For years, scientists have been watching the Arctic Ocean with a mounting sense of unease. Sea ice on the very northern tip of our planet is melting -- and it has been doing so much more quickly than expected.
By September 2007, in fact, the area in the Arctic covered by sea ice was only half as big as Europe, a 40 percent reduction from the mid-1990s, as calculated by the National Snow and Ice Data Center in the US. Glaciers on Greenland are likewise disappearing at an alarming rate. And the Arctic Ocean itself has been warming up since 1995, a trend that has only accelerated since the beginning of this decade. In the summer of 2007, water temperatures in the Bering Sea between Alaska and eastern Siberia were 5 degrees Celsius (9 degrees Fahrenheit) higher than average -- warmer than ever before.
So much for the data. The question has long been: why is the Arctic heating up so fast? Climate models project what ought to be a much slower rise in temperature for the Arctic region. An increase in greenhouse gas emissions and the resulting warming of the Earth's atmosphere is not enough to explain the phenomenon.
Point of No Return
A new study completed by a team of US, Norwegian and German researchers may now provide some clues. Published in the scientific journal Geophysical Research Letters in November, the study posits that a dramatic change in atmospheric circulation patterns has taken place since the beginning of the decade, with centers of high pressure in winter shifting toward the north-east. The new pattern of sudden climate change is characterized by "poleward atmospheric and oceanic heat transport," the authors write in the study, a transport which drives temperature increases in the Arctic. The discovery was made using specialized filters that allow one to follow changes to high pressure centers over time.
Behind the complex language and impenetrable calculations upon which the study is based, however, is a frightening possibility: climate change in the Arctic could already have reached the point of no return. Climate researchers have long been warning of such "tipping points," and that crossing them could mean irreversible developments for eco-systems and humanity. In the case of the Arctic, that could mean a complete disappearance of ice in the region during the summer months. Such an eventuality would then further magnify global warming, due to the fact that bright white ice reflects sunlight back into the atmosphere whereas dark colored land and ocean absorbs heat.
"In the case of Arctic Sea ice, we have already reached the point of no return," says the prominent American climate researcher James Hansen, director of the Goddard Institute for Space Studies at NASA.
Warm Air for the Arctic
The waters around the North Pole are heavily influenced by the currents coursing through the Atlantic and Pacific Oceans. Those currents are driven by conflicting pressure systems in each ocean: in the Pacific, the low pressure zone located near the Aleutian Islands extending west from Alaska is doing battle with a subtropical high pressure zone further south; in the Atlantic the currents are determined by the Azores High and the Icelandic Low.
Winter in the Arctic has long been determined by what researchers refer to as a "tri-polar" pattern. The interaction among the Icelandic Low, the Azores High and the subtropical high in the Pacific led to primarily east-west winds, a pattern which effectively blocked warmer air from moving northward into the Arctic region.
But since the beginning of the decade, the patterns have changed. Now, a "dipolar" (bipolar) pattern has developed in which a high pressure system over Canada and a low pressure system over Siberia have the say. The result has been that Artic winds now blow north-south, meaning that warmer air from the south has no problem making its way into the Arctic region. "It's like a short-circuit," says Rüdiger Gerdes, a scientist at the Alfred Webener Institute for Polar and Marine Research and one of the five authors of the study.
The influx of warm air from the south was especially intense during the winter of 2005-2006, the study says. During that period, 90 terawatts of energy flowed into the Artic Ocean from the North Pacific -- an amount that far exceeds the needs of the entire industrial world. Gerdes has no doubt that the ice will "quickly disappear if the new pressure patterns stay the way they are." He says that the Arctic Ocean would still freeze during the winter, but the ice pack would be too small to survive the warmer summer months.
Dramatic Disappearance of Arctic Ice
James Overland from the Pacific Marine Environmental Laboratory in Seattle agrees. In the scientific journal Tellus the oceanographer, together with colleagues, also points to the new north-south flow patterns in the Arctic. "If the current flows stay the way they are, then we will see the disappearance of Arctic sea ice 40 years earlier than we would as a result of greenhouse-gas emissions alone," Overland told SPIEGEL ONLINE. Even if the Arctic circulation were to return to normal and would switch to the "dipolar" pattern just once in a decade, the situation would look grim, he said. "Each time we would see a loss of so much ice that it would be impossible to return to the initial state."
Overland says that the dramatic disappearance of Arctic ice observed in 2007 was no exception. The summer of 2008 was just as bad, he says. The progression is clear: sooner or later the ice cap will become so small that it will not be able to survive the warm summer months.
Gerdes and his co-authors fear that the changes in the Arctic could mean that a "new era of global-warming-forced climate change" has begun. The volume of greenhouse gas emissions like CO2 and methane into the Earth's atmosphere could have resulted in a permanent change in the global climate system.
The series of warm winters experienced in the Arctic this decade, it should be noted, is not the first time in recent history the region has been visited by mild weather. In the 1930s, there was a similar "dipolar" pattern that pushed warm air into the Arctic, as researchers now know. Back then, though, it was air from the North Atlantic and not from the North Pacific. Furthermore, says Gerdes, the warm air did not penetrate beyond 75 degrees north latitude, which roughly marks the previous limits of the ice cap. Today, the heat spreads through the entire Arctic.
It could be that the new patterns of air circulation in the Arctic are caused by natural climate variations. But given the dramatic ice melt currently being observed, such an explanation is not enough to satisfy researchers. The American scientist Overland, for his part, has no doubts: the dramatic change in pressure systems in the Northern Hemisphere combined with Arctic warming is, he says, "a clear signal of warming."