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MOSAIC: A dangerous hobby
Large-scale salinity changes 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 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. 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 vapour — 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 vapour to the atmosphere, which transports it toward both poles. At higher latitudes, that water vapour 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 per cent of the Earth’s water, experience 86 per cent of planetary evaporation, and receive 78 per cent 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. Analysis of 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 — 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. 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. — Samina Iqbal | ||
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