Cooler Climate May Hit N. America, Europe Next Decade

Brian Handwerk
for National Geographic News
April 30, 2008
Shifting ocean currents could throw some cold water on global warming over the next decade, a new study suggests.

Europe and North America may soon experience chillier temperatures, thanks to natural North Atlantic variations that could temporarily mask the effects of human-driven, or anthropogenic, climate change.

"We believe that ocean currents and systems could, in the short term, change global warming patterns and even mean temperatures," said Noel Keenlyside of the Leibniz Institute of Marine Sciences in Kiel, Germany.

Keenlyside explained that since record keeping began in the 19th century, the North Atlantic climate has changed in natural cycles that last a decade or more.

These shifts are likely associated, at least in part, with natural variations in ocean currents. (Related: "Ocean 'Thermostat' May Be Secret Weapon Against Warming [February 8, 2008].)

A new forecasting model, based on past and present sea surface temperatures, suggests the imminent onset of a cool-down cycle for currents in both the North Atlantic and tropical Pacific.

Keenlyside and colleagues, whose study appears in this week's issue of the journal Nature, hope to further quantify this effect and incorporate it into future climate predictions on the decade scale.

"I think it's just naive to think that there won't continue to be multi-decadal fluctuations in the [ocean] climate," he said.

Temporary Change

Ocean current systems move heat around the globe, but they do not remain static. Their fluctuations are largely driven by seawater density, which is in turn governed by factors like temperature and salinity.

The massive North Atlantic current called the thermohaline circulation brings warm water north, where it releases its heat, and then transports cooled water south again. (Related: "Global Warming May Alter Atlantic Currents, Study Says" [June 25, 2007].)

When this current is naturally strong, climate in the North Atlantic warms. When it is weak, as Keenlyside predicts for the coming decade, temperatures cool.

"We're pretty convinced that this has an influence on the surface temperatures," said Richard Wood of the Met Office Hadley Centre in Exeter in the United Kingdom.

"We don't expect climate change to be just a smooth warming of the climate, each year after the next. It's superimposed on natural variations of the climate," added Wood, who penned a Nature commentary on the study.

"Especially if you are interested in things that are going to happen on the scale of a decade or two, those natural variations in the climate could be just as important on those time scales as the global warming signal," he said.

Wood also stressed that our understanding of ocean fluctuations—and thus our ability to include them in climate models—is currently in its infancy.

New ocean monitoring technologies—such as a network of automated sensor buoys—are only beginning to deliver the data that will help scientists to understand the ocean climate's intricacies and to better forecast climate on the decadal level.

Meanwhile, Keenlyside cautioned the effect he describes is a baseline natural fluctuation that will not deter larger global warming trends.

"We want to make very clear that we don't want to say that [anthropogenic] global warming is not here," he said.

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