Evapotranspiration and the water cycle
Climate change is able to affect evapotranspiration—the evaporation of moisture from soil, from the surface of plants and trees, and from water bodies; and transpiration, the use and release of water from plants—through its influence on temperature, wind, humidity, and water availability. Evapotranspiration is the second largest component of the water cycle after precipitation, and changes to this process have far reaching consequences, because it serves to regulate land surface soil moisture, groundwater recharge and runoff.
Scientists use the difference between the amount of moisture the atmosphere demands from soil and water surfaces—a quantity known as potential evapotranspiration—and precipitation to measure dryness and drought. Changes in potential evapotranspiration are likely to occur where temperatures are the highest and can lead to soil moisture deficits and more severe droughts.
The rate of evapotranspiration increased globally between 1982 and 1997 but stopped increasing, or has decreased, since about 1998. In North America, the observed decrease in evapotranspiration occurred in water-rich rather than water-limited areas.
Scientists have identified several factors that are likely contributing to the decrease in evapotranspiration, including wind speed, decreasing solar energy at the land surface due to increasing cloud cover and concentration of small particles (aerosols), increasing humidity, and declining soil moisture.
Potential evaporation—the amount of moisture that the air demands—is anticipated to increase as the world warms, but much more research is needed to confidently identify historical trends, causes, and implications for future evapotranspiration trends. This represents a critical uncertainty in projecting the impacts of climate change on regional water cycles.