Saturday, September 10, 2011

Record arctic melt

The sea ice news of the past week has been stunning. We've hit new lows in sea ice volume, area, and extent. Neven is unbeatable on Arctic sea ice, and he cuts to the heart of the matter:
The incredible has happened. In the past week the 2011 melting season has started to surpass record year 2007. First, the good people from the Polar Science Center informed us of the fact that their PIOMAS model is showing a new sea ice volume record. A day later a new all-time low on the Cryosphere Today sea ice area graph was reached. And two days after that the same thing happened on the University of Bremen sea ice extent chart. . . . Four years ago, weather conditions that on average occur every 20 years or so, brought huge amounts of heat into the Arctic via air and water, flushed large amounts of ice through Fram and Nares Strait and - to top if off - compacted the ice pack so hard at the end of the melting season that the minimum extent was finally reached in the last week of September. Up until mid-July this year's melting season resembled that of 2007, but after that things fell apart on the atmospheric front. The heat had been brought in alright, but the flushing through Nares (which opened late) and Fram was slow, and in these last weeks of the season there isn't much compaction to speak of, as the winds are too fickle to stay in place for a prolonged period. Despite all this 2011 is right down there battling it out with 2007 on almost every graph. This is a sure sign that the ice is very weak and thin in large parts of the ice pack, which means that perfect weather conditions conducive to melting and compacting are no longer necessary to break records. The ice will melt out, regardless of what the weather does.
It's been known since Arrhenius' hand-calculated climate model that Arctic melting will be a positive feedback causing further warming, as highly reflective ice is replaced by highly absorptive water. But the number turns out to be difficult to calculate as distinct from the overall climate sensitivity implied by a given set of model runs. I personally have been looking for hard numbers on this for a while, and -- early Christmas present! -- "Estimating the global radiative impact of the sea ice–albedo feedback in the Arctic" from last month's Journal of Geophysical Research has my back:
A simple method for estimating the global radiative forcing caused by the sea ice–albedo feedback in the Arctic is presented. It is based on observations of cloud cover, sea ice concentration, and top-of-atmosphere broadband albedo. The method does not rely on any sort of climate model, making the assumptions and approximations clearly visible and understandable and allowing them to be easily changed. Results show that the globally and annually averaged radiative forcing caused by the observed loss of sea ice in the Arctic between 1979 and 2007 is approximately 0.1 W m−2; a complete removal of Arctic sea ice results in a forcing of about 0.7 W m−2, while a more realistic ice-free summer scenario (no ice for 1 month and decreased ice at all other times of the year) results in a forcing of about 0.3 W m−2, similar to present-day anthropogenic forcing caused by halocarbons. The potential for changes in cloud cover as a result of the changes in sea ice makes the evaluation of the actual forcing that may be realized quite uncertain since such changes could overwhelm the forcing caused by the sea ice loss itself, if the cloudiness increases in the summertime.
Losing that sea ice is going to cause a number of problems; especially by accelerating Greenland's melt and the release of permafrost carbon, both CO2 and methane. But the direct effect is pretty gobsmackingly awful. In the short term (decades) another 0.25C of warming -- which may not sound like much, but which represents about a third of the warming seen so far and a fifth of the distance between here and 2C. The long-term effects -- 0.6 W m-2 -- mean an eventual warming of about 1.5C. Which means the process we've set in motion at the polls, which is likely irreversible on human timescales without geoengineering, is going to take us past +2C all by itself.

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