Droughts: European scale
Droughts are the second most important natural disaster after floods. Between 1950 and 2014 droughts affected 2.2 billion people (10). For Europe, the damage caused by droughts between 1950 and 2014 has been estimated to be EUR 621 Mio on average per event (10). To date, 11% of the European population and 17% of the area of the EU have been affected by water scarcity (11).
From 2006-2010, on average 15% of the EU territory and 17% of the EU population have been affected by meteorological droughts each year (15). In the 1990s and 2000s the drought hotspots were the Mediterranean area and the Carpathian Region (16).
Four types of droughts
Four different types of drought have been defined (9):
- Meteorological droughts relate to a deficiency of precipitation.
- Hydrological droughts reduce streamflow and low water levels of reservoirs and lakes. Hydrological droughts mainly affect water resources management, power plant cooling, irrigation, and inland navigation. Groundwater droughts are a special case of hydrological droughts (van Lanen and Peters 2000, Kumar et al 2016). They occur when water deficiencies reach deep subsurface storages resulting in exceptionally low groundwater levels, groundwater recharge and baseflow. They reduce the supply of fresh water, where groundwater is the major source for drinking water supply.
- Agricultural droughts are characterized by low soil water availability for plants, potentially leading to reduced biomass and yield or crop failure.
- Socioeconomic droughts can emerge from all of the aforementioned drought types. It is characterized by a shortfall of water supply (water scarcity) leading to monetary losses.
Global trends over past decades
Man-made climate change has caused shifts in temperature and rainfall globally. It is to be expected that these shift have affected droughts on a global scale as well. Yet it is very complicated to detect human influence on global drought trends. This is due to the large natural variability compared with the climate change trends, and due to the fact that our records of drought observations are relatively short. The challenge to find a human influence on droughts in the last decades has been addressed on a global scale over the period 1900-2017 (33). Drought severity over this period (expressed as the Palmer drought severity index) has been reconstructed from tree ring data, and these data were compared with drought observations and climate model reconstructions. Two important conclusions can be drawn from these analyses.
- A human influence, but not straightforward: First, the analyses do reveal a human influence on droughts since 1900, but not a continuous increasing trend. Three distinct periods were identified. In the first half of the century (1900-1949), a signal of greenhouse-gas-forced change is robustly detectable. Multiple observational datasets and reconstructions using data from tree rings confirm that human activities were probably affecting the worldwide risk of droughts as early as the beginning of the twentieth century. Around mid-century, however, from 1950 to 1975, tree ring data and climate reconstructions do not show any greenhouse gas influence on droughts. On the contrary: a trend of drought decrease is revealed. According to the authors of this study, it looks like air pollution (aerosols) and volcanic eruptions blocked sunlight and influenced rainfall such that the drought trend was reversed. The influence of greenhouse gases on drought increase re-emerged in recent decades (1981 to present) when air pollution declined, and this signal is likely to grow stronger in the next several decades (33).
- Particularly clear at the global scale: Second, clear signals of drought increase, and decrease around mid-century, are only revealed by looking at the global scale. Simultaneous drying in Australia, Mexico and the Mediterranean, for example, is a stronger signal than drying in any one of those regions in isolation. On a regional scale, for instance focusing on Europe or North America only, natural variability is too large compared to the signal we're looking for. Still, changes at the regional scale do reflect the global signal: positive trends in the early half of the twentieth century, negative trends at mid-twentieth century, and positive trends again since the 1980’s (33).
European trends over past decades
Globally, dry areas are growing, by a factor of more than two over the past 40 years (1). However, currently available records suggest that, while increasing summer dryness has been observed in Central and Southern Europe since the 1950s, no consistent trends can be seen over the rest of Europe (2,3,4). According to the IPCC there is medium confidence that anthropogenic influence has contributed to some changes in drought patterns since the 1950s and in particular trends towards more intense and longer droughts in Southern Europe (3).
The frequency of meteorological droughts in Europe has increased since 1950 in parts of southern Europe and central Europe (Austria and Hungary), but droughts have become less frequent in northern Europe and parts of eastern Europe. Trends in drought severity also show significant increases in the Mediterranean region (in particular the Iberian Peninsula, France, Italy and Albania) and parts of central and south-eastern Europe, and decreases in northern and parts of eastern Europe (17).
