When records were being set for loss of summer Arctic sea ice area (2007) and sea ice volume (2008), the deniers spent all their time talking about how quickly the ice refroze in the ensuing months. Now, they are strangely quiet on the remarkably slow refreezing we’re seeing.
Why the slow refreezing this year? I’ll post the answer from the National Snow and Ice Data Center at the end. First, some background.
“The recent sea-ice retreat is larger than in any of the (19) IPCC [climate] models,” as Tore Furevik, Vice director at Bjerknes Centre for Climate Research, pointed out in a May 2006 talk (big PPT here) on climate system feedbacks.
And that was before another staggering drop in Arctic sea-ice area in 2007 (see “Arctic Ice shrinks by an Alaska plus a Texas“).
And then we hit a record low volume in 2008 (see here), as this remarkable figure shows:
Then we saw some recovery in 2008 to the third lowest area on record, and, I expect, the second lowest volume. The Arctic ice loss is not monotonic, but reflects an overall warming trend and local weather conditions.
“What drove the dramatic retreat of arctic sea ice during summer 2007” is the title of a GRL analysis published last year. It found:
A model study has been conducted of the unprecedented retreat of arctic sea ice in the summer of 2007. It is found that preconditioning, anomalous winds, and ice-albedo feedback are mainly responsible for the retreat. Arctic sea ice in 2007 was preconditioned to radical changes after years of shrinking and thinning in a warm climate. During summer 2007 atmospheric changes strengthened the transpolar drift of sea ice, causing more ice to move out of the Pacific sector and the central Arctic Ocean where the reduction in ice thickness due to ice advection is up to 1.5 m more than usual. Some of the ice exited Fram Strait and some piled up in part of the Canada Basin and along the coast of northern Greenland, leaving behind an unusually large area of thin ice and open water. Thin ice and open water allow more surface solar heating because of a much reduced surface albedo, leading to amplified ice melting. The Arctic Ocean lost additional 10% of its total ice mass in which 70% is due directly to the amplified melting and 30% to the unusual ice advection, causing the unprecedented ice retreat. Arctic sea ice has entered a state of being particularly vulnerable to anomalous atmospheric forcing.
And so the warming-driven thinning makes the Arctic much more vulnerable to local weather conditions.
What to make of the recent slow refreezing? I put that question to NSIDC, and Research Scientist Walt Meier replied:
I think the lesson is to not make too much from only a few data points. There is significant natural variability in the climate system, and particularly the sea ice. It is only by looking at long-term trends, after the short-term variability is averaged out, that you can make any judgments on long-term climate factors. It is particularly unjustified to draw any conclusions about sea ice from only a few days or weeks.
In the short term, winds can play an important role in the sea ice extent. If there are winds pushing the ice edge “inward”, then you either increase the seasonal decline or slow (or even temporarily reverse) the seasonal increase, depending on the season. The slow-down in the increase in October was due to winds blowing the ice northward from the Siberian coast. It looks like there is another slowdown, also likely due to winds.
What about NSIDC director’s Mark Serreze “death spiral” of Arctic ice metaphor? I noted that the NYT’s Revkin has written recently:
Meier writes:
Andy Revkin’s comment is quite apt. We don’t expect to see a continual downward trend, there will some fits and starts, but the overall trend will continue to be down.
Serreze wrote me that “I share Walt’s views.”
Finally, we have the Met Office’s recent statement, which the deniers are trumpeting:
Modelling of Arctic sea ice by the Met Office Hadley Centre climate model shows that ice invariably recovers from extreme events, and that the long-term trend of reduction is robust — with the first ice-free summer expected to occur between 2060 and 2080. It is unlikely that the Arctic will experience ice-free summers by 2020.
Analysis of the 2007 summer sea-ice minimum has subsequently shown that this was due, in part, to unusual weather patterns. Arctic weather systems are highly variable year-on-year and the prevailing winds can enhance, or oppose, the southward flow of ice into the Atlantic. Consequently, the sea ice has not declined every year, but has shown considerable variability — both in extent and thickness.
The high variability has made it difficult to attribute the observed trend to man-made emissions of greenhouse gases, although there is now enough data to detect a human signal in the 30-year trend. The trend and observed variability, including the minimum extent observed in 2007, is consistent with climate modelling from the Met Office.
About half of the climate models involved in the Intergovernmental Panel on Climate Change fourth assessment report, show that ice declines in steps — failing to recover from extreme years. The observed temporary recovery from the 2007 minimum in 2008 and 2009 indicates that the Arctic ice has not yet reached a tipping point, if such exists. We expect Arctic ice to continue to decline in line with increasing global temperatures. If the rate of global temperature rise increa
ses then so will the rate of Arctic sea-ice decline.
First, how disappointing that the Met Office appears unaware of the analysis showing that the Arctic did not experience a “recovery” from 2007 to 2008, unless losing 2000 cubic kilometers of sea ice is a recovery.
Serreze responds:
I agree with the Met Office that going seasonally ice free by 2020 is unlikely. I still think were looking at somewhere around 2030.
He makes one final point:
One thing we need to come to some agreement on is what we call “ice free.” Do we mean no ice at all in mid September or something like less than 1 million square km? “Ice free” is one of these terms like “tipping point” that tends to get tossed around without being all that well defined. I’m guilty of this myself.
I’m also moving towards the view that the bigger issue is not the when we go ice free, for we seem destined to do so, but the ecological and climatic implications of losing the ice.
I’d certainly second that last point, especially since the major climatic implication is accelerated warming of the carbon-rich permafrost (see here).
