Summer Sea Ice Cover Is Smaller, Younger, Thinner
As summer Arctic sea ice melts away to nothing, it’s important to remember that not only is the ice shrinking in size - it’s also getting younger and thinner.
Looking at not just the area, but also the age and thickness of sea ice is a key aspect of understanding why an ice-free Arctic summer could occur in far less time than previously thought - perhaps less than a decade from now. It is also a level of complexity often ignored by the media.
And this is not to say that the area loss in itself is not significant. Nalân Koç, research director at the Norwegian Polar Institute in Tromso, said that the extent of the Arctic sea ice has plummeted over the course of more than 30 years of satellite monitoring, losing 13 percent of its total extent per decade.
In Scandinavian terms, Koç said that since 1979 the total yearly average sea ice cover has shrunk by about the size of eight Norways put together - or more than 3 million km². Between 2007 and 2012 alone, the two lowest record years for Arctic sea ice cover, the two record minimum years for average sea ice cover, an area the size of two Norways disappeared.
Despite the overwhelming evidence, there are plenty of voices in the media that would deny the severity of the situation. In recent days, some have pointed to the fact that the current level of sea ice coverage is less than it was this time last year.
But that type of interpretation ignores the complexity of the melting trend, in which year-to-year variability is normal within the downward slide. Sebastian Gerland, sea ice expert at NPI, said that even if June 2013 isn’t quite as low as 2012, it is important to remember that 2012 marked the lowest Arctic sea ice extent in recorded history, and that the 30-plus-year long negative trend in Arctic summer sea ice extent is substantial.
“One can also see that the 2013 development is so far not very different from the 2007 development at this time of year, the green line,” he said, commenting on the graph pictured below.
Nalân Koç suggested that the lower and lower records of recent years may be skewing public perception.
“We are getting so used to the records that they are losing their meaning,” she said.
And to add to the obscured public understanding, the age and thickness of the sea ice is often left out of the discussion entirely. But Koç said it is just as important as the extent.
The graph and images below show that even in the winter season, more and more of the total sea ice cover has been getting younger and younger over time. And younger ice is thinner ice, neither of which bodes well for maintaining long-term sea ice cover.
“In 1981, we had multi-year ice, the green part, old ice - and that could be up to four or five, in some cases six meters thick,” Koç said. The graph shows that upward of 35 percent of the total winter sea ice cover was older than 2 years in 1981, whereas by 2011 it was around 15 percent of the total - or half of its percent total just 30 years previously.
“Today we are in a situation where most of the Arctic Ocean is covered with first-year ice, which is just 1.5 or 2 meters thick,” she said.
This thickness - 1.5 to 2 meters - is the same as the average thickness of first year ice. Koç said that NPI manages a project that has been measuring sea ice passing through the Fram Strait (between Svalbard and Greenland) for more than 20 years, using moored “upward-looking sonars” that can gauge the thickness of the ice as it passes throughout the year.
“What we are seeing when we look at the record of sea ice thickness is that we are losing the thickest part of the ice,” she said. “Most of the thickness of the ice that was passing through in 2005 was 3.2 meters. In 2007 it went down to 2.2 meters. And now it’s around less than 2 meters.”
Koç said that a “threshold value” has been reached, when multiyear ice thickness equals the first-year ice thickness, after which an ice-free summer is on the horizon.
Why is this threshold value so important?
For one thing, without the effects of time, the surface of younger ice is not only thinner than older ice; it is also smoother. This means that when melting begins to occur, instead of concentrating in small areas between the ridges and creases in older ice, melt ponds quickly spread across vast areas of smooth, thin ice. Water is a darker surface than ice and snow, and darker surfaces absorb more light and heat than lighter ones. So when melting begins to occur on young, smooth, thin ice, the melt ponds can actually accelerate the melting process as the winter changes to summer.
This is another area where some voices in the media have suggested that winter sea ice recovery is actually increasing, neglecting to mention the context that summer melting is also accelerating.
As Gerland explains, because the summer decline in sea ice is still much sharper than the winter decline, the total increase in ice extent from summer to the following winter has been larger in recent years. Taking the data out of context makes it appear as though the winter recovery is actually getting stronger.
But in reality, the ice is simply rebounding from a lower and lower summer minimum.
“The recovery each autumn increases on average over time, since the negative summer trend is stronger than the negative winter trend,” Gerland said. “Winters in the Arctic are still cold enough to produce large amounts of ice.”
Thankfully, although winter sea ice is also decreasing, an ice-free winter is still many, many years off.
“As long as we have cold winters, it will freeze,” Koç said.
Scientists have only very recently voiced their concern that a sea-ice free Arctic summer may be just around the corner. Only a few years ago in 2007, the Intergovernmental Panel on Climate Change predicted that this landmark shift would not happen until at least 2050, even as late as the end of the century.
“It is all changing must faster than our present abilities to model it,” Koç said.