Most stream gauges in Europe show a decrease in summer low flows over the second half of the 20th century (15). However, current data availability is insufficient for attributing this trend to global climate change (18).
Vulnerabilities - Global assessment
Projected drought risk in 1.5°C and 2°C warmer climates
The goal of the Paris agreement is to hold global warming well below 2°C and to pursue efforts to limit to 1.5°C above preindustrial temperature. Global changes in drought risk under the 1.5°C and 2°C warming targets have been assessed from simulations based on a model designed specifically to test the climate impacts associated with 1.5°C and 2°C warming scenarios (14). The study focused on key drought-prone regions in North and South America, Europe, Africa, Asia, and Australia. In this study, the future period of 1.5°C and 2°C warming refers to the second half of this century. Changes were compared with the present-day reference period 1967-2016:
- Higher risk in Mediterranean, central Europe, the Amazon, and southern Africa. According to the results of this study drought risk increases significantly for both warming targets in the Mediterranean, central Europe, the Amazon, and southern Africa. Moreover, for these four regions the additional 0.5°C of warming from 1.5°C to 2°C leads to significantly drier mean conditions and higher risk of consecutive drought years. Southern Australia has a comparable increase in drought risk between 1.5°C and 2°C, while Southeast Asia sees no significant change in drought risk under any future scenario. The results of this study indicate that when warming is limited to 1.5°C or 2°C, projected increases in precipitation and temperature/evaporative demand may balance each other over the U.S. Southwest and Central Plains, leading to little change in drought risk relative to present day (14).
- Drought events that last for several years. The risk of a drought “event” that lasts for four consecutive years, roughly similar to the recent 2012-2015 drought in California, substantially increases for the Mediterranean under the 1.5°C and 2°C warming targets. For central Europe, this risk of consecutive years of drought is lower but also increases for a warming of 2°C relative to 1.5°C. The Amazon experiences the strongest increase of the risk of such a drought “event”, while the response for Southern Africa is similar to the Mediterranean. Southern Australia also experiences an increase in risk of consecutive drought years under both 1.5°C and 2°C scenarios relative to present day. No major changes are simulated for the U.S. Southwest and Central Plains, and Southeast Asia (14).
- A high-end scenario of climate change. What if global warming exceeds the 2°C warming target? Simulations with a high-end (RCP 8.5) scenario of climate change show that the risk of consecutive drought years increases substantially in all regions, including the U.S. Southwest and Central Plains but except Southeast Asia. In Southeast Asia, evaporation strongly increases but this increase is balanced by a more-or-less comparable increase of precipitation. The latter is also shown in simulations for the U.S. Southwest and Central Plains under the 1.5°C and 2°C warming targets, but his balance does not hold for stronger warming, causing the U.S. Southwest and Central Plains to dry under a high-end (RCP 8.5) scenario of climate change (14).
Prolonged droughts globally
A similar study was carried out on a global scale, focused on changes in drought conditions over 12-month periods for 1.5 and 2°C global warming levels, and for 3°C global warming (20). The latter is closer to what is expected by the end of the 21st century if current emission trends are retained. This was done for a number of climate projections that, in combination, address the uncertainty in the climate projections of a large number of climate models.
This study shows that as a result of 1.5 to 2°C global warming, long-term droughts will happen 5 to 10 times more frequent for most of Africa, aside the zone around the equator, but also the Caribbean, Central America, Central and West Asia, Oceania, and north-west China. They will also occur far more frequent in southern Europe, large parts of western and eastern Europe, southern and central United States, Chile, and central and eastern regions of Brazil. Current 1-in-100-year droughts would occur every two to five years. At these warming levels, the magnitude of droughts is likely to double in 30% of the global landmass (20).
Approximately two thirds of the world population will experience progressively longer and more frequent droughts. The study indicates that global mean drought length progressively increases with warming. Global warming has already reached almost 1°C. A further increase up to 1.5°C will increase the length of these droughts by a month. The increase of global mean drought length rapidly accelerates when global warming further increases up to 3°C of global warming: up to 4.2 months for each °C warming. This acceleration in drought duration could lead to more frequent mega-droughts (a decade-scale drought) like we haven’t observed often in the past. Examples of these mega-droughts are the dust bowl in the United States, the persistent drought in the Sahel, or the recent “Millennium Drought” in southeast Australia (20).
