Aridity and drought in Germany
Questions and answers on the water cycle, evaporation, and precipitation
The trend towards drier conditions and increased droughts in Germany is the result of simple physical principles and can be clearly attributed to climate change.
How does aridity develop?
How dry a region is depends on the difference between precipitation and evaporation. If precipitation exceeds the amount of water that can evaporate, a region is considered humid. If more water can evaporate than falls as precipitation, a region is considered arid. Much of this water is released as water vapour through transpiration via the stomata of plants. That is why it is noticeably cooler on a hot day in a forest than, for example, in the shade of a building.
Evaporation requires energy, and a substantial amount of it. That becomes clear when you consider that it takes about a quarter of an hour to fully evaporate one litre of water on a kitchen stove. As the surface becomes warmer, more water can evaporate. Of course, this is only possible if the soil retains enough water and that water is accessible, for example through plant roots. Energy availability and temperature therefore determine only potential evaporation, meaning the amount of water that could evaporate if sufficient water were available.
In dry regions, potential evaporation serves only as a reference value; actual evaporation is much lower because too little water is available. Potential evaporation depends primarily on the distribution of solar radiation, which in turn is influenced by cloud cover and aerosols, or, more simply, by air pollution from fine particulate matter. Solar radiation varies only moderately across Germany, whereas actual evaporation differs considerably from region to region.
Precipitation in Germany falls predominantly along the north-western coasts, in the low mountain ranges, and in the Alpine foothills. This creates a clear pattern of dry zones across the country: the western part and the mountainous regions are quite humid, whereas the eastern part and certain areas in the south are dry.
What is the difference between aridity and drought?
Aridity is a climatological characteristic determined by the long-term average of precipitation and potential evaporation. In Germany, however, precipitation fluctuates sharply from year to year. When there is less rainfall, the sun often shines for longer periods; thus, more water could potentially evaporate but less is available. As a result, a year with a relatively high amount of sunshine is drier than the climatological average. Such a pronounced deviation from the mean state is defined as a drought. Contrary to occasional claims, turbulence directly downwind of wind turbines plays no role in this process.
Droughts have varied impacts. The severity of their consequences depends, among other things, on the season. Plants cope far better with them in winter than in summer because they evaporate less water during the colder months. The impacts of a drought also depend on how land surfaces are utilised within a region.
Why is climate change exacerbating aridity and droughts?
Climate change alters both evaporation and precipitation. The atmospheric greenhouse effect is intensified by the CO2 that humanity has released since the onset of industrialisation. Like water vapor, CO2 absorbs a portion of the outgoing longwave radiation emitted by Earth’s surface toward space. These gases then re-emit the radiation in all directions. A fraction is therefore radiated back to Earth, causing additional warming.
The magnitude and importance of the greenhouse effect is illustrated by the temperature differences between day and night on Earth compared to the Moon. On Earth, daytime temperatures are rarely more than 20°C warmer than at night. On the Moon, however, temperatures reach up to 120°C on the daylight side and plunge to minus 170°C on the night side.
Because atmospheric downward radiation has increased, surface temperatures have risen compared with the pre-industrial era, allowing more water to evaporate. In addition, solar radiation has increased over the past 30 years, mainly due to reduced air pollution and thus fewer aerosols in the atmosphere. Cloud cover has also decreased, so more energy reaches the surface. Together, these factors have increased potential evaporation by roughly ten per cent over the past 30 years.
Mean precipitation in Germany varies considerably from year to year and has not yet shown a definitive trend. Because potential evaporation has increased while average precipitation has remained unchanged, Germany has become drier overall. As a result, droughts are becoming more frequent, since precipitation now fluctuates around a generally drier climatic baseline.
How is precipitation changing?
Although no change is currently observed in average precipitation volumes, the pattern of precipitation events has altered measurably: they are becoming shorter and more intense. This, too, is a direct consequence of climate change. For one, warmer air absorbs more water vapour. This is confirmed by observations of atmospheric water vapour content in Germany, which is on the rise. When this water vapour condenses in the atmosphere, it forms water droplets that create clouds. During condensation, the energy that was required for evaporation is released back into the air as heat. This warming creates buoyancy within the cloud and generates air movement that draws in even more water vapour. This effect is self-reinforcing: more water vapour brought in leads to more condensation, which causes more air movement, drawing in yet more water vapour. As a result, precipitation events become heavier and shorter. With more water vapour in a warmer atmosphere, more energy is released and more buoyancy is generated, ultimately causing more water to fall as precipitation.
What role does wind play?
Air movement plays an important role in evaporation because evaporated water must be transported into the atmosphere. Otherwise, saturation would occur, and evaporation would come to a halt. When the sun warms the Earth’s surface, it also heats the air, generating buoyancy. Water vapour is carried upward with the rising air. Buoyancy therefore also depends on the energy of incoming solar radiation. If that energy increases, more buoyancy is generated. This can be observed in summer as a heat haze over hot car parks, where warm air rises in a turbulent flow and the turbulence distorts the path of light through the air.
Sunlight thus plays a key role both in evaporation and in the transport of moisture into the atmosphere. While windy conditions can increase transport and allow slightly more evaporation, this effect is limited. Evaporation primarily requires energy, so improved transport plays only a minor, subordinate role.
Do wind turbines increase evaporation?
Wind turbines are sometimes wrongly blamed for droughts because they generate turbulence that supposedly increases evaporation. That claim is misleading for several reasons. First, the main driver of evaporation is energy input, which comes primarily from the sun. Second, wind near the ground is already weakened and made turbulent by surface friction. Although wind turbines do create additional turbulence locally, they also extract energy from the wind overall. As a result, turbulence is ultimately reduced. Wind turbines therefore have virtually no effect on evaporation.
Conclusion
The aridity of a region depends on the balance between evaporation and precipitation. The amount that can evaporate is determined by the available energy, which comes from solar radiation and from longwave radiation emitted back toward the surface by greenhouse gases such as water vapour and CO2. Potential evaporation is increasing with global warming because more energy is available and warmer air can hold more water vapour. This higher water vapour content is already being measured. By contrast, average precipitation has not yet changed, although precipitation events are becoming shorter and more intense. As a result, Germany has become drier and droughts occur more frequently; and this a clear consequence of global climate change.