Arlington, Va. -- Tropical ocean waters have become dramatically saltier over the past 40 years, while oceans closer to Earth’s poles have become fresher, scientists report in the December 18th issue of the journal Nature. These large-scale, relatively rapid oceanic changes suggest 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 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, U.K.; and Igor Yashayaev of the Bedford Institute of Oceanography in Dartmouth, Canada.
"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. "This is consistent with global warming hypotheses that suggest ocean evaporation will increase as Earth's temperature does. These issues are particularly important as pressure on freshwater resources has become critical in many areas around the world."
An acceleration of Earth's global water cycle can potentially affect global precipitation patterns that govern the distribution, severity and frequency of droughts, floods and storms. It would also exacerbate global warming by rapidly adding more water vapor—itself a potent, heat-trapping greenhouse gas—to the atmosphere. And it could continue to freshen North Atlantic Ocean waters to a point that could disrupt ocean circulation and trigger further climate changes.
The oceans and atmosphere continually exchange fresh water. Evaporation over warm, tropical and subtropical oceans transfers water vapor to the atmosphere, which transports it toward both poles. At higher latitudes, that water vapor precipitates as rain or snow and ultimately returns to the oceans, which complete the cycle by circulating fresh water back toward the equator. The process maintains a balanced distribution of water around our planet.
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. Because evaporation concentrates salt in the surface ocean, increasing evaporation rates cause detectable spikes in surface ocean salinity levels. In contrast, salinity decreases generally reflect the addition of fresh water to the ocean through precipitation and runoff from the continents.
Curry, Dickson, and Yashayaev analyzed a wealth of 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—in some cases radically—over the five decades for which reliable and systematic records of ocean measurements are available, the scientists report.
They observed that surface waters in tropical and subtropical Atlantic Ocean regions became markedly saltier. Simultaneously, much of the water column in the high latitudes of the North and South Atlantic became fresher.
This trend appears to have accelerated since 1990—when 10 of the warmest years since records began in 1861 have occurred. The scientists estimated that net evaporation rates over the tropical Atlantic have increased by five percent to ten percent over the past four decades.
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.
Among other possible climate impacts, an accelerated evaporation - precipitation cycle would continue to freshen northern North Atlantic waters. 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 significantly moderates wintertime air temperatures, especially in Europe.
If the North Atlantic becomes too fresh, its waters would stop sinking and the Conveyor could slow down. Analyses of ice cores, deep-sea sediment cores, and other geologic evidence have clearly demonstrated the Conveyor has abruptly slowed down or halted many times in Earth's history. That has caused the North Atlantic region to cool significantly and brought long-term drought conditions to other areas of the Northern Hemisphere over time spans as short as years to decades.
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 precipitation in higher latitudes, contributing to the freshening of North Atlantic waters and increasing the possibility of slowing the Conveyor.
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.
The research was also supported by the Framework V Programme of the European Community, the National Oceanic and Atmospheric Administration's Consortium on the Ocean's Role in Climate, and the Ocean and Climate Change Institute at the Woods Hole Oceanographic Institution.
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