A 14-year NASA mission has confirmed that a massive redistribution of freshwater is occurring across Earth, with middle-latitude belts drying and the tropics and higher latitudes gaining water supplies.
The results, which are probably a combination of the effects of climate change, vast human withdrawals of groundwater and simple natural changes, could have profound consequences if they continue, pointing to a situation in which some highly populous regions could struggle to find enough water in the future.
"To me, the fact that we can see this very strong fingerprint of human activities on the global water redistribution, should be a cause for alarm," said Jay Famiglietti, a researcher at NASA's Jet Propulsion Laboratory and one of the authors of a study published in Nature on Wednesday.
The results emerge from the 2002-2016 GRACE mission, which is short for Gravity Recovery and Climate Experiment, supplemented with additional data sources.
The GRACE mission, which recently ended but will soon be replaced by a "Follow-On" endeavor, consisted of twin satellites in orbit that detected the tug of Earth's gravity below them — and monitored mass changes based on slight differences in measurements by the two satellites.
Among the massive features on Earth, water and ice are the ones that change most regularly. Thus, the GRACE data has been used to detect the vast losses of ice in Greenland, Antarctica and Alaska, as well as changes in ocean currents and the scale of the California drought.
The new research, led by NASA's Matthew Rodell, pulls together these and other findings to identify 34 global regions that gained or lost more than 32 billion tons of water between 2002 and 2016.
As the study notes, 32 billion tons is about the amount of water contained in Lake Mead, which is in Nevada and Arizona. So all 34 areas saw very large changes.
The resulting map of the findings shows an overall pattern, in which ice sheets and glaciers lose by far the most mass at the poles, but at the same time, middle latitudes show multiple areas of growing dryness even as higher latitudes and the tropical belt tend to see increases in water.
The study emphasizes that the 34 separate changes that it detects do not all have the same cause — not even close.
There's strong suspicion that the melting of glaciers and ice sheets is tied to climate change. On land, it's possible that some droughts and rainfall increases might also be, though the study is cautious about that, noting that natural variability can also be a major factor.
Still, the idea of mid-latitude drying and higher- and lower-latitude wetting is a common feature of climate change models.
"We only have 15 years of data from GRACE, but it sure as heck matches that pattern, it matches it now," Famiglietti said. "That's cause for concern."
Further data from a new launch of the GRACE "Follow-On" mission will contribute to a longer data record that may help better identify trends, Famiglietti said.
And there are other human-induced changes, relating not to climate change but, rather, to direct withdrawals of water from the landscape.
In northern India, the northern China plain and the Caspian and Aral seas in Central Asia, among other regions, human withdrawals for agriculture have subtracted enormous amounts of water from Earth. The changes in the Aral Sea region, previously documented by NASA, have been particularly intense.
There are also major cases of humans increasing water storage in the landscape, particularly in China, where massive dam construction has created enormous reservoirs.
Mainly, though, what's striking about the map is the way that a combination of human-driven water withdrawals and droughts seem to be punishing the central latitudes of the Northern Hemisphere, in particular, but also the Southern Hemisphere to a significant extent.
"I think we have forgotten, society has forgotten, how much water it takes to produce food," Famiglietti said.
"We've taken its availability for granted. And you know, now we're at a point in many of these aquifers where we can't take it for granted anymore. Population is too great, groundwater levels are too low. … we're at tipping points."
Still, it's important to bear in mind that, while the GRACE data has given a new panoptic view of the changing distribution of water around the globe, the data remains coarse and the causes behind the trends in many cases remain a matter of interpretation, said Peter Gleick, an expert on climate change and water who is president emeritus the Pacific Institute.
"Without a doubt the GRACE system proved that we can see very significant changes in water storage around the planet," Gleick said. "Figuring out what drives those changes is a harder thing."
The next GRACE satellite mission will provide better data, he said.
"We're in this transition between not really having a global overview and someday having an incredibly high-resolution, sophisticated, remote-sensing overview," Gleick said. "That's where we are."
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