Navigating Uncertain Seas

By Phil Berardelli
ScienceNOW Daily News
11 February 2008

Ocean circulation patterns might well be shifting due to climate change, but one researcher argues that those alterations remain well beyond the capabilities of scientists to detect. If he's right, researchers might have to wait years if not decades for the right answers.

Ever since scientists began worrying about the effects of global warming, they have planted sensing equipment all over the planet designed to help track changes in the air and sea. In many parts of the oceans, such equipment includes arrays of buoys that transmit data on current speed, direction, and depth, along with other measurements. Researchers feed the incoming data into computer models, which churn out predictions about future climate developments. Based on the data and the models, some scientists have concluded that major ocean currents--such as those that slowly circulate deep-ocean water to the surface and back down again--are beginning to change speed and direction, presumably in response to the warming climate. It's a critical issue because significant changes in ocean currents could impact regional weather patterns all over the globe--think El Ni%26ntilde;o.

Not so fast, says oceanographer Carl Wunsch of the Massachusetts Institute of Technology in Cambridge. He has analyzed the available historical data on ocean currents and compared them with calculations predicting the currents' natural variability--or how they change direction, speed, and temperature periodically--without climate-change-induced influences. As Wunsch reports online this week in Nature Geoscience, the data aren't nearly comprehensive enough to permit researchers to draw even a preliminary conclusion about how or if climate change is affecting ocean currents.

The problem, Wunsch explains, is that "the ocean is very noisy," meaning it is "always changing, everywhere, all the time." The behavior of every cubic kilometer of ocean is governed by a combination of Earth's rotation, solar radiation, wind, temperature, salinity, depth, and ocean-floor and shoreline topography, as well as other factors, all responding to very long time scales. But because the historical database is so relatively short, he says, it's almost impossible to find a recognizable trend "that is not just a temporary shift." His recommendation: Researchers need to collect more data and for decades longer.

Physical oceanographer Brian Arbic of the University of Texas, Austin, says the paper is important because it takes the first "quantitative look at a large potential source of error" in the database of ocean currents. Wunsch's findings demonstrate how difficult it is to "directly estimate changes in the ocean's overturning circulation," adds physical oceanographer Terrence Joyce of Woods Hole Oceanographic Institution in Massachusetts. The results are bound to be bad news for "government end users and popular media wanting quick, simple answers" on global warming's impact, he says.

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