An international team of scientists using observations from NASA-German satellites found evidence that Earth’s total amount of freshwater dropped abruptly starting in May 2014. The rapid decline continued until March 2016. By 2023, the freshwater levels have not yet recovered.
The researchers used observations from the Gravity Recovery and Climate Experiment (GRACE) satellites. These were operated by the German Aerospace Center, German Research Centre for Geosciences, and NASA. The original GRACE satellites flew from March 2002 to October 2017. The successor GRACE–Follow On (GRACE–FO) satellites launched in May 2018.
Follow On (GRACE-FO) missions, global terrestrial water storage (TWS), excluding ice sheets and glaciers, declined rapidly between May 2014 and March 2016. By 2023, it had not yet recovered. with the upper end of its range remaining 1 cm equivalent height of water below the upper end of the earlier range.
1,200 cubic kilometres of water were lost
From 2015 through 2023, satellite measurements showed that the average amount of freshwater stored on land — that includes liquid surface water like lakes and rivers, plus water in aquifers underground — was 290 cubic miles (1,200 cubic km) lower than the average levels from 2002 through 2014, said Matthew Rodell, one of the study authors and a hydrologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
“That’s two and a half times the volume of Lake Erie lost.”
A series of droughts, warmer ocean temperatures and global warming seem to be the reason
The global freshwater decline outlined in the study began with a severe drought in northern and central Brazil, followed by major droughts across Australasia, the Americas, Europe, and Africa. Warmer Pacific ocean temperatures from late 2014 to 2016, culminating in a major El Niño, disrupted weather and rainfall globally. Despite El Niño’s end, freshwater levels did not recover, with 13 of the 30 most intense droughts recorded since 2015. Researchers suggest global warming may be exacerbating this trend.
Global warming increases atmospheric water vapor, causing more extreme precipitation, explained NASA meteorologist Michael Bosilovich. However, extended dry periods between heavy rainfall allow soil to dry and compact, reducing its ability to absorb water when it rains.
“The problem when you have extreme precipitation,” Bosilovich said, “is the water ends up running off,” instead of soaking in and replenishing groundwater stores. Globally, freshwater levels have stayed consistently low since the 2014-2016 El Niño, while more water remains trapped in the atmosphere as water vapor. “Warming temperatures increase both the evaporation of water from the surface to the atmosphere, and the water-holding capacity of the atmosphere, increasing the frequency and intensity of drought conditions,” he noted.
Some uncertainty still remains
Linking the abrupt drop in freshwater to global warming is challenging due to uncertainties in climate predictions, noted Susanna Werth, a hydrologist at Virginia Techa, who was not affiliated with the study. “Measurements and models always come with errors,” she said.
Whether global freshwater levels will rebound, stabilize, or continue declining remains unclear. However, with the nine warmest years coinciding with this decline, Rodell suggested, “We don’t think this is a coincidence and it could foreshadow what’s ahead.”
Further information about the GRACE-FO mission can be found here.
