A new study published in the journal Nature identified a possible cause for this discrepancy. It identifies a natural, cyclical flow of atmospheric energy around the Arctic Circle. A team of researchers, led by Rune Graversen of Stockholm University, conclude this energy flow may be responsible for the majority of recent Arctic warming.
The study specifically rules out global warming or albedo changes from snow and ice loss as the cause, due to the "vertical structure" of the warming ... the observed warming has been much too weak near the ground, and too high in the stratosphere and upper troposphere.
This study follows hot on the heels of research by NASA, which identified "unusual winds" for rapid Arctic ice retreat. The wind patterns, set up by atmospheric conditions from the Arctic Oscillation, began rapidly pushing ice into the Transpolar Drift Stream, a current which quickly sped the ice into warmer waters.
A second NASA team, using data from the the GRACE (Gravity Recovery and Climate Experiment) satellite, recently concluded that changes in the Arctic Oscillation were "mostly decadal in nature", rather than driven by global warming.
quote: In the Southern Hemisphere, the land-area mean temperature has slowly but surely declined in the last few years. The city of Buenos Aires in Argentina received several centimetres of snowfall in early July, and the last time it snowed in Buenos Aires was in 1918! Most of Australia experienced one of its coldest months of June this year. Several other locations in the Southern Hemisphere have experienced lower temperatures in the last few years. Further, the SSTs (sea surface temperatures) over world oceans are slowly declining since mid-1998, according to a recent world-wide analysis of ocean surface temperatures
quote: Recent Sea-Level Contributions of the Antarctic and Greenland Ice SheetsAndrew Shepherd1 and Duncan WinghamScience 16 March 2007:Vol. 315. no. 5818, pp. 1529 - 1532After a century of polar exploration, the past decade of satellite measurementshas painted an altogether new picture of how Earth's ice sheets are changing. As global temperatures have risen, so have rates of snowfall, ice melting, and glacier flow. Although the balance between these opposing processes has varied considerably on a regional scale, data show that Antarctica and Greenland are each losing mass overall. Our best estimate of their combined imbalance is about 125 gigatons per year of ice, enough to raise sea level by 0.35 millimeters per year. This is only a modest contribution to the present rate of sea-level rise of 3.0 millimeters per year. However, much of the loss from Antarctica and Greenland is the result of the flow of ice to the ocean from ice streams and glaciers, which has accelerated over the past decade. In both continents, there are suspected triggers for the accelerated ice discharge—surface and ocean warming, respectively—and, over the course of the 21st century, these processes could rapidly counteract the snowfall gains predictedby present coupled climate models.
quote: Our study confirms many changes seen in upper Arctic Ocean circulation in the 1990s were mostly decadal in nature, rather than trends caused by global warming, " said Morison.
quote: The researchers found that most of the warming is happening high above ground. At midsummer, the data shows that the air that has warmed the most is 2 kilometres above land.This, says Graversen, rules out the theory that Arctic warming is being accelerated by melting ice. Although the researchers remain unsure what is accelerating Arctic warming, they suggest it might be related to how fast energy is being transported towards the North Pole by cyclones.The team calculated the flow of energy into the Arctic Circle using meteorological data, and looked at how this flow has changed since the 1980s.They found that the amount of energy transported from the tropics into the Arctic has increased and that the increase corresponds to the rise of temperatures in the region. "We are not saying this is the only explanation," says Graversen, "this could explain maybe 25% of the amplification of warming in the Arctic."
quote: "This assumption is that if this [heat flow] has increased—which we see in the data that it has—then it has contributed to the warming in the Arctic, not only at the surface, but higher in the atmosphere," Graversen said.Increased moisture in northward-moving air also plays a role, he said, because when the water vapor condenses into clouds and snow, it releases energy, warming the air.Nobody knows how much of this change is the result of human emissions of planet-warming gases such as carbon dioxide, but it's likely that they play a role."Many models suggest an increase in energy transport when more greenhouse gases are introduced into them," he said. "Changes in the circulation in the atmosphere might have had a much larger effect than previously thought, but these changes may also have been induced by greenhouse gases."
quote: by Rovemelt on January 7, 2008 at 12:03 AMWhat part of this sentence from Graversen don't you understand?"Many models suggest an increase in energy transport when more greenhouse gases are introduced into them," he said.
quote: The recent warming of the Earth’s surface is most probably due to an increase of atmospheric greenhouse-gas concentrations. (8) Although most greenhouse gases are fairly uniformly distributed around the globe, the temperature response to greenhouse-gas forcing is thought to be larger in polar than equatorial regions.
quote: Our results do not imply that studies based on models forced by anticipated future CO2 levels are misleading when they point to the importance of the snow and ice feedbacks. It is likely that a further substantial reduction of the summer ice-cover would strengthen these feedbacks and they could become the dominant mechanism underlying a future Arctic temperature amplification. Much of the present warming, however, appears to be linked to other processes, such as atmospheric energy transports.
quote: As scientists, we understand the dangers of nuclear weapons and their devastating effects, and we are learning how human activities and technologies are affecting climate systems in ways that may forever change life on Earth,” said BAS member Stephen Hawking, the renowned cosmologist and mathematician.“As citizens of the world, we have a duty to alert the public to the unnecessary risks that we live with every day, and to the perils we foresee if governments and societies do not take action now to render nuclear weapons obsolete and to prevent further climate change.
quote: But the models we have right now are our best tools to explain what is happening and what will happen in the future.
quote: The last 25 years constitute a period of more complete and accurate observations and more realistic modeling efforts. Yet the models are seen to disagree with the observations. We suggest, therefore, that projections of future climate based on these models be viewed with much caution.