November 24, 2011
Arctic Sea Ice Loss Setting Record
The National Snow and Ice Data Center reports that Arctic sea ice continues to shrink each summer to nearly record low levels, with this year´s ice loss the second-lowest ever since records first began being taken in 1979.
Despite only a few decades worth of records, a new study, published in the British journal Nature, found that “both the duration and magnitude of the current decline in sea ice seem to be unprecedented for the past 1,450 years,” reported the authors. The decline of sea ice is “unparalleled” in relation to previous patterns of natural variation, they added.
“The recent sea ice decline “¦ appears to be unprecedented,” said Christian Zdanowicz, a glaciologist at Natural Resources Canada, who co-led the study and is a co-author of the paper published Wednesday online.
“We kind of have to conclude that there's a strong chance that there's a human influence embedded in that signal,” he said.
In September, Germany´s University of Bremen reported that sea ice was at a record low, based on Japanese sensor data from NASA´s Aqua satellite. The US NSIDC used a different satellite data set for its records, and reported that the sea ice coverage in 2011 was the second-lowest on record; 2007 was the lowest.
Many factors have driven the rapid decline of summer sea ice in recent years. Many of these factors have never all coincided in previous historical periods of major sea ice loss, according to Christophe Kinnard, of the Center for Advanced Studies in Arid Zones in La Serena, Chile, lead author of the report.
Kinnard and his colleagues used proxy data -- such as ice core records, tree ring data, lake sediments and historical evidence -- to reconstruct Arctic sea ice cover of the past. They found that unusually warm Atlantic Ocean water flowing into the Arctic is the cause of the recent sea ice decline.
“Everything is trending up — surface temperature, the atmosphere is warming, and it seems also that the ocean is warming and there is more warm and saline water that makes it into the Arctic,” said Kinnard. Basically, the sea ice is being bombarded from both top and bottom. It is being “eroded from below and melting from the top.”
In the past, sea ice loss was mostly driven by an influx of warm, salty water from the North Atlantic into the Arctic due to a change in ocean currents, and wasn´t necessarily linked to periods of warmer air temperatures, said Kinnard.
In contrast, Zdanowicz said, temperature has come to dominate control of the sea ice.
Zdanowicz and colleagues had some questions in light of the recent dramatic decline of Arctic sea ice: “Is this exceptional? Is this unique? Is this part of a longer cycle?”
To try to find an answer to these questions, the team compiled data from more than 60 sources which all provided information about climatic and sea ice changes over hundreds or thousands of years. About 80 percent of the data came from ice cores from polar glaciers, and about a third of those were Canadian.
Their model showed that when the sea ice extent was at its lowest historically, at the beginning of that period, at least 5.25 million square miles of sea ice covered the Arctic in late summer, the time of year when sea ice is usually at its lowest extent.
“Today, we´re lower than eight,” Kinnard said.
The information from the study may be useful to scientists who make predictions about sea ice loss and have so far been largely underestimating the rate of its decline, said Kinnard. Their underestimations could indicate that “the models are missing something.”
Zdanowicz added that climate models are tested by seeing how well they are able to reproduce the past -- and the new reconstruction allows for that.
The team also noted that sea ice has a strong effect on the overall climate. Because sea ice is so bright, for example, it reflects sunlight, reducing warming; while the ocean water is dark, absorbing sunlight and increasing warming, said Anne de Vernal, a researcher at the University of Quebec in Montreal who also co-authored the report.
Gathering information now about sea ice could be useful in predicting other aspects of the future climate. But the current sea ice loss makes it harder to guess how other systems will respond, said Vernal.
“What we are experiencing at the moment seems to be very exceptional“¦. This means that we are entering into the world which has no equivalent in the past,” she added.
It remains difficult to determine whether the observed decline of Arctic sea ice over the past few decades is due to natural variability or not, Kinnard said. He and his colleagues conclude, however, that it is consistent with human-caused global warming.
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