As our planet warms and the climate changes, scientists have predicted that droughts and floods will occur more frequently and be more severe, but it has been challenging to detect these changes on a regional and continental scale. According to a recent NASA-led study, major droughts and pluvials—periods of excessive precipitation and water storage on land—have been happening more frequently after all.
Two NASA scientists looked at 20 years of data from the NASA/German GRACE and GRACE-FO satellites to identify extreme wet and dry events in their study, which was published on March 13, 2023, in the journal Nature Water. More than 20% of the economic losses brought on by extreme weather events in the U.S. each year are attributable to floods and droughts. The economic effects are similar around the world, but the human cost is typically greatest in underdeveloped countries and slum areas.
The researchers also discovered a strong correlation between global warming and the global intensity of these extreme wet and dry events, which combines extent, duration, and severity.
The number of extreme wet and dry events increased from three per year in the previous 13 years to four per year from 2015 to 2021, seven of the nine warmest years in the modern record. According to the authors, this makes sense because, during dry events, warmer air causes more moisture to evaporate from the Earth's surface. Warm air can also hold more moisture, which can fuel extreme snowfall and rainfall events.
From 2002 to 2021, 1,056 extreme wet and dry events were observed by the GRACE and GRACE-Follow-On satellites, which were used in the study by Bailing Li of Goddard and Rodell. The satellites measure the Earth's gravity field precisely to look for unusual patterns in the amount of water that is stored in soils, aquifers, lakes, rivers, snow cover, and ice.
Rodell compared it to checking the water level in a bathtub. "You don't need to know how much water is in the tub to see how much it rises and falls." GRACE and GRACE-FO offer a fresh map of global water storage anomalies each month, giving users a thorough understanding of the gravity of hydrological events and how they change over time.
Rodell and Li used an "intensity" metric in their research that takes into account the severity, duration, and geographic scope of extreme dry events and droughts. They found the global total intensity of extreme events increased from 2002 to 2021, mirroring Earth's rising temperatures over the same period.
A pluvial that started in central Africa in 2019 and is still going on was by far the most intense event found in the study. More than one meter of Lake Victoria's level has risen as a result of it. Brazil experienced its worst drought in the previous 20 years between 2015 and 2016, which left reservoirs empty and forced water rationing in some Brazilian cities.
The Brazilian drought occurred in 2016, the warmest year on record, reflecting the effects of global warming, according to Bailing Li, a University of Maryland hydrologist at Goddard. "Both events were associated with climate variability," she added. The recent droughts in the southwest United States and southern Europe were among the most severe ones ever, in part because of anthropogenic global warming.
"Global warming has had broad and profound effects on terrestrial water storage, including reduced annual snowfall in high elevations and groundwater depletion by humans during times of scarcity of surface waters," Li threw in. "GRACE data give us a distinctive view of how hydrological extremes have changed globally because they reflect these changes."