There are, however, also regions in the world that will not suffer from increasing drought. In parts of the globe the supply of water outweighs continental drying and the water balance will not progressively decline. These regions include northern Europe, the south-eastern part of South America, Central Africa, Canada, the Russian Federation, and China (except the north-west) (20).
Drought risk will increase this century, despite future emission reductions. In general, there is a greater propensity to severe droughts in the Mediterranean ecosystems by the end of the century, which may be especially pertinent when specific impacts on human activities are taken into consideration, such as livestock farming, agricultural yields and household subsistence (22).
Droughts in the root zone under 1.5 and 2 °C global warming
Global drought trends under 2 °C compared with 1.5 °C global warming have been estimated based on multi-model simulations of climate change. A distinction was made between two types of droughts. Meteorological droughts describe precipitation deficiencies resulting from the combined effect of changes in precipitation and evapotranspiration. Droughts in the soil, often called ‘agricultural droughts’, refer to the impact of meteorological droughts on the soil water that is available for plants (32). In this study a dry period is called a ‘drought event’ when dry conditions last for at least three continuous months. Whether conditions are considered ‘dry’ was derived from the quantification of four drought indicators. These indicators describe precipitation deficiency, net precipitation deficiency (including evapotranspiration), soil moisture at the surface, and soil moisture at the root zone, respectively.
Globally, the frequency and duration may increase much faster for meteorological droughts than for surface soil moisture droughts and root zone soil moisture droughts, according to this study. Meteorological droughts are projected to occur about 36% more frequent under 1.5 °C global warming, and 62% more frequent under 2 °C warming. For surface soil moisture droughts the estimated frequency increases are 5% (1.5 °C warming) and 14% (2 °C warming), respectively (32).
On average, meteorological droughts are projected to last about 15% and 20% longer under 1.5 °C and 2 °C global warming, respectively. For surface soil moisture droughts the estimated duration increases are 5% (1.5 °C warming) and 2% (2 °C warming), respectively. The spread around these average numbers are large, however. Projected drought increase in the root zone is generally less than at the soil surface (32).
Surface soil moisture drought risk will probably increase in America, South America, Europe, southern Africa, southern China and Australia. For Europe, the projected frequency increase of surface soil moisture droughts is 32% (± 5%) and 51% (± 6%) for 1.5 °C and 2 °C global warming, respectively. The increase of root zone soil moisture drought risk will be less, however. Root zone soil moisture drought frequency in Europe is projected to increase by 5% and 15% for 1.5 °C and 2 °C global warming, respectively. The areas affected by droughts will increase less than 10%; this increase will be largest in Africa, Oceania and South America (32).
For food production and terrestrial biodiversity soil moisture in the root zone is particularly relevant. This study shows that changes in meteorological and soil moisture droughts do not have to be the same. In fact, root soil moisture droughts may not increase as much as meteorological droughts, but still seem to increase in some areas. Limiting globally averaged temperature rise below 1.5 °C will substantially reduce drought risk in duration and frequency relative to the 2 °C level (32).
What is driving future water scarcity: climate change or population growth?
The frequency of droughts will increase in the next several decades, according to several studies (4). This will probably increase humans’ exposure to water scarcity as well. In addition, population growth or decline may increase or decrease exposure to water scarcity, and these effects may vary from one country to another. What is driving future water scarcity: climate change or population growth?
For the world as a whole and for individual countries, this has been assessed by studying the impacts of (a) population growth, (b) increased incidence of droughts due to climate change, and (c) the combined influence of the two through their interaction (12). Both a low and high emissions scenario of climate change was assumed (the so-called RCP 4.5 and RCP 8.5 scenarios), and a large number of models were used to adequately cover the spread of uncertainties in the results of different models. For population growth, a medium fertility population growth scenario was assumed (United Nations, 2013), producing a world population of approximately 10.5 billion by the end of the century. Drought was characterized on a monthly basis by calculation the difference between precipitation and potential evapotranspiration for each month in the previous two years and accumulating these differences.
