Warmer Climate Accelerating Planet's Fresh Water Cycle

ARLINGTON, Virginia, December 19, 2003 (ENS) - Tropical ocean waters of today are much saltier than they were 40 years ago, while oceans closer to the Earth's poles have become fresher, a team of international scientists reports in today's issue of the journal "Nature."

These large, rapid oceanic changes mean that recent climate changes, including global warming, may be altering the fundamental planetary system that regulates evaporation and precipitation and cycles fresh water around the globe, the scientists warn.

The study was conducted by Ruth Curry of the Woods Hole Oceanographic Institution (WHOI); Bob Dickson of the Centre for Environment, Fisheries, and Aquaculture Science in Lowestoft, UK; and Igor Yashayaev of the Bedford Institute of Oceanography in Dartmouth, Nova Scotia, Canada.

An acceleration of Earth's global water cycle may affect global precipitation patterns that govern the distribution, severity and frequency of droughts, floods and storms, the three scientists say.

Water cycle acceleration would also intensify global warming by quickly adding more water vapor, a potent, heat trapping greenhouse gas, to the atmosphere.

And, say Curry, Dickson and Yashayaev, it could continue to freshen North Atlantic Ocean waters to a point that could disrupt ocean circulation and trigger further climate changes.

"This study is important because it provides direct evidence that the global water cycle is intensifying," said Elise Ralph, associate director of the National Science Foundation's (NSF) physical oceanography program, which funded the research.


Tropical ocean waters, like this stretch on the Oahu coast, are becoming saltier (ENS File Photo)
"This is consistent with global warming hypotheses that suggest ocean evaporation will increase as Earth's temperature does," Ralph said. These issues are particularly important as pressure on freshwater resources has become critical in many areas around the world."

The oceans contain 96 percent of the Earth's water, experience 86 percent of planetary evaporation, and receive 78 percent of planetary precipitation, and thus represent a key element of the global water cycle for study, the scientists said.

The three scientists analyzed salinity measurements collected over recent decades along a key region in the Atlantic Ocean, from the tip of Greenland to the tip of South America.

Their analysis showed the properties of Atlantic water masses have been changing radically over the five decades for which reliable and systematic records of ocean measurements are available.

The trend toward freshening of water at the poles and increased salinity in the tropical oceans has accelerated since 1990 when 10 of the warmest years since recordkeeping began in 1861 have been documented.

These results indicate that fresh water has been lost from the low latitudes and added at high latitudes, at a pace exceeding the ocean circulation's ability to compensate, say the scientists.

Taken together with other recent studies revealing parallel salinity changes in the Mediterranean, Pacific, and Indian Oceans, a growing body of evidence suggests that the global hydrologic cycle has revved up in recent decades.

An accelerated evaporation-precipitation cycle would continue to freshen northern North Atlantic waters, the three scientists say.


North Atlantic ocean off the coast of England. (Photo courtesy FreeFoto)
The North Atlantic is one of the few places on Earth where surface waters become dense enough to sink to the abyss. The plunge of this great mass of cold, salty water helps drive a global ocean circulation system, often called the Ocean Conveyor.

This Conveyor helps draw warm Gulf Stream waters northward in the Atlantic, pumping heat into the northern regions that moderates wintertime air temperatures, especially in Europe.

Melting glaciers and Arctic sea ice, another consequence of global warming, are other sources of additional fresh water to the North Atlantic.

An accelerated water cycle also appears to be increasing rain and snow in higher latitudes, contributing to the freshening of North Atlantic waters and increasing the possibility of slowing the Conveyor. If the North Atlantic becomes too fresh, its waters would stop sinking and the Conveyor could slow down, the scientists warn.

Analyses of ice cores, deep sea sediment cores, and other geologic evidence have demonstrated the Conveyor has abruptly slowed down or halted many times in Earth's history. That has caused the North Atlantic region to cool and brought long term drought conditions to other areas of the Northern Hemisphere within a few years.

A cooling of the North Atlantic region would slow the melting process, curtail the influx of fresh water to the North Atlantic. The Conveyor would again begin to circulate ocean waters.

But global warming and an accelerated water cycle would continue to bring fresh water to high latitudes, possibly enough to maintain a cap on the Conveyor even if the Arctic melting ceased.

Monitoring Earth's hydrological cycle is critical, the scientists said, because of its potential near term impacts on Earth's climate.

Support for this research also came from the Framework V Programme of the European Community, the U.S. National Oceanic and Atmospheric Administration's Consortium in the Ocean's Role in Climate, and the Ocean and Climate Change Institute at the Woods Hole Oceanographic Institution.