Reason #1: Because the extent is more than two standard deviations below the long term average.
Reason #2: Because the extent is lower than at the same point in 2007, which reached the lowest extent on record (though which had an usually rapid melt in late June).
Reason #3: Because it is only measuring extent, not volume.
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.
…
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″ standâ€
Though the extent of the September sea ice did bounce back a little from 2007’s nadir, in every year since then the minimum has been lower than it was in every year before 2007. And this year 2007’s record has not been merely broken, it has been smashed.
…
This is all the more surprising because 2012 has in other ways been a pretty ordinary year in the Arctic. In 2007 the summer weather was particularly inimical to the persistence of ice, with lots of warm southerly winds and clear skies that allowed the sunshine to do its worst. This year has seen far less in the way of special circumstances. It is true that a powerful cyclone chewed up a lot of ice in the East Siberian and Chukchi seas in early August—but the rate of ice loss outstripped that seen in 2007 both before the storm and after it.
The data is from the U.S. National Snow and Ice Data Center
Canada should have equally good Arctic science data available online.
Reason #1: Because the extent is more than two standard deviations below the long term average.
Reason #2: Because the extent is lower than at the same point in 2007, which reached the lowest extent on record (though which had an usually rapid melt in late June).
Reason #3: Because it is only measuring extent, not volume.
Also, due to the positive feedbacks involved in warming and ice melting. Melted ice begets more melting of ice.
The latest:
Why Climate Models Underestimated Arctic Sea Ice Retreat: No Arctic Sea Ice in Summer by End of Century?
ScienceDaily (Oct. 12, 2011) — In recent decades, Arctic sea ice has suffered a dramatic decline that exceeds climate model predictions. The unexpected rate of ice shrinkage has now been explained by researchers at CNRS, Université Joseph Fourier and Massachusetts Institute of Technology. They argue that climate models underestimate the rate of ice thinning, which is actually about four times faster than calculations. This model bias is due to the poor representation of the sea ice southward drift out of the Arctic basin through the Fram Strait. When this mechanism was taken into account to correct the discrepancy between simulations and observations, results from the new model suggested that there will be no Arctic sea ice in summer by the end of the century.
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.
…
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″ standâ€
http://www.realclimate.org/index.php/archives/2012/04/arctic-sea-ice-volume-piomas-prediction-and-the-perils-of-extrapolation/
Though the extent of the September sea ice did bounce back a little from 2007’s nadir, in every year since then the minimum has been lower than it was in every year before 2007. And this year 2007’s record has not been merely broken, it has been smashed.
…
This is all the more surprising because 2012 has in other ways been a pretty ordinary year in the Arctic. In 2007 the summer weather was particularly inimical to the persistence of ice, with lots of warm southerly winds and clear skies that allowed the sunshine to do its worst. This year has seen far less in the way of special circumstances. It is true that a powerful cyclone chewed up a lot of ice in the East Siberian and Chukchi seas in early August—but the rate of ice loss outstripped that seen in 2007 both before the storm and after it.
…
It is still possible that changes in wind patterns and longer-term natural climate shifts may slow the currently tumultuous process of decline. But according to Mark Serreze of NSIDC the system has entered a “new regime†in which, eventually, most of the ice will come and go every year, with little lasting the whole summer. September ice cover of less than 1m km2 could be normal within decades. That’s just France and Germany.