According to this assessment, population growth alone increases the average monthly number of people exposed to extreme drought from 85.5 now to 121.0 million (+35.5 million) at the end of this century (2081–2100), worldwide. Climate change alone may increase this number from 85.5 to 315.5 million (+230.0 million) (under the high end scenario of climate change RCP 8.5). Combining population growth and climate change, population exposure reaches 472.3 million (+386.8 million relative to the present). These results show that population growth is responsible for only 9.2 % of the increase in numbers exposed to extreme drought while climate change alone accounts for 59.5 % of the increase. The interaction between climate change and population growth explains the remaining 31.3 % of change in population exposure—reflecting the fact that the growing population tends to be in regions with more frequent droughts (12).
The results above refer to one scenario of population growth, one scenario of climate change, and the end of this century. These results seem to be robust, however. Similar results are obtained under a higher population growth scenario, under the low-end scenario of climate change (RCP 4.5), and for the middle of the century (2041–2060): climate change plays the primary role (12).
Similar to the global situation, climate change is the main source of increasing drought exposure in most individual countries. There are exceptions to the rule. In Russia, for instance, substantial population decline will dominate over more droughts under climate change. In Spain, on the other hand, the population decline is being dominated by the impact of climate change (12).
The exact numbers should not be taken literally given the very large uncertainties in this assessment. Still, the authors conclude that climate change mitigation should be the main policy response in efforts to alleviate numbers exposed to future extreme droughts. Plausible future decreases in population growth will not make a major dent in the very large increase of numbers likely to be exposed to extreme drought through the rest of this century in most parts of the world (12).
The impact of urbanization, excluding climate change
The impact of urbanization on the extent of urban areas exposed to flood and drought hazards globally has been assessed, without factoring in the potential impacts from climate change (7). According to this assessment, the urban extent in dry lands will increase strongly. Across all regions and all dry-lands, most urban expansion is expected to occur in semi-arid regions of China. The most urban expansion in hyper-arid regions is expected in Northern Africa. For Western Europe the (projected) percent urban extent in dry-lands are 5% (2000) and 6% (2030), and for Eastern Europe 11% in both years (7).
The impact of climate change
The impact of human water use and climate change on future low flows and associated hydrological drought has been assessed on a global scale for 1971–2099 (based on climate projections from five general circulation models (GCMs) and four RCP emission scenarios) (6). In this assessment low flow is defined as the discharge that is equalled or exceeded 90% of the time, and human water use includes irrigation and reservoir regulation, but excludes industrial and domestic water use. No future changes in population and land use were included. The results show that climate change has a negative impact on the low flow regime (decrease of 10% or more) in South-America, Australia, Southern-Africa, Southeast Asia and the Mediterranean, for all four emission scenarios. Positive impacts on the low flow regime are found in Northwest Africa and large parts of Northern Europe, Russia and Canada (6). According to the results of this assessment human influence can account for almost 100% of the changes in future hydrological drought in areas such as Asia, Middle East and North-Africa (Mediterranean). These areas are heavily impacted by water abstraction and reservoirs are not enough to compensate for this. In these regions low flows are expected to be even lower in future and droughts will likely become more severe (6).
Globally, both precipitation and evapotranspiration (= physical evaporation plus biological transpiration) are expected to increase with warming. The combined effect has been projected (with a number of General Circulation Models) for the end of the 21st century for a high-end climate change scenario (the RCP8.5 scenario). The relative contributions from projected changes in moisture supply (precipitation) versus evaporative demand determine whether droughts will become more severe in the future and will cover wider areas (5). The projected climate changes suggest increased drying, driven primarily by increases in (potential) evapotranspiration. This will likely have significant ramifications for globally important regions of agricultural production in the Northern Hemisphere mid-latitudes, including Europe. The percentage of global land area projected to experience at least moderate drying by the end of the 21st century may increase from 12 to 30 % (5).
Vulnerabilities - Assessment Europe
Will drought events become more frequent and severe in Europe?
A picture of drought trends in Europe until the end of the 21st century has been presented, based on a combination of indicators that characterize drought trends through changes in precipitation and evapotranspiration (and thus high temperatures). The indicators have been calculated from the results of a large multi-model ensemble of high-resolution climate projections. Two future scenarios of climate change were used: a moderate (RCP4.5) and a high-end scenario (RCP 8.5) (23).
