Study Suggests Atlantic Responds to Global Warming

Ruth Curry finds northern waters are fresher; tropics are saltier


Tropical seas have become saltier over the past 40 years, while waters closer to Earth’s poles have grown fresher, according to a recent study led by Ruth Curry, a research specialist in the WHOI Physical Oceanography Department. These trends suggest that recent climate changes—including global warming—may be altering the fundamental systems that regulate the movement of fresh water around the globe.

“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,” Curry and colleagues wrote in the December 17 issue of Nature. Curry collaborated on the study with Bob Dickson of the Centre for Environment, Fisheries, and Aquaculture Science (United Kingdom) and Igor Yashayaev of the Bedford Institute of Oceanography (Canada).

The research team analyzed measurements of salt content (salinity) collected along a transect from the tip of Greenland to the tip of South America. They found that surface waters in the tropical and subtropical Atlantic have become markedly more saline. Simultaneously, much of the water column in the high latitudes became fresher.

The oceans and atmosphere continually exchange fresh water. Evaporation over warm tropical and subtropical oceans transfers water vapor to the atmosphere, which then 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.

But Earth’s warming in recent decades may be intensifying evaporation over oceans in the low latitudes, raising salinity concentrations there and transporting more water vapor toward Earth’s poles. The scientists estimated that net evaporation rates over the tropical Atlantic have increased by 5 to 10 percent over the last four decades. This trend appears to have accelerated since 1990; ten of the warmest years on record (since 1861) have occurred in that period.

An acceleration of Earth’s global water cycle could affect global precipitation patterns, altering the distribution, severity, and frequency of droughts, floods, and storms. It also could exacerbate global warming by rapidly adding more water vapor—itself a potent, heat-trapping greenhouse gas—to the atmosphere.

This warming of the planet and the movement of more water vapor to high latitudes has contributed to the melting of glaciers and Arctic sea ice, pouring additional fresh water into the North Atlantic. Increasing precipitation in higher latitudes already seems to be contributing to the freshening of North Atlantic waters.

Among other possible climate impacts, an accelerated evaporation-precipitation cycle could continue to freshen waters in the North Atlantic to a point that could disrupt ocean circulation. 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 waters 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, where it significantly moderates wintertime air temperatures, especially in Europe.

If the North Atlantic becomes too fresh, its waters will stop sinking, and the conveyor could slow down. Analyses of ice cores, deep-sea sediment cores, and other geologic evidence clearly demonstrate that the Conveyor has abruptly slowed 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.

“Our 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,” Curry, Dickson and Yashayaev wrote. 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 changed in recent decades.