I expected Alun Anderson’s After the Ice: Life, Death, and Geopolitics in the New Arctic to mostly contain information I had seen elsewhere. In fact, it is chock full of novel and interesting details on everything from marine food webs to international law to oil field development plans. I read the first 200 pages in one sitting.
One chapter goes to some length in describing how we know what we do about Arctic sea ice volume. It is harder to measure than the extent of sea ice, which can be observed in all sorts of ways by satellites (optical instruments, synthetic aperture RADAR, passive microwave emissions, etc). One effort to estimate how ice volume is changing was based on multibeam SONAR on submarines. An 11 day survey conducted by Peter Wadhams, using the nuclear-powered HMS Tireless concluded that 40% of Arctic sea ice has been lost since the 1970s. Another team, led by Drew Rothrock, used previously secret US submarine data to confirm that figure for all areas that submarines have been visiting.
Anderson also describes the importance of the cold halocline layer: a thin layer of cold water that insulates the bottom of Arctic ice from the warmer Atlantic waters underneath. Without this layer, multiyear Arctic ice would be doomed. For a number of reasons, climate change threatens to undermine it. If it does, the complete disappearance of summer sea ice could occur faster than anyone now expects.
There are many reasons to worry about the vanishing Arctic ice, from the increased absorption of solar radiation that accompanies lost albedo to the danger of invasive species entering the Atlantic from the Pacific. I’ve written previously about ‘rotten’ ice, and many other issues in Arctic science.
“Satellites have long been used to track ice extent (area), but calculating the thickness of the marine floes requires the overflying spacecraft to gauge the difference between the top of the ice surface and the top of the water – a relatively simple calculation then gives the overall volume.
Radar altimeters flown on missions previous to Cryosat have not had the resolution to do this as precisely as researchers would like, and have not flown far enough to the north to get a full view of the Arctic basin.
Cryosat, however, will see finer detail, and its “vision” will cover virtually all of the Arctic. Its orbit will leave just a 400km-wide circle at the pole that is out of sight of its instrument.”
the cold halocine layer
I think you mean “halocline”. I’m no expert; this post introduced me to the term and I had to look it up.
Fixed, thanks.
The eventual demise of the summer sea ice is a common feature of nearly every climate model projection (the exceptions are models with very inappropriate initial conditions). But the question of when the Arctic will be ‘ice-free’ is imprecise and calls for a clear definition of what ice-free means. Does it mean completely ice-free, or is there a minimum threshold implied? Does it mean the first time the summer sea ice goes beneath this threshold or does it imply a probability of encountering low-ice conditions over a period of time? (e.g. high likelihood of Septembers with less than 106 km2 of ice in a 10-year period). Regardless of whether the concept is actually useful for any practical purpose (say for planning shipping across the Arctic), it is nevertheless a powerful image in communicating the dramatic changes that are under way in the Arctic.
Once defined, predictions of when an ice-free Arctic will occur seem justified. In the published literature there are several papers specifically targeting such predictions (Zhang and Walsh, 2006, Wang and Overland, 2009, Boe et al. 2009, Zhang et. al. 2010) while others include discussion about the timing of ice-free summers (e.g. Holland et al. 2006). Some address the fact that the CMIP3/IPCC AR4 simulations show sea ice declines less rapid than the observations and attempt to correct for it. Published projections, though with varying definitions of what constitutes ice-free, all project an ice-free Arctic ocean somewhere between 2037 (Wang and Overland, 2009) and the end of the century. Predictions of earlier ice-free dates so-far seem to be confined to conference presentations, media-coverage, the blogosphere, and testimony before to the UK parliament.
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In summary, we think that expressing concern about the future of the Arctic by highlighting only the earliest estimates of an ice-free Arctic is misdirected. Instead, serious effort should be devoted to making detailed seasonal-to-interannual (initial-value) predictions with careful evaluations of their skill and better estimates of the climate-forced projections and their uncertainties, both of which are of considerable value to society. Some effort should also target the formulation of applicable and answerable questions that can help focus modeling efforts. We believe that substantially skillful prediction can only be achieved with models, and therefore effort should be given to improving predictive modeling activities. The best role of observations in prediction is to improve, test, and initialize models.
But when will the Arctic be ice free then? The answer will have to come from fully coupled climate models. Only they can account for the non-linear behavior of the trajectory of the sea ice evolution and put longer term changes in the context of expected natural variability. The sea ice simulations in the CMIP5 models are currently being analyzed. This analysis will reveal new insights about model biases, their causes, and about the role of natural variability in long-term change.It is possible that this analysis will change the predicted timing of the “ice free summers” but large uncertainties will likely remain. Until then, we believe, we need to let science run its course and let previous model-based predictions of somewhere between “2040 and 2100″ stand”
http://www.realclimate.org/index.php/archives/2012/04/arctic-sea-ice-volume-piomas-prediction-and-the-perils-of-extrapolation/