Observed trends since the 1950s: From the 1950s to the 2010s, southern Europe experienced increasing drought frequency and severity (24) with the Mediterranean region as a hotspot especially in spring and summer (25); a clear increase was also evident in the Carpathian region (26). Northern Europe, on the contrary, showed a tendency towards wetter conditions (27), whilst central and eastern Europe act as a transition zone (28).
Future trends at moderate global warming: Under the moderate scenario of global warming, the increasing drought trend is projected to continue and grow stronger until the end of the 21st century over southern and western Europe, while northern Europe is projected to revert the past trend to see more frequent and severe droughts. Central Europe, eastern Europe, and southern Scandinavia show only a moderate increase, and Iceland is characterized by a moderate decrease in drought frequency. Only over the Mediterranean area drought events will become significantly more severe (23).
For southern Europe, the projected increase of drought frequency and severity is due to more frequent extreme rainfall events combined with a general decrease of total precipitation (29) and a projected temperature rise, including a higher probability of severe heat waves (30). Over northern Europe, the projected temperature rise, resulting in an increased evaporative demand, outbalances the projections of increasing precipitation (31), leading to a projected increase of droughts over northernmost Scandinavia.
Future trends at strong global warming: Under the high-end scenario of global warming, drought frequency is projected to increase over the whole of Europe, with a few exceptions: moderate increase over Switzerland, Hungary, Poland, Belarus, Lithuania, and central Scandinavia, and mixed tendencies over Iceland. The largest increases are projected over southern Europe where droughts and extreme droughts could be up to 50% more frequent, as well as over France and the British Islands (up to +40%), but also over central Europe and Russia (up to +35%). Droughts will become much more severe over the southern third of Europe and over northernmost Scandinavia. Excluding central Iceland and southern Norway, the entire European continent will be affected by more frequent and severe droughts as the century passes (23).
Future seasonal trends: According to the projections in this study, northern Europe will experience fewer winter droughts in the future than during the last decades but more frequent droughts in spring and autumn. Western Europe will also experience fewer winter droughts, and more frequent droughts in other seasons, especially in the summer. In central and eastern Europe there will be more frequent droughts in spring and summer. Southern Europe will experience more frequent droughts in all seasons (23).
Drought hotspots: According to the authors, their findings reinforce the view of the Mediterranean area as a drought hotspot for the entire 21st century: southern Europe shows both the largest drought frequency values over the recent past and the largest increase in future periods. The Mediterranean region also stands out as the area with Europe’s largest increase of total drought severity as the 21st century progresses, both under the moderate and high-end scenario. Other possible hotspots for drought hazard and risk in future decades are northern and northeastern Scandinavia, southern England, and western Europe. Like the Mediterranean region, these regions are also projected to face the largest increase of drought frequency and severity under both scenarios of climate change (23).
Other studies on droughts in Europe
For the future, summer dryness is expected to increase in Central and Southern Europe during the 21st century, leading to enhanced risk of drought, longer dry spells and stronger soil moisture deficits (1,4,21). Based on the balance of evidence coming from multi-model experiments, the following changes in summer dryness are expected in the course of the 21st century (1):
- Mediterranean, Southern and Central Europe: summer dryness is expected to increase during the 21st century – longer dry spells, stronger soil moisture deficits;
- Northern Europe: no major changes in dryness are expected until the end of this century.
Climate model projections indicate that the frequency and duration of extreme meteorological droughts will significantly increase in the future (19). These projections showed the largest increases in frequency for extreme droughts in parts of the Iberian Peninsula, southern Italy and the eastern Mediterranean, especially at the end of the century with respect to the baseline period 1971-2000. The changes are most pronounced for the RCP8.5 high emissions scenario and slightly less extreme for the moderate (RCP4.5) scenario.
Changes in river flow extremes at a +2°C global warming are currently of central interest as this is the global target defined by policymakers to lower international greenhouse gases emissions. The impacts of a +2°C global warming on hydrological droughts has been assessed for Europe for a combination of different models (global and regional climate models, hydrological models); the changes in floods and droughts were compared with the reference period 1971 – 2000. The results show that drought magnitude and duration may increase in Spain, France, Italy, Greece, the Balkans, south of the UK and Ireland. For the rest of Europe, the projections generally show a decrease of drought magnitude and duration (8).
