Freshwater is flowing into Earth’s oceans in greater amounts every year, a team of researchers has found, thanks to more frequent and extreme storms linked to global warming. All told, 18 percent more water fed into the world’s oceans from rivers and melting polar ice sheets in 2006 than in 1994, with an average annual rise of 1.5 percent.
“That might not sound like much – 1.5 percent a year – but after a few decades,
it’s huge,” says Jay Famiglietti, University of California-Irvine Earth system
science professor and principal investigator on the study. He notes 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 says. “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.”
In essence, he explains, the evaporation and precipitation cycle taught in
grade school is accelerating dangerously because of greenhouse gas-fueled
higher temperatures, triggering monsoons and hurricanes. Hotter weather above
the oceans causes freshwater to evaporate faster, which leads to thicker clouds
unleashing more powerful storms over land. The rainfall then travels via rivers
to the sea in ever-larger amounts, and the cycle begins
The pioneering study, which is ongoing, employs NASA and other world-scale
satellite observations rather than computer models to track total water volume
each month flowing from the continents into the oceans.
“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,” says Famiglietti and lead author Tajdarul Syed of
the Indian School of Mines, formerly of UCI.
“This paper uses satellite records of sea level rise, precipitation and
evaporation to put together a unique 13-year record – the longest and first of
its kind. The trends were all the same – increased evaporation from the ocean
that led to increased precipitation on land and more flow back into the
The researchers caution that although they have analyzed more than a decade of
data, that still is 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 say, and is under way.
Earth's Water Cycle Accelerating
November 3, 2010