The amount of water flowing into the oceans has slowly but steadily increased in recent years, signifying a possible speeding up of the water cycle due to climate change.
These results came out of a research paper published on Oct. 4 in Proceedings of the National Academy of Sciences. It marks the first time satellites were used to quantify global river flows.
Between 1994 and 2006, the scientists measured an 18% increase in freshwater discharge into the oceans. The source of that water included river runoff and melting ice caps. It averaged out to an additional 540 cubic kilometers of water per year.
Don Chambers, an associate professor of physical oceanography at the University of South Florida and a co-author of the study, calculated that the volume was the equivalent of the Great Lakes losing six feet of water every year.
“The biggest implication [is what this] means for climate,” said Jay Famiglietti, a co-author and professor of earth system science at the University of California at Irvine. The results fit the expectation that climate change will accelerate the global water cycle, the process where water is moved around the world through evaporation, precipitation and runoff.
As the planet warms, heat will increase evaporation from the oceans. The water condenses into clouds, and much will fall as precipitation over land. With more rain comes floods and extra river runoff flowing into the ocean—but not everywhere.
“It’s not as simple as saying if global total rainfall increases, it will be wetter everywhere. It’s likely that wet regions will get even more rain while semi-arid regions become drier,” said Matthew Rodell, a hydrologist at NASA Goddard Space Flight Center, who was not involved with the study.
Famiglietti said increased evaporation and precipitation could lead to more extreme weather, such as prolonged droughts and more intense floods.
The researchers’ data spanned 13 years. “Admittedly, it isn’t that long,” said Famiglietti. “I want to be clear that this is an emerging trend…there are many ups and downs in the data, [but] if these trends persist, then they will be very much an indication that the water cycle is intensifying.”
Famiglietti and his colleagues were limited by the scarcity of on-the-ground data. Most countries lack river flow measurement gauges, either for want of resources or political reasons.
“A lot of nations are unwilling to share their data. They don’t want other nations to know what they’re doing with their water, especially if that water crosses international boundaries,” said Rodell.
Instead, the scientists used satellite and ocean temperature data to measure changes in the ocean’s mass over time. Combined with satellite and ground-based datasets on precipitation and evaporation, they could then calculate the global rate of water flow into the oceans.
“[The research] does a pretty nice job of bringing together satellite-based observation datasets [and existing measurements] to look at discharge to the oceans in a new way,” said Rodell.
Since high-quality satellite data only became available in 1994, “it’s really important to wait for a longer data record,” Famiglietti explained, describing how the study might be improved with time.
Famiglietti also wants to break down the data to find out how much of the water flow originated from river runoff versus land-based glaciers or melting ice caps.
Another component is the water that comes from human-induced changes. Under normal circumstances, groundwater is essentially taken out of the water cycle because it’s trapped underground for sometimes thousands of years. But when humans mine the water for above-ground consumption, they add extra water to the global water cycle—water that will eventually discharge into rivers as runoff.
Famiglietti sees the study as “an early warning” for one potential consequence of climate change. Further research could pinpoint the exact source of increased water flows and establish longer global trends.
“It’s very difficult given the short record we’ve had…to diagnose a trend,” Rodell agreed. “And I think [the authors are] pretty clear about that…you can’t say for sure that it’s going to continue. Nevertheless, it does seem to fit with other indicators of climate change.”
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