The recent loss of sea ice in the Arctic is greater than any natural variation in the past 1½ millennia, a Canadian study shows.
Ice and sediment cores
Direct information about sea ice coverage can be obtained by counting fossils of sea ice dwellers, such as microorganisms called dinoflagellates. When they die, their bodies become part of the sediment on the sea floor. Taking cores of the sediment can therefore provide historical information about the sea ice cover, said Anne de Vernal, a microfossil specialist at the University of Quebec in Montreal.
However, sediment cores are very difficult to collect in the Arctic because of the sea ice and the lack of sediment in many areas, so there is relatively little data available.
Also, the sediment is deposited slowly, which means thin layers can represent huge amounts of time, said glaciologist Christian Zdanowicz.
That's why the information from sediment cores is compared to a much bigger set of ice cores from glaciers, which are deposited more quickly and provide information on smaller time scales.
The chemistry of the ice is influenced by nearby sea ice, providing indirect information about its extent. For example, heavy sea ice cover limits the amount of salt from ocean water that sprays into the atmosphere and gets deposited in the ice. Also, microorganisms in sea ice produce a chemical called methane sulfonic acid that is deposited on nearby glaciers in higher quantities when there is a lot of sea ice nearby. By analyzing the ice for chemicals such as salt and methane sulfonic acid and comparing it to other data such as sediment cores and tree rings, the researchers were able to reconstruct the amount of sea ice across the Arctic in the past.
"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 in Nature.
"We kind of have to conclude that there's a strong chance that there's a human influence embedded in that signal."
In September, Germany's University of Bremen reported that sea ice had hit a record low, based on data from a Japanese sensor on NASA's Aqua satellite. The U.S. National Snow and Ice Data Center, using a different satellite data set, reported that the sea ice coverage in 2011 was the second-lowest on record, after the record set in 2007.
What makes recent sea ice declines unique is that they have been driven by multiple factors that never all coincided in historical periods of major sea ice loss, said Christophe Kinnard, lead author of the new report.
"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," Kinnard said, "and so the sea ice is eroded from below and melting from the top."
In the past, he said, sea ice loss was driven mostly 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.
In contrast, Zdanowicz said, temperature has come to dominate control of the sea ice.
Most of the current data about the recent, rapid sea ice loss comes from satellite measurements that began in the 1970s.
Other reports of sea ice variability come from sources such as ship logs and only go back around 130 years, said Kinnard, a research scientist at the Centro de Estudios Avanzados en Zonas Áridas in La Serena, Chile, who conducted most of the research while doing his PhD and working at the Geological Survey of Canada under Zdanowicz and fellow glaciologist David Fisher.
Zdanowicz said he and his 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?"
The researchers compiled data from more than 60 sources, including ice core records, tree rings and lake and ocean sediments, which all provide information about climatic and sea ice changes over hundreds or thousands of years. About 80 per cent of the data came from ice cores from polar glaciers, and about a third of those were Canadian.
Using those historical data records and statistics derived from modern data correlating sea ice to other factors, the researchers managed to reconstruct sea ice changes over the past 1,450 years — since about 600 A.D.
The model showed that when the sea ice extent was at its lowest historically, at the beginning of that period, at least 8.5 million square kilometres 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.
Data will improve predictions
Kinnard said the information about the causes of past sea ice losses might be useful to scientists who make predictions about sea ice loss and have so far been largely underestimating the rate of its decline: "Which probably indicates 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.
Sea ice also has a strong effect on the overall climate, the scientists noted. For example, it is bright and so it reflects sunlight, reducing warming, while 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.
Fisher added that it also affects water and atmospheric chemistry. That in turn could produce feedback that somehow speeds up further sea ice loss.
That means information about sea ice could be useful in predicting other aspects of the future climate.
On the other hand, de Vernal said, the unique nature of the current sea ice loss makes it harder to guess how other systems will respond.
"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."