less than the record 2012 minimum only 55% of the time in the late twenty-first century. At 2.0 °C this will be the case in almost all September months. The likelihood of any September to be completely ice-free under a 2.0 °C scenario is 35% by 2100, according to this study. If warming is limited to 1.5 °C, this likelihood is only a few percent. Besides, under scenarios that reach or exceed 2.0°C, ice-free conditions for multiple summer months and for several years in a row also become possible by the late twenty-first century (185).

Still, even when warming is limited to 1.5 °C, the Arctic summer sea-ice cover experiences significant reductions compared to today's cover. Nevertheless, to limit warming to 1.5 °C rather than 2.0°C or more has the potential to avoid frequent ice-free conditions as well as to prevent a loss of multiyear sea ice in the Arctic (185).

For 100 cities spread across Europe – from southern Scandinavia to the south of Spain and from Ireland to the Baltic States and Greece – the potential to reduce the urban heat island effect has been calculated. The scientists used a combination of urban climate modeling and statistical modeling. They calculated three characteristics of the urban climate: the current heat island effect, the cooling potential of more greening and less pavement, and the cooling that is currently already effective because of the vegetation and the absence of pavement that is already part of the current city layout. If you subtract this latter value from the current heat island effect, you get an impression of what the heat island effect could have been in a worst case without vegetation and much more pavement. Their results are an average over a 10-year time period, from 2008 to 2017, and for the summer season only (261).

According to this study, the average urban heat island effect for all 100 cities is 1.43°C. The values for the individual cities range between 0.57 and 2.54°C. These values include the current cooling of an average of 0.45°C (range individual cities: between 0.03 and 1.82°C). There is still a large potential for extra cooling from more greening and less pavement, ranging between 0 and 1°C, with an average value for all 100 cities of 0.49°C, slightly higher than the estimated current cooling. There are no clear trends in the size of the urban heat island effect and the potential for additional cooling across Europe (261).

The values for the urban heat island effect presented in this study are relatively low when compared to the results of previous studies. According to the authors this is due to the fact that they looked at 10-year average air temperature values and left out the 10% highest values in their results (261).

Seasons 

The cooling potential of trees varies from one season to another (234). In general, in European cities, the cooling is highest during summer. In several Southern European and Turkish cities, the cooling during spring is higher than or very close to the cooling during summer. The cooling during autumn is lowest in all cities and regions in comparison to the cooling in summer and spring.

Hot extremes

The cooling effect of trees is especially high in cities in Central and Eastern Europe, and on the British Isles, where a cooling effect of 8 to 12 °C has been measured on hot days. Remarkably, the cooling effect on hot days was ‘only’ up to 4 °C in the Mediterranean, Turkey and the Iberian Peninsula. The relatively low cooling effect of urban trees on a hot day in cities in Southern Europe is due to the limited availability of soil moisture and thus the amount of evapotranspiration, the combination of evaporation and transpiration of water from the leaves (235).

As a result, in Southern Europe, trees have a stronger cooling effect during average summertime conditions than during hot extremes. In contrast, in Scandinavia, the British Isles, North-Western Europe and parts of the Alps, the cooling provided during hot extremes is at times even higher than average summertime cooling.

Future projections

Projections indicate that European summers will become dryer. As a result, the cooling potential of urban trees on a hot day will probably decrease in many cities throughout Europe (236). This reduction will be strongest in Southern and Southeastern Europe. Irrigation could help to maintain the cooling provided by trees in these regions but may be limited by future water scarcity.

Different trees, different effects

Since evapotranspiration is one of the main mechanisms of urban cooling by trees, the type of trees strongly influences urban cooling. Trees with a large root depth, for instance, allow higher exploitation of soil moisture and higher evapotranspiration when the upper soil layers are dry (237). Different types of trees may also exhibit differences in roughness to the airflow. These differences result in differences in the efficiency of heat convection, and hence cooling at street level. The roughness of trees also interacts in complex ways with the surrounding urban structure: trees may reduce the roughness and be less effective in cooling when they are smaller than surrounding buildings and increase the roughness when they are higher (238).

Limitations of this study

One major limitation of this study is the fact that all data refer to approximately 10:15 a.m., being the Landsat satellite acquisition time over Europe. This early moment may not adequately capture the differences in temperature on a hot day. In addition, this data is mainly derived during cloud-free conditions. Land surface temperature differences between vegetated land and urban fabric could not be quantified during cloudy conditions. The results will also be influenced by differences in urban morphology from one city to another, such as differences in the ratio of building height and street width. This ratio can strongly influence the cooling effect of trees.