The team, led by Tajdarul Syed of the Indian School of Mines, Dhanbad, India, and formerly with the University of California, Irvine, used NASA and other world-scale satellite observations to track total water volume flowing from the continents into the ocean each month. They found 18 percent more water fed into the world's ocean from rivers and melting polar ice sheets in 2006 than in 1994. The average annual rise was 1.5 percent.
"That might not sound like much – 1.5 percent a year – but after a few decades, it's huge," said Jay Famiglietti, UC Irvine Earth system science professor and principal investigator on the study, published this week in the Proceedings of the National Academy of Sciences. He noted that while freshwater is essential to humans and ecosystems, the rain is falling in all the wrong places, for all the wrong reasons.
"In general, more water is good," Famiglietti said. "But here's the problem: Not everybody is getting more rainfall, and those who are may not need it. What we're seeing is exactly what the Intergovernmental Panel on Climate Change predicted – that precipitation is increasing in the tropics and the Arctic Circle with heavier, more punishing storms. Meanwhile, hundreds of millions of people live in semiarid regions, and those are drying up."
Famiglietti said the evaporation and precipitation cycle taught in grade school is accelerating dangerously because of higher temperatures fueled by greenhouse gases. Hotter weather above the ocean causes freshwater to evaporate faster, which leads to thicker clouds unleashing more powerful storms over land. The resulting rainfall then travels via rivers to the sea in ever-larger amounts, and the cycle begins again.
"Many scientists and models have suggested that if the water cycle is intensifying because of climate change, then we should be seeing increasing river flow. Unfortunately, there is no global discharge measurement network, so we have not been able to tell," wrote Famiglietti and Syed.
Satellite records of sea-level rise, precipitation and evaporation were used to create a unique 13-year record – the longest and first of its kind. The trends the researchers found were all the same: increased evaporation from the ocean that led to increased precipitation on land and more flow back into the ocean.
Among the NASA data used in the ongoing study are measurements from the NASA/European Topex/Poseidon and Jason-1 satellite altimeters and the NASA/German Aerospace Center Gravity Recovery and Climate Experiment (GRACE) satellites. The study is funded by NASA and Earth system science fellowships.
"As we turn up the thermostat on planet Earth, it's not just higher temperatures we have to think about," said co-author Josh Willis of NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Long-term changes in rainfall will be a part of climate change too. What we've shown here is that we now have the tools to see global climate change as it occurs – not just the warming, but changes in the hydrological cycle as well."
The researchers cautioned that although they had analyzed more than a decade of data, it was still a relatively short time frame. Natural ups and downs that appear in climate data make detecting long-term trends challenging. Further study is needed, they said, and is underway.
Other authors of the study include Don Chambers of the University of South Florida, Tampa, Fla.; and Kyle Hilburn of Remote Sensing Systems, Santa Rosa, Calif.