NASA Satellites Reveal Abrupt Drop in Global Freshwater Levels

NASA Satellites Reveal Abrupt Drop in Global Freshwater Levels
by James R. Riordon
Washington DC (SPX) Nov 16, 2024

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 and has remained low ever since. Reporting in Surveys in Geophysics, the researchers suggested the shift could indicate Earth's continents have entered a persistently drier phase.

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."

During times of drought, along with the modern expansion of irrigated agriculture, farms and cities must rely more heavily on groundwater, which can lead to a cycle of declining underground water supplies: freshwater supplies become depleted, rain and snow fail to replenish them, and more groundwater is pumped. The reduction in available water puts a strain on farmers and communities, potentially leading to famine, conflicts, poverty, and an 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 decrease in freshwater using observations from the Gravity Recovery and Climate Experiment (GRACE) satellites, operated by the German Aerospace Center, German Research Centre for Geosciences, and NASA. GRACE satellites measure fluctuations in Earth's gravity on monthly scales that reveal changes in the mass of water on and under the ground. The original GRACE satellites flew from March 2002 to October 2017. The successor GRACE - Follow On (GRACE - FO) satellites 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 into 2016, culminating in one of the most significant El Nino events since 1950, led to shifts in atmospheric jet streams that altered weather and rainfall patterns around the world. However, even after El Nino subsided, global freshwater failed to rebound. In fact, Rodell and team report that 13 of the world's 30 most intense droughts observed by GRACE occurred since January 2015. Rodell and colleagues suspect that global warming might be contributing to the enduring freshwater depletion.

Global warming leads the atmosphere to hold more water vapor, which results in more extreme precipitation, said NASA Goddard meteorologist Michael Bosilovich. While total annual rain and snowfall levels may not change dramatically, long periods between intense precipitation events allow the soil to dry and become more compact. That decreases the amount of water the ground can absorb when it does rain.

"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 Nino, 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.

While there are reasons to suspect that the abrupt drop in freshwater is largely due to global warming, it can be difficult to definitively link the two, 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 come with errors."

It remains to be seen whether global freshwater will rebound to pre-2015 values, hold steady, or resume its decline. Considering that the nine warmest years in the modern temperature record coincided with the abrupt freshwater decline, Rodell said, "We don't think this is a coincidence, and it could be a harbinger of what's to come."