The findings of a new study, conducted by scientists at the Max Planck Institute for Meteorology (MPI-M), have indicated that neither natural fluctuations nor self-acceleration offer adequate explanations for Arctic sea ice retreat. Instead, the team, whose work has been published in the journal Geophysical Research Letters
, contends that the recent evolution of Arctic sea ice demonstrates a strong, physically plausible correlation with increasing concentrations of greenhouse gases.
The researchers, who set out to identify the main driver for the observed sea ice loss in the Arctic, took data from historical records detailing natural variations in sea ice extent between the early 1950s and the late 1970s. This was a novel approach as the majority of similar studies have employed complex mathematical models in order to test the validity of the anthropogenic climate change hypothesis.
"Sea ice is so thin that it reacts very sensitively to the large natural fluctuations of weather and climate that prevail in the Arctic," explained lead author of the study, Dr Dirk Notz. "Because these fluctuations are inherently chaotic, their specific timing cannot be reproduced by standard climate models. Such models, therefore, aren't necessarily the best tools to examine [whether] natural fluctuations did cause the observed sea ice loss."
Dr Notz and his colleagues compared the information collected from historical records, with data recorded by satellites since the late 1970s. Their analysis demonstrated that there exists only a minute chance that the recently observed extreme sea-ice retreat happened by chance. As a result, the scientists were able to rule out the possible causes of self-acceleration and natural fluctuation.
"Whenever we had a strong sea ice loss from one year to the next, the ice cover always recovered somewhat in the following year," said Dr Notz, who pointed out that such a pattern could not have resulted from self-acceleration. In light of this, the team set about finding a plausible explanation for the sea ice loss.
"Having excluded natural fluctuations and self acceleration as the main drivers for the sea ice retreat, it was clear to us that some external driver was responsible for the observed sea ice decline," said the Director of MPI-M, Prof Dr Jochem Marotzke, who co-authored the study. "We therefore set out to find an external driver that showed a physically plausible relationship with the observed sea ice retreat."
The team investigated whether or not a variation in solar radiation might have played a role in this process. However, for such theory to be validated, there would need to have been an increase in the intensity of solar radiation during recent years, and in fact, its strength has slightly decreased. The scientists also failed to establish any link between Arctic sea ice loss and prevailing wind patterns, volcanic eruptions or oceanic heat transport.
"In the end, only the increase in greenhouse gas concentration showed a physically plausible link with the observed sea ice retreat," said Dr Notz. "We expect a decreasing sea ice cover for increasing greenhouse gas concentration, which is exactly what is observed.
"Greenhouse gases increase the down-welling thermal radiation," he added. "This radiation, in turn, is the major player in the heat budget of Arctic sea ice."
Although their evidence strongly indicated a link between greenhouse gas concentrations and Arctic sea ice loss, there remained a difficult question for the researchers to answer: 'Why, in the Antarctic, is this situation different?' The team found that the reason behind this discrepancy was the variation in land-mass distributions. In the Arctic Ocean, ice is locked-in by surrounding land masses, and so its extent is predominantly influenced by its melting and freezing. Antarctic sea ice, on the other hand, is free to drift in the Southern Ocean, and so is primarily governed by the prevailing wind patterns.
"Our results show that greenhouse gas concentration is currently not a major driver for sea-ice extent in the Southern Ocean, where winds and currents clearly are important," explained Prof Dr Marotzke. "In the land-locked Arctic Ocean, however, greenhouse gas concentration appears to play the dominating role for the observed sea ice evolution."