Human activity and, in particular, the production of greenhouse gases can be linked definitively to warming in parts of the Arctic and Antarctic, according to a new study that makes the controversial connection for the first time.
Scientists used models to determine the causes of climate change in the two polar regions, finding that only when they included human influences could they explain the rise in temperatures in both areas.
Nathan Gillett, who co-wrote the study that appeared yesterday in the online journal Nature Geoscience, said they compared four different models using artificial versus naturally occurring factors affecting temperatures.
Their stark discovery was that only with the influence of artificial greenhouse gases, such as carbon dioxide, could they simulate the warming trend in parts of the remote regions.
"It makes clear that the warming that we're seeing definitely can be linked to human influence in the Arctic and the Antarctic," said Dr. Gillett of the Canadian Centre for Climate Modelling and Analysis at Environment Canada in Victoria.
"We could only explain the warming when we included greenhouse gases and human climate influences."
The study tracked data from weather stations in both areas over the past several decades and simulated the various influences on the environment in their models. Natural factors might include volcanoes or solar brightness, which can alter temperatures.
But the researchers said the models showed that the two-degree temperature rise in the peninsula of Antarctica over the past three decades could be reproduced only when CO{-2} was factored into the model.
"That's why this study is so important: because it formally demonstrates the human contribution for the first time," said Andrew Monaghan of the National Centre for Atmospheric Research in Boulder, Colo.
But some in the scientific community disagree, adding to an already splintered array of opinion on the causes of climate change and whether the Antarctic is actually warming.
John Christy, an atmospheric scientist at the University of Alabama, has done studies on climate models and says they are extremely limited tools in trying to mimic what happens in nature.
He said they are unable to reproduce all of the naturally occurring influences and, as a result, give a false picture of what might be causing changes in the environment. Clouds, for example, can dampen warming in the real world, but he said models have been known to show them amplifying warming.
"They overstate the confidence of what they have in that result because we have too many examples of models that fail," Dr. Christy said from Huntsville, Ala. "We have shown that climate models just don't have the variability that nature provides to us."
Dr. Christy disputed whether the bulk of continental Antarctica is warming, saying that it is, in fact, cooling. The report looks largely at the Antarctic peninsula - which makes up 2 per cent of the continent - and the eastern and western coastal regions, where scientists have found warming.
The report focuses on temperature changes going back to 1900 and up to the present, but doesn't include earlier periods when areas in the Arctic were actually warmer than they are today and were not affected by artificial greenhouse gases, Dr. Christy said. "Just 1,000 years ago the Arctic was much warmer than it is today, so it's interesting that they would use the term conclusively," he said. "Natural variability can account for warming since the Arctic has been warmer before."
