California Change in Evapotranspiration
Evapotranspiration is one of the major factors regulating land surface moisture. It is the process by which surface water enters the air, which includes soil and water evaporation and plant consumption. In California, research indicates there has been a recent increase in evapotranspiration linked to warmer temperatures and changing circulation patterns that are increasing dryness in the region and increasing drought risk.
Evapotranspiration and the water cycle
Climate change is able to affect evapotranspiration—the evaporation of moisture from soil, on 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.
There is scientific consensus that global warming has worsened drought in California through increased evapotranspiration. In one study, scientists analyzed 432 different combinations of precipitation, temperature, wind and radiation data gathered between 1901 and 2014 to simulate monthly changes in soil moisture across the state. They found that between 8 and 27 percent of the overall drought severity between 2012 and 2014 was due to human-caused warming. The findings suggest that within a few decades, continually increasing temperatures and resulting moisture losses will make California even drier.
Scientists are still working to understand how evapotranspiration in California responds to increased greenhouse-gas concentrations, because of the complex response of cloud cover and humidity to climate change. Clouds keep the planet cooler than it would be otherwise by reflecting sunlight back to space. However, a study published in April 2016 suggests that clouds may not offer as much protection against sunlight as once thought.
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.