Abrupt drop in global freshwater levels

Nov. 15 () –

The total amount of fresh water on Earth declined sharply starting in May 2014. and has remained low since then.

In a report based on satellite data published in Surveys in Geophysicsan international team of scientists suggests that the change could indicate that Earth’s continents have entered a persistently drier phase.

From 2015 to 2023, satellite measurements showed that the average amount of freshwater stored on Earth (which includes liquid surface water like lakes and rivers, plus water in underground aquifers) was 290 cubic miles (1,200 cubic kilometers) lower than levels average from 2002 to 2014, he said in a statement Matthew Rodell, one of the study’s 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.”

In times of drought, coupled with the modern expansion of irrigated agriculture, farms and cities must rely more heavily on groundwater, which can lead to a cycle of declining groundwater supplies: reserves. freshwater runs out, rain and snow fail to replenish it, and more groundwater is pumped. The reduction in available water puts pressure on farmers and communities, which can lead to famine, conflict, poverty and increased risk of disease when people turn to contaminated water sources, according to a UN report on water stress published in 2024.

The team of researchers identified this abrupt, global decline in freshwater using observations from the GRACE (Gravity Recovery and Climate Experiment) satellites, operated by the German Aerospace Center, the German Research Center for Geosciences and NASA. The GRACE satellites measure fluctuations in Earth’s gravity on monthly scales that reveal changes in the mass of water above and below the ground. The original GRACE satellites flew from March 2002 to October 2017. The successor GRACE-Follow On (GRACE-FO) satellites were launched in May 2018.

The decline in global freshwater reported in the study began with a massive drought in northern and central Brazil, and was followed shortly by a series of major droughts in Australasia, South America, North America, Europe and Africa. Warmer ocean temperatures in the tropical Pacific from late 2014 to 2016, culminating in one of the most significant El Niño events since 1950, caused changes in atmospheric jet streams that altered weather and precipitation patterns around the world.

However, even after El Niño subsided, global freshwater did not recover. In fact, Rodell and his team report that 13 of the world’s 30 most intense droughts observed by GRACE have occurred since January 2015. Rodell and his colleagues suspect that global warming could be contributing to persistent freshwater depletion.

Global warming causes the atmosphere to hold more water vapor, resulting in more extreme precipitation, said meteorologist Michael Bosilovich of NASA’s Goddard Center. While total annual rainfall and snowfall levels may not change dramatically, extended periods between heavy precipitation events allow the soil to dry out and become more compact. That decreases the amount of water the soil can absorb when it rains.

“The problem when there is extreme rainfall,” Bosilovich said, “is that the water ends up running off,” instead of being absorbed and replenishing groundwater reserves. Globally, freshwater levels have remained consistently low since the 2014-2016 El Niñowhile more water remains trapped in the atmosphere in the form of water vapor. “Increasing temperatures increase both the evaporation of water from the surface to the atmosphere and the water holding capacity of the atmosphere, which increases the frequency and intensity of drought conditions,” he pointed out.

Although there is reason to suspect that the sharp drop in freshwater is largely due to global warming, it may be difficult to definitively link the two factors, said Susanna Werth, a hydrologist and remote sensing scientist at Virginia Tech, who was not affiliated with the study. “There are uncertainties in climate predictions,” Werth said. “Measurements and models always have errors.”

It remains to be seen whether global freshwater will recover to pre-2015 values, remain stable, or resume its decline. Noting that the nine warmest years in the modern temperature record coincided with the abrupt drop in fresh water, Rodell said: “We don’t believe this is a coincidence, and it could be a harbinger of things to come.”