Drought risk management should be based on 3 key pillars (13):
- Monitoring, early warning and information delivery. Nations need to establish an integrated drought monitoring and early warning system that compiles information on the status of all segments of the hydrologic cycle: precipitation deficiencies, temperature anomalies, ground and surface water supplies, soil moisture, snowpack, vegetation status, long-term climate forecasts. This information must be delivered to decision makers at all levels in a timely fashion so risks can be mitigated and reduced.
- Assessment of risk, vulnerability and impacts. Vulnerability refers to the degree of resilience to drought in a society or its ability to withstand the effects of a drought episode. Focus must be on different sectors, population groups and regions.
- Mitigation and response. Mitigation refers to proactive measures that are identified and implemented that increase the resilience of an individual, population group, community or nation and, thus, reduce or eliminate the negative impacts of drought.
In general, adaptive measures that may prove particularly effective include rainwater harvesting, conservation tillage, maintaining vegetation cover, planting trees in steeply-sloping fields, mini-terracing for soil and moisture conservation, improved pasture management, water re-use, desalination, and more efficient soil and irrigation-water management. Restoring and protecting freshwater habitats, and managing natural floodplains, are additional adaptive measures that are not usually part of conventional management practice (4).
The increased drought vulnerability as a direct result of human water abstraction can be compensated by reservoirs. Reservoirs retain the water for some time and thus lead to a smoothed hydrograph, with lower peak flows and higher low flows. This is especially the case for large parts of the United States and Europe, where the number of reservoirs is large (6).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Europe.
- Norwegian Meteorological Institute (2013)
- Kiktev et al. (2003); Alexander et al. (2006); Van der Schrier et al. (2006); Sheffield and Wood (2008); Dai (2011a), all in: Norwegian Meteorological Institute (2013)
- IPCC (2012), in: Norwegian Meteorological Institute (2013)
- IPCC (2014)
- Cook et al. (2014)
- Wanders and Wada (2015)
- Güneralp et al. (2015)
- Roudier et al. (2016)
- WMO (2006); Mishra and Singh (2010), both in: Zink et al. (2016)
- Guha-Sapir et al 2015, in: Zink et al. (2016)
- European Commission (2007, 2010), in: Zink et al. (2016)
- Smirnov et al. (2016)
- Wilhite (2016)
- Lehner et al. (2017)
- European Environment Agency (2017)
- Sepulcre-Canto et al. (2012); Spinoni et al. (2016), both in: European Environment Agency (2017)
- Gudmundsson and Seneviratne (2015); Spinoni et al. (2015, 2016), all in: European Environment Agency (2017)
- Stahl et al. (2010, 2012), in: European Environment Agency (2017)
- Stagge et al. (2015), in: European Environment Agency (2017)
- Naumann et al. (2018)
- Ruosteenoja et al. (2018)
- Carrão et al. (2018)
- Spinoni et al. (2018)
- Briffa et al. (2009); Vicente-Serrano et al. (2014); Gudmundsson and Seneviratne (2015); Spinoni et al. (2015a, 2015b), all in: Spinoni et al. (2018)
- Hoerling et al. (2012); Spinoni et al. (2017a), both in: Spinoni et al. (2018)
- Spinoni et al. (2013, 2014), both in: Spinoni et al. (2018)
- Bordi et al. (2009); Seneviratne (2012); Kingston et al. (2015), all in: Spinoni et al. (2018)
- Spinoni et al. (2015a, 2015b, 2017a), all in: Spinoni et al. (2018)
- Nikulin et al. (2011); Heinrich and Gobiet (2012); Kjellström et al. (2013); IPCC (2014a, 2014b); Madsen et al. (2014), all in: Spinoni et al. (2018)
- Beniston et al. (2007); Jacob et al. (2014); Russo et al. (2014); IPCC (2014a), all in: Spinoni et al. (2018)
- Heinrich and Gobiet (2012); Forzieri et al. (2014); Sein et al. (2014); IPCC (2014a), all in: Spinoni et al. (2018)
- Xu et al. (2019)
- Marvel et al. (2019)