Web Extra Friday, January 16, 2004

Saltier sea

Oceanographers have documented the growing saltiness of the Atlantic in the tropics, in opposition to freshening of the polar oceans. The changes in the sea-surface salinity indicate changes in the hydrologic cycle, the researchers say, which may be attributable to human-induced climate changes.

For more than a century, scientists have logged ocean surface salinity, which is considered a fingerprint for the hydrologic cycle. In the case of increasing salinity, more evaporation can translate to higher atmospheric temperatures (the warmer it is, the more water the atmosphere can hold) or increased winds. Less salinity indicates dumping of more freshwater through rain, possibly from more storms than usual, or by melting polar ice.

Ruth Curry and colleagues at Woods Hole Oceanographic Institution published an analysis of sea-surface salinity and temperature data from the 1950s to the 1990s, in the Dec. 18/25 Nature. The measurements the team used were taken along a north-south transect skirting the east coast of the Americas by a variety of ships traversing the region. Over the past 50 years, Curry calculates, the Atlantic has experienced a gradual 5 percent increase in evaporation rate. But a closer look at the data shows that the changes in salinity increased more quickly over the past 15 years, and that the ocean warmed more quickly in the past two decades than it did over the entire 50-year period.

The key, Curry says, is that "water vapor is an important greenhouse gas." Increased evaporation rates creates more vapor in the atmosphere, "which in turn should accelerate global warming," she says, and there is a possibility that the changes to the hydrologic cycle could become a runaway process.

That feedback loop could have serious consequences, though perhaps not as extreme as shutting down the so-called "conveyor belt" in the North Atlantic, originally suggested by Wally Broecker of Lamont Doherty Earth Observatory, that drives ocean mixing. Nonetheless, the research "is very much in line with what Wally says," says Peter Rhines of the University of Washington, Seattle. "Regional changes in the oceans are occurring and seem to be a response to this." Salinity and temperature changes paralleling the North Atlantic shifts have also been recorded in the Indian Ocean and elsewhere, the authors point out.

Rhines calls the work "an exciting piece of the puzzle to do with global change." The research shows what should be expected "in the simplest case of global warming," not natural cyclicity, he says.

Harry Bryden of the University of Southampton (United Kingdom) says that Curry's database is excellent, using carefully vetted measurements from a variety of sources. "I think it's clear that over the last 40 years that the tropical and subtropical waters have gotten saltier and polar ones have gotten fresher," he says, representing "a substantial change in the hydrologic cycle." However, he says he is less inclined to conclude that the dramatic shifts are immediately attributable to anthropogenic global warming. "I'm a little bit more concerned that these are oscillations" over longer time scales, he says, "rather than trends."

Naomi Lubick

"What if the conveyor were to shut down?" Wally Broecker, GSA Today, January 1999, Abstract
Model of the conveyor at United Nations Environmental Programme Web site

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