NASA scientists have studied 17 years of gravity observations of Earth to understand how the global water cycle is changing.
The rate at which plants and the land surface release moisture into the air has increased globally between 2003 and 2019. These processes are collectively known as evapotranspiration, and a new study by NASA has calculated its increase by using observations from gravity satellites.
By gauging the mass change of water between the oceans and the continents, the researchers were able to determine that evapotranspiration’s rate of increase is up to two times higher than previous estimates. This is very important because evapotranspiration represents a critical branch of the global water cycle – a cycle that creates the conditions for life on land.
The rate of evapotranspiration affects the global water cycle because as moisture from the oceans circulates through the atmosphere, a portion falls as precipitation over the continents. Some of the water goes into rivers as runoff, and some seeps into soils. The remaining water evaporates from the land and transpires from plants into the air.
Finding that evapotranspiration is increasing at a faster rate than previously thought has implications for understanding how climate change could impact Earth in the future. A warming world will accelerate evapotranspiration, speeding up the drying of land and vegetation. Weather patterns can also be affected because increased evaporation from land can create droughts in some regions.
To get a global estimate of how evapotranspiration is changing, researchers found a new way to use data collected by the pair of Gravity Recovery and Climate Experiment (GRACE) satellites that operated from 2002 to 2017 and the successor pair, GRACE Follow-On that launched in 2018.
Because water has mass and therefore contributes to the Earth’s gravity signal, these spacecraft are very sensitive to the movement of water around the world, from tracking changes in ice sheets to water stored on land to variations in ocean mass.
“When the gravity signal decreases, it means the land is losing water. Some of the loss is through rivers flowing back into the oceans, but the rest of it goes up into the atmosphere as evapotranspiration,” said JT Reager, a JPL scientist.