The verdict is in. Warming near the poles is caused by human activity, according to new research.
"The polar regions exhibit the largest climatic variability on Earth," said Andrew Monaghan of the National Center for Atmospheric Research, Boulder, Colo., who wrote a commentary accompanying the study, published in Nature Geoscience. "Detecting and attributing climate change has been more difficult than elsewhere, and the issue has been confounded by comparatively short and sparse temperature records in both the Arctic and Antarctic."
Antarctica was the only continent where evidence for human-caused climate change was inconclusive, according to the most recent assessment report from the Intergovernmental Panel on Climate Change, published in 2007.
The new study finally makes the link: "The main message of this paper is that we're able for the first time to directly attribute warming in both the Arctic and the Antarctic to human influences on climate," said lead author Nathan Gillett, who completed the work while at the University of East Anglia in Norwich, U.K.
Greenhouse gases, primarily carbon dioxide, are transported globally and persist in the atmosphere, so they exert a warming effect even in uninhabited polar regions.
A global team of researchers used climate data collected from the Arctic and the Antarctic over 100 years and 50 years, respectively, and compared the measurements with predictions from four different climate models.
In one test, they used only natural influences on climate, like variations in the sun's intensity and volcanic eruptions, in the simulations. In another, they added human influences, including greenhouse gas emissions and the ozone hole, which tends to have a cooling effect.
"What came out of that is that there was a clear detection in both the Arctic and Antarctic of a human influence on climate. We've shown that we detect the human fingerprint in both ... regions," said author Peter Stott of the Met Office Hadley Center in Exeter , U.K.
The strongest effect is in the Arctic, where temperatures have warmed by over 3.5 degrees Fahrenheit in the last 40 years. Summer minimum sea ice coverage in the Arctic was the second-lowest on record in 2008, behind 2007, which broke the previous record low by more than 20 percent.
The sea ice losses in the Arctic contribute to warmer land temperatures in the area, which in turn can lead to increased permafrost melting and the release of additional greenhouse gases, Monaghan said.
The Antarctic shows a smaller but still significant overall warming trend. While some parts of the continent appear to be cooling, such as over the South Pole, warming is pronounced over West Antarctica and the Antarctic Peninsula.
Warming in these coastal regions may lead to collapse into the ocean of ice sheets — including the 2002 loss of the larger-than-Rhode-Island Larsen B ice sheet — which contribute to sea level rise. Measuring trends in the Antarctic is more difficult because there are fewer than 20 data stations, and they have been operating for fewer years than in the Arctic.
"In my mind, the most important issue is the potential implication of polar ice sheet melt on sea level rise," Monaghan said. "At the same time we have this increased sense of urgency to gain a better understanding of the various factors that influence polar climate variability and especially whether humans are playing a role. So that's why this study is so important. It formally demonstrates the human contribution for the first time."