New research eliminates all other possible causes for shrinking polar ice cap
By Summit Voice
SUMMIT COUNTY — Rather than building another complex climate model to study dwindling Arctic sea ice, scientists at Germany’s Max Planck Institute for Meteorology did a little old-fashioned detective work.
One by one, they eliminated all the possible causes of the decline: Wind patterns, volcanic eruptions, oceanic heat transport, or cosmic rays. That left only one possibility — rising atmospheric temperatures caused by increasing concentrations of greenhouse gases.
As a result of their study, the researchers are ready to say they’ve shown a strong, physically plausible correlation between the buildup of heat-trapping gases and the vanishing ice cap.
“Sea ice is so thin that it reacts very sensitive to the large natural fluctuations of weather and climate that prevail in the Arctic,” said Dirk Notz, lead author of the study published in the journal Geophysical Research Letters. “Because these fluctuations are inherently chaotic, their specific timing cannot be reproduced by standard climate models. Such models therefore aren’t necessarily the best tool to examine if natural fluctuations did cause the observed sea-ice loss.”
They started by comparing natural variations in sea-ice extent between the early 1950s and late 1970s with the magnitude of fluctuations as measures by satellites since the late 1970s. That helped them determine there’s only a slim probability that the recently observed extreme sea-ice minima simply happened by chance. And they were able to exclude self-acceleration as the main driver for sea-ice loss.
“Whenever we had a strong sea-ice loss from one year to the next, the ice cover always recovered somewhat in the following year,” explains Dirk Notz. “This would not be the case if the sea-ice retreat were indeed self-accelerating,” he said.
“Having excluded natural fluctuations and self acceleration as the main driver for the sea-ice retreat, it was clear to us that some external driver was responsible for the observed sea-ice decline,” said Jochem Marotzke, director of the respected research institution.
“We therefore set out to find an external driver that showed a physically plausible relationship with the observed sea-ice retreat,” Marotzke said.
They looked, or course, at solar radiation.
“Here, a physically plausible link to the observed sea-ice retreat can only be established if solar radiation had increased in recent years,” Marotzke said.
However, solar radiation has slightly decreased in the past decades. Its fluctuations are therefore very unlikely to be the main driver of the observed sea ice loss. The scientists could not find a plausible link to changes in prevailing wind patterns, volcanic eruptions, oceanic heat transport, or cosmic rays, either.
“In the end, only the increase in greenhouse gas concentration showed a physically plausible link with the observed sea-ice retreat. We expect a decreasing sea-ice cover for increasing greenhouse gas concentration, which is exactly what is observed,” Notz explained. The physical link between greenhouse gas concentration and sea ice is quite straightforward, he adds: “Greenhouse gases increase the downwelling thermal radiation. This radiation, in turn, is the major player in the heat budget of Arctic sea ice.”
In the Antarctic, the situation is different. Here, the sea-ice cover has slightly increased in recent years. This increase is clearly incompatible with greenhouse gas concentration being the main driver for the sea-ice evolution down South.
The major reason for this discrepancy lies in the different land-mass distributions, the scientists found. In the Arctic Ocean, the ice is virtually locked by the surrounding land masses, and its extent is primarily governed by its melting and freezing. Therefore, greenhouse gases play such an important role up in the high North. In the Antarctic, by contrast, the sea ice is free to drift around in the open Southern Ocean. Hence, the ice extent there 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 more important,” Marotzke said. “In the land-locked Arctic Ocean, however, greenhouse gas concentration appears to play the dominating role for the observed sea-ice evolution”.