Sea Ice Was So Much Older Then
by Bill Chameides | July 20th, 2011
posted by Erica Rowell (Editor)
We’re well into the 2011 melt season for Arctic sea ice. How goes it?
The extent of the sea ice in the Arctic has arguably been one of the most contentious of the metrics used by climate scientists to monitor the pace and severity of global warming. Since the 1970s the maximum extent of ice coverage in the Arctic Ocean (measured every March) has declined, with some short-term upward bumps along the way but with an overall average rate of loss of about three percent per decade. The minimum extent (measured in September) has declined more rapidly — at about 11.5 percent per decade.
The loss of sea ice is troubling because it could set up a positive feedback with global warming. (Because ice is reflective and has a high albedo, it tends to cool the planet, so as more and more ice melts, its becomes less reflective and so absorbs more heat from the sun leading to more warming and more ice melting, and so forth.) Some scientists have warned that the loss of sea ice could pass a tipping point — once enough ice melts, the positive feedback might take over leading the inevitable loss of all the ice.
Concern about such a tipping point reached a peak, so to speak, in the fall of 2007, when the melt season ended not just at historical lows in sea ice extent, but with an unprecedented one-year drop.
There were those who claimed that this was a sign we had passed the tipping point and predicted the rapid loss of all the remaining summer sea ice in the Arctic. (See here, here and here.) Others suggested this possibility was unlikely.
Events since 2007 have tended to validate the cooler heads. Since 2007, sea ice extent at the end of the melt season (September) has recovered. That’s the good news, but:
- Sea ice extent, still remains well below historic amounts; and
- While sea ice extent at the end of the melt season in 2008 and 2009 saw an increase relative to the previous year, the 2010 extent was less than that at the end of 2009’s melt season — perhaps indicative of a resumption of the long-term downward trend.
Which begs the question: How’s the ice doing in 2011? The most up-to-date data that we have comes from the National Snow and Ice Data Center. At this year’s maximum in March the sea ice extent was below that of 2010 and was the 2nd lowest on record (March 2006 was the lowest). And NSIDC’s current map for July 17 shows that sea ice extent has been below the 2007 levels for much of the 2011 melt season. However, bear in mind that a lot can happen between now and September.
Age Before Extent
Sea ice extent is important — and easy to relate to visually — but also important is its age. Older ice tends to be thicker and therefore less likely to disappear over a melt season. This makes the ice cover more stable and less likely to melt in an anomalously warm year. To get a real picture of what’s happening in the Arctic some argue we should be concerned about age (or thickness) as well as extent. (See here and here.)
Now, determining the record of sea ice age is not all that easy compared to total sea ice extent; the latter can at least in principle be determined visually. The former requires knowing either what’s going on below or where the ice has been.
James Maslanik of the University of Colorado at Boulder and co-authors took up the challenge. Using a combination of observational data from satellites and drifting buoys, they were able to build a record of the position and extent of ice aged two-to-five or more years for March (maximum) and September (minimum) over the period from 1980 to 2011. They reported their result last week in the journal Geophysical Research Letters.
Their analysis suggests that sea ice age trends have been pretty close to what we’ve been seeing for total sea ice extent. There was a huge 40 percent decrease in the March (and September) extent of multi-year ice (i.e., ice that is two years or older) in 2008 relative to 2007 — reflecting the minimum in sea ice extent at the end of the 2007 melt season.
But since 2007, the extent of multi-year ice has been on an upward trend — the largest increase being in the two and three year old ice. The authors note that this recovery in multi-year ice “is consistent with an ice pack that has not passed a tipping point across the Arctic Ocean as a whole.“
That’s the good news. But there are also some sobering notes:
- While the extent of multi-year ice has been on an uptick, the extent of the oldest ice (five or more years) has continued to decline since 2008.
- The amount of multi-year ice in March 2011 was below that of any year before 2008.
- And even with the uptick, the amount of multi-year ice in March 2011 is consistent with the multi-decadal downward long-term trend.
This last point suggests that the huge decrease in sea ice extent in 2007 and age in 2008 was an anomaly but that the slow decline in overall sea ice age since the 1970s continues unabated.
This of course begs the question of what happened in 2007. Data suggests that 2007 was an anomaly caused in part by unusual wind patterns that pushed ice out of the Arctic basin. (See here [xml] and here.)
Since 2007, conditions have returned to what Maslanik et al call “more ice favorable” and hence the partial recovery.
So where are we headed in 2011? Will this year’s melt season end lower or higher than last year? Will it challenge the record low of 2007? Will sea ice get older or younger? We don’t know, but at least we won’t be younger than that now.filed under: Arctic, climate change, faculty, global warming
and: climate science, ice, ice age, sea ice