Oct 01 2007
Arctic Sea Ice Anomaly
Tamino has a new post about the lack of sea ice currently in the arctic. I’m not sure why arctic sea ice extent has been featured so much lately in the blogging community, but I best get on the same page as everyone else, right?
Below is a figure stolen borrowed from the most recent (as of 1 October 2007) Tamino sea ice extent post.
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Notice the last three data points; they are well below any of the other measured points. It appears that each data point represents a monthly average of sea ice extent anomaly. The post does not clarify if it is indeed monthly data (it should be since three points have been added in the last three months), but more importantly, it does not say if it is the mean anomaly or the minimum. It doesn’t really matter, but I’m curious.
The post makes it seem that this degree of sea ice loss is unprecidented. Indeed, the magnitude from the mean is unprecidented. However, the rate of change does not appear to be any greater than in the past. For instance, near 1996 there is a sudden increase in the sea ice extent, followed quickly by a sudden decrease in the sea ice extent. I don’t have the data, so I’ll make some estimates based entirely on visual inspection of the graphs.
At the start of the rise in temperatures around 1996 the sea ice extent anomaly is around -0.5 million km^2. In four months, it jumps to over +1.0 million km^2. This represents a rate of growth of 0.375 million km^2 / month. Next, it decreased back down to around -0.5 million km^2 in 2 months. This is a growth rate of -0.75 million km^2 / month.
If we look at the latest 4 months of data, we see that sea ice extent has decreased by about 2.0 million km^2. This represents a growth rate of -0.5 million km^2 per month. This is the same rate of loss of sea ice extent as was calculated for the spike near 1996.
As the Northern Hemisphere moves from summer to winter, the arctic sea ice will grow. How much will it grow? Will it reach the same maximum extent as last winter? I don’t know. It’s clear that the long-term trend is downward; in 10 years there will be less ice than this year. However, we should be cautious in interpreting this as anything other than what it is: 3 months of data in a clearly noisy dataset. I think it is best to stress the long-term loss of arctic sea ice instead of the monthly fluctuations.
In the comments of the thread at Tamino, jre asks some good questions:
Given what we have observed recently in the Arctic ice sheet, is it possible — even likely — that rapid changes in a land-bound ice sheet could occur with even a small perturbation? How about an ice sheet that rests on bedrock under sea level?
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2 Responses to “Arctic Sea Ice Anomaly”
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I find jre’s question to contain a confusion in terms. What the graph presents is Arctic SEA ICE anomalies. It is seawater that has frozen and is floating on the ocean. Ice sheets are quite different. They are glacial in origin, i.e. the results of accumulated snow fall on land, sliding into the sea. Whether an ice sheet rests on sea floor or is floating will certainly change its behavior, but it will remain very different from sea ice, follow different dynamics and indicate other things than frozen sea water.
What happened this year to sea ice does not necessarily give any usable info to predict the behavior of ice sheets such as the ones in Greenland.
[...] previously posted about the arctic sea ice anomaly, stressing that we should be cautious in interpreting the short-term fluctuations in a clearly [...]