Expect More Floods as Global Water Cycle Speeds Up

National Geographic, By Sandra L. Postel, National Geographic Freshwater Fellow, October 8, 2010

There is nearly 20 percent more freshwater flowing into the world's oceans than there was 10 years ago--a sign of climate change and a harbinger of more flooding.

This post is part of National Geographic's Freshwater Initiative.

A new indicator has joined the century-long rise in temperature to signal that the planet's climate is changing: the global water cycle is speeding up. Using satellite observations, NASA and university researchers have found that rivers and melting ice sheets delivered 18 percent more water to the oceans in 2006 than in 1994.

The findings, which appear in this week's Proceedings of the National Academy of Sciences, suggest that the volume of water running off the land toward the sea is expanding by the equivalent of roughly one Mississippi River each year.

On the face of it that might sound like a good thing--more water in rivers means more water to tap for agriculture, industry, and growing cities. But most of the increase is occurring in places where extra water isn't needed, like the wet tropics or the remote Arctic, or is being delivered through torrential storms that overwhelm human infrastructure and coping capacities. Though no single weather episode can be pinned to climate change, the massive rains that recently flooded a fifth of Pakistan is the kind of event scientists expect to see more of--and that nations should prepare for.

Image of flooding in Sukkur, Pakistan, on August 18, 2010 by NASA.

(See more pictures of the Pakistan flood.)

Why is the water cycle speeding up? As the atmosphere warms from the addition of carbon dioxide and other greenhouse gases, it can hold more moisture. As a result, more water evaporates from the oceans, leading to thicker clouds that then dump more rainfall over the land. That heavier-than-normal rain can then produce massive flooding as it runs back toward the sea, where the cycle begins all over again.

Scientists have expected global warming to speed up the water cycle in this way, but the use of satellite data allowed the trend to be observed and measured for the first time. The research team, led by Jay Famiglietti of the University of California at Irvine, used satellite records of sea level rise, precipitation, and evaporation to compile a unique 13-year record, the first of its kind.

As the scientific evidence mounts that more severe floods and droughts are on the horizon, getting on with ways of adapting to climatic change becomes just as urgent as slowing the pace of that change.

Sandra Postel directs the independent Global Water Policy Project and lectures, writes, and consults on international water issues. She is also Freshwater Fellow of the National Geographic Society, and serves as lead water expert for the Society's freshwater initiative.

Sandra is the author of several acclaimed books, including Pillar of Sand: Can the Irrigation Miracle Last? (W.W. Norton, 1999) and Last Oasis (W.W. Norton, 1997), which appears in eight languages and was the basis for a 1997 PBS documentary. She is also co-author, with Brian Richter, of Rivers for Life. Her essay "Troubled Waters" was selected for Best American Science and Nature Writing. She is a 1995 Pew Scholar in Conservation and the Environment.

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