Since Arctic sea ice extent plummeted to a record low in September, shattering the previous record set in 2007, much has been made of the fact that sea ice has declined at a much faster pace than what most scientists had anticipated. Until recently, most computer models failed to capture the precipitous decline in Arctic sea ice that has occurred since satellite observations began in 1979.
In order to make improved projections, scientists are fine-tuning their understanding of the many influences on sea ice trends, including both man-made global warming and natural climate variability. The emerging science points to a complex interplay between manmade global warming, natural climate variability, and sea ice dynamics that scientists are only just beginning to truly understand.
For example, one new study shows that the melt ponds that form on top of sea ice floes in June and July can dramatically accelerate sea ice melt. These ponds, which form as snow and ice melt under the Arctic sun, can dramatically increase the amount of solar radiation the ice absorbs. This warms the surface and eventually allows more heat to ocean waters below, in effect melting sea ice from the top and bottom.
The study found that the melt rate beneath pond-covered ice is up to three times greater than that of bare ice.
“In a crude sense, pond-covered ice is more akin to open water. So, because ponded ice reflects less of the solar radiation, there is more heat available to melt the surface of the ice,” said Daniel Feltham, a researcher at the Center for Polar Observation and Modeling at the University College London, and co-author of the study, in an email.
The study, published in the Journal of Geophysical Research, showed that computer models incorporating melt pond information tend to perform better in simulating historical conditions than models that don’t take such ponds into account.
The study also points to an unexpected relationship between snow cover and sea ice loss. The more snow on top of the ice at the beginning of the sea ice melt season, the greater potential there is for melt ponds to form, and thereby speed up sea ice loss, the study found.
“More snow means more snowmelt in [the] spring, resulting in larger melt ponds at that time,” which lowers the reflectivity of the sea ice, Feltham said. “As a result of this, the net effect of more snow may actually be more melt, rather than less.”
Julienne Stroeve, a research scientist at the National Snow and Ice Data Center in Boulder, Colorado, said that during a recent trip to the Arctic she noticed that sea ice floes that were covered with melt ponds tended to be one and a half feet thinner than ones without them.
Are Model Trends Catching Up to the Real World?
Newer computer models that scientists are using to help prepare the next IPCC report, which is scheduled to be released in 2014, do a much better job capturing the historical sea ice trends than the previous generation of models did, several sea ice experts told Climate Central.
“... The argument that the ice loss is outpacing model projections no longer seems to be true,” said Marika Holland, a researcher at the National Center for Atmospheric Research, in an email conversation.
Recent studies have also zeroed in on the relative contribution of manmade global warming to sea ice loss compared to natural climate factors. The research has shown that manmade global warming is likely responsible for 50 to 60 per cent of Arctic sea ice decline during the satellite era. Mark Serreze, the director of the NSIDC, said that making a clear separation between manmade and natural factors is “very difficult,” since as the sea ice thins, it become more vulnerable to melting spurred by weather patterns.
This occurred in 2007, when prevailing winds flushed sea ice out of the Arctic Ocean through the Fram Strait, leaving it to melt in the iceberg graveyard that is the North Atlantic Ocean.
Accelerated melting due to weather factors may also have occurred during the past melt season, when a massive Arctic storm swept across the Arctic Ocean in August. This may have helped break up ice floes and encourage melting, although no formal studies of this event have yet been completed.
One projection that the newer models have not honed in on, however, is the approximate date when the Arctic Ocean will be seasonally ice free during the summer months. This is a subject of intense media speculation, and is of special interest to oil and gas companies eager to explore the Arctic Ocean for natural resources, as well as for shipping companies hoping to take advantage of new and shorter trade routes.
Stroeve said that sea ice volume, which incorporates measurements of ice extent as well as thickness, is a more important metric than sea ice extent alone. Ice extent, she said, will exhibit considerable variability even after the arrival of ice-free summers, whereas ice volume trends would be more consistent. “Because you could have an anomalously cold summer that allows a thin layer of ice to remain and increase the extent. In our climate model simulations we don’t see evidence of a tipping point, so it appears that even though the Arctic may become ice-free, the ice could also recover for a few years.”
Projections of when the Arctic will be, in effect, ice-free in summer (some sea ice is expected to linger in spots, such as north of Greenland and the Canadian Archipelago) still range from sometime during the next decade to as late as the end of this century. In general, the newer projections show the Arctic becoming seasonally ice free earlier than previous forecasts did.