by Catherine Brahic, Vienna,
From issue 2653 of New Scientist magazine, 24 April 2008, page 12

THERE may be more to global warming than we thought. On top of the effect of human-made carbon emissions, natural changes in the warm ocean currents travelling to the icy north may be helping to heat up the entire northern hemisphere.

Temperatures in the Arctic are rising far faster than in other parts of the world. Climate models produced by the Intergovernmental Panel on Climate Change (IPCC), which are tuned to reproduce the human-made greenhouse effect, predict the region should have warmed by 1.4 °C between 1960 and 2000. In fact, the Arctic's average air temperature rose by 2.2 °C.

Vladimir Semenov of the Obukhov Institute of Atmospheric Physics in Moscow, Russia, says that ocean currents carrying warm water from lower latitudes into polar regions could have played a part in this increase. He analysed air temperature data from the north Atlantic, which revealed a cyclic pattern of highs and lows over the past century. He argues the length of such cycles must be explained by ocean currents, which also fluctuate over a timescale of decades.

To find out if these currents might affect climate beyond the Arctic, Semenov plugged the cyclic temperature data into one of the IPCC's leading climate models, along with his assumption that the pattern was caused by ocean currents. Sure enough, the model confirmed that when the flow of warm water from the tropics was particularly strong, it not only accelerated Arctic melting, but also triggered an additional effect on the atmosphere: when the icy lid capping the Arctic ocean was smaller and thinner, more heat escaped from the sea into the frigid air above it.

The resulting increase in Arctic air temperatures would reduce the temperature gradient between the pole and warmer latitudes, says Semenov, which would in turn reduce the atmospheric circulation that carries heat away from warmer latitudes and into the north. As a result, the entire hemisphere ends up warmer.

Between 1970 and 2000, the average temperature of the northern hemisphere increased by 0.5 °C. Semenov calculates that the natural process he outlined may have been responsible for around 0.2 °C of that increase. He attributes the remaining 0.3 °C to human-made global warming.

"It's an interesting idea -- and the first time I've come across it," says Peter Wadhams, head of the Polar Ocean Physics Group at the University of Cambridge, who chaired the session in which Semenov presented his findings at the European Geosciences Union annual meeting in Vienna, Austria, last week. Although the research is speculative, it is a plausible process, Wadhams says.

Semenov says his findings show that the latest cycle has peaked, suggesting the effect may become less pronounced in the next few decades. That may mean that IPCC climate models have overestimated the speed at which the planet will warm in future.

Link to article: http://environment.newscientist.com/article/mg19826533.900-arctic-currents-may-be-warming-the-world.html