Heat relief

Even with rising carbon dioxide levels and global temperatures, some regions may cool slightly over the next decade.

During the next decade, short-term variations in climate will cool some regions of the globe despite the continued planetwide warming expected over the long-term. Temporary winners in this climate scenario include large swaths of western Europe and North America, the new analysis suggests.

Several aspects of the climate in areas surrounding the North Atlantic Ocean — including temperature, the frequency of hurricanes and the amount of rainfall — vary irregularly on cycles that can last decades. Those variations are potentially predictable, but scientists typically don’t have enough data to accurately initialize their models, says Noel S. Keenlyside, a climate modeler at the Leibniz Institute of Marine Sciences in Kiel, Germany. For example, the ocean currents that carry immense amounts of heat from the tropics to polar regions can run hundreds of meters deep, but most temperature observations, especially those made by satellites, pertain only to surface waters.

Even when climate models simulate a short period of time — say, 10 years — researchers typically simulate an extended interval before the period of interest, an interval that often can be measured in decades. This run-up period ensures that the model is stable during the period of interest, says Keenlyside.

The new model developed by Keenlyside and his colleagues differs from others because during its run-up period, the researchers constantly adjust the sea-surface temperatures predicted by the simulation to match those actually observed by oceanographers and satellites. Other models don’t perform such adjustments, Keenlyside notes. The researchers validated their new model using extensive ocean temperature and weather data gathered worldwide between 1955 and 2005.

In several regions, including large parts of western Europe and North America, results of the team’s new temperature-predicting model are consistently better than those produced by models that don’t adjust sea-surface temperatures during a run-up period. Results of the model allow the researchers to infer changes in the North Atlantic’s ocean currents.

Using the data, the researchers gazed into the climate crystal ball for the 10 years leading up to 2015. Overall, the Gulf Stream and other currents that carry heat from the tropics and the Caribbean to the North Atlantic will slow during the decade, thereby cooling parts of western Europe, the researchers report in the May 1 Nature. Also, from 2005 to 2015, parts of North America and the tropical Pacific will cool, driving global average temperature down about 0.24 degrees Celsius as compared to simulations that didn’t adjust sea-surface temperatures.

Although the team’s model predicts climate well in some regions, in others it performs rather poorly, says Richard Wood, a climate modeler at the Met Office Hadley Centre in Exeter, England. “There’s a long way to go before a climate model can produce accurate results in all regions,” he notes.

If, as the model predicts, temperatures in the tropical Pacific are cooler than normal between now and 2015, says Wood, the probability of an El Ni±o — a climate condition that affects weather patterns worldwide and is characterized by higher-than-average sea-surface temperatures in that region — decrease considerably.

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