Over the last few decades, and particularly in recent years, the area of the Arctic Ocean covered by a skin of sea ice has steadily shrunk. But it’s not just this extent that matters—the volume of sea ice, which takes into account its thickness, is also important, but traditionally much more difficult to measure.
The 2010 launch of the European Space Agency’s CryoSat-2 satellite finally allowed scientists to take a wide-scale view of Arctic sea ice volume, and the first five years of data have yielded some surprises.
The volume of sea ice left at the end of the summer melt season seems to vary more from year to year than had perhaps been previously appreciated; after declining for several years, sea ice volume shot up after the unusually cool summer of 2013, the data revealed.
The authors of a new study reviewing the volume data, detailed on Monday in the journal Nature Geoscience, are quick to caution, though, that one single year of rebound doesn’t suggest any sea ice recovery, as the overall trend is still downward. Rather, the view afforded by CryoSat-2 will help them get a better handle on the ice’s future.
Eye on volume
The Arctic is one of the fast-warming spots on the planet, with average temperatures that have risen twice as fast as the global as a whole. That warming has led to the melting of sea ice in a self-reinforcing cycle: As ice melts, it exposes more open ocean, meaning more incoming sunlight is absorbed by dark waters instead of reflected by the bright, white ice.
Since 1979, the summer minimum Arctic sea ice extent has declined at a steady clip of nearly 14 percent per decade, as measured by satellites that can view changes in ice area.
These same satellites, though, are stymied when it comes to measuring volume, which is a key measure as thick ice is insulating, helping to regulate the transfer of heat from the ocean to the atmosphere. That in turn can affect atmospheric circulation patterns that influence weather.
Ice volume is also important to have a handle on because understanding all the forces that affect sea ice can inform sea ice prediction models and shipping routes. (Though unlike land-based ice, sea ice melt doesn’t affect sea level rise as sea ice already displaces water, like ice in a glass.)
CryoSat-2, by virtue of its orbit and onboard radar, can resolve ice thickness. It does this by bouncing its signal back from both the ocean water and the ice surface, and then calculating the thickness from that difference.
“It’s brilliant,” lead author Rachel Tilling, a PhD student at University College London, said. “We wouldn’t have been able to do this without CryoSat.”
Not a recovery
While five years of data isn’t a lot in the grand scheme of sea ice trends, it did allow Tilling and her colleagues to make some interesting findings.
While the sea ice volume decreased 14 percent from 2010 through 2012, after the relatively cool summer of 2013—which had conditions more akin to the 1990s—the volume jumped by 41 percent compared to the previous year.
What the scientists think happened was that the traditionally older, thicker ice around Greenland and the Canadian archipelago “just didn’t melt away as much as it usually would” during the cooler summer conditions, “and it kind of just remained over the summer melt season,” Tilling said.
What she and her colleagues have taken away from this finding is that the sea ice might be more resilient than had been assumed, able to rebound somewhat after a cooler melt season.
“We’ve almost wound the clock back a few years,” Tilling said.
But, she stresses, this doesn’t mean the sea ice is recovering, “because it’s not in any way a recovery.” One year of increased volume has “not reversed this long-term decline,” she said.
Because the data only covers five years, it necessarily limits what conclusions can be drawn. Julienne Stroeve, a sea ice researcher at the National Snow and Ice Data Center in Boulder, Colo., said that while it’s good to see the CryoSat-2 data published, “it's a bit premature to make big conclusions about volume changes from only five years of data without accounting for all the uncertainties.” Stroeve was not involved in the new study.
Tilling and her colleagues are looking forward to the data to come from CryoSat-2, which is slated to last until 2017, but with no similar missions planned to follow it. They are hoping that this work will point to how valuable such data is and provide an impetus to come up with a successor mission.
“We want more,” Tilling said.