A new global, satellite-based study of Earth’s freshwater distribution found that wet areas are getting wetter and dry areas drier. The data suggest that this pattern is due to a variety of human and natural factors, including people’s use and management of water, human-caused climate change, and natural climate cycles.
The study, recently published in the journal Nature, also incorporated satellite precipitation data from the ESSIC-led Global Precipitation Climatology Project; Landsat imagery from NASA and the U.S. Geological Survey; irrigation maps; and published reports of human activities related to agriculture, mining and reservoir operations. Using data taken from 2002 to 2016, the study suggests that changes in two-thirds of the 34 regions, from California to China may be linked to climate change or human water use, such as large scale pumping of groundwater for farming.
Freshwater is present in lakes, rivers, soil, snow, groundwater and glacial ice. Its loss in the ice sheets at the poles—attributed to climate change—has implications for sea level rise. On land, it is one of Earth's most essential resources. While some regions' water supplies are relatively stable, others normally experience increases or decreases. But the current study revealed a new and distressing pattern.
"What we are witnessing is major hydrologic change," said co-author James Famiglietti of NASA's Jet Propulsion Laboratory. "We see, for the first time, a very distinctive pattern of the wet land areas of the world getting wetter—those are the high latitudes and the tropics—and the dry areas in between getting dryer. Embedded within the dry areas we see multiple hotspots resulting from groundwater depletion."
Famiglietti noted that while water loss in some regions is clearly driven by warming climate, such as the melting ice sheets and alpine glaciers, it will take more time before other patterns can be unequivocally attributed to climate change.
"The pattern of wet-getting-wetter, dry-getting-drier is predicted by the Intergovernmental Panel on Climate Change models for the end of the 21st century, but we’ll need a much longer dataset to be able to definitively say that climate change is responsible for the emergence of a similar pattern in the GRACE data," Famiglietti said. "However, the current trajectory is certainly cause for concern."
The twin GRACE satellites launched in 2002 measured changes in Earth's gravity field caused by movements of large volumes of water or other forms of mass on the planet below. Using this method, variations in terrestrial water storage were tracked until the GRACE mission ended in October 2017. However, the GRACE satellite observations alone couldn’t tell the research team what was causing the apparent trends.
"We examined information on precipitation, agriculture and groundwater pumping to find a possible explanation for the trends estimated from GRACE," said Beaudoing, who has joint appointments at UMD and NASA Goddard.
The team found that across numerous regions one of the big causes of groundwater depletion was agriculture, which can be complicated by natural cycles. California, which in 2017 produced more than half of the total vegetable production in the U.S., was a prime example. Decreases in freshwater caused by the state’s severe drought from 2007 to 2015 were compounded by groundwater withdrawals to support the farms in the state’s Central Valley and elsewhere. A majority of California's freshwater comes in the form of rainfall and snow that collects in the Sierra Nevadas and then is managed through a series of reservoirs as it melts. When natural cycles led to dry years with diminished snowpack and surface waters, farmers and other Californians relied more heavily on groundwater.
Natural cycles of rainy and dry years also can cause large decreases and increases in regional amounts of freshwater. For example, in Africa the western Zambezi basin and Okavango Delta, is a vital watering hole for wildlife in northern Botswana. And during the 14-year study period water storage in this region increased at an average rate of 29 gigatons (126 million Olympic swimming pools) per year from 2002 to 2016. This wet period during the GRACE mission followed a dry period of at least two decades. Lead author of the paper Matt Rodell, who is chief of the Hydrological Sciences Laboratory at NASA’s Goddard Space Flight Center, said he believes this is a case of natural variability that occurs over decades in this region of Africa.
"This is the first time we’ve assessed how freshwater availability is changing, everywhere on Earth, using satellite observations," said Rodell. "A key goal was to distinguish shifts in terrestrial water storage caused by natural variability—wet periods and dry periods associated with El Niño and La Niña, for example—from trends related to climate change or human impacts, like pumping groundwater out of an aquifer faster than it is replenished."
The twin GRACE satellites, launched in 2002 as a joint mission with the German Aerospace Center (DLR), precisely measured the distance between the two satellites to detect changes in Earth's gravity field caused by movements of mass on the planet below. Using this method, GRACE tracked variations in terrestrial water storage on monthly to yearly timescales until its science mission ended in October 2017. A successor mission, called GRACE Follow-On is undergoing final preparations for launch.