Monday, September 24, 2012
Retrieved from National Snow and Ice Data Center (NSIDC)
On September 16, 2012, the arctic reached a record low for sea ice cover. Now, it is finally expanding as autumn arrives, but it is still well below average.
The sea ice cover this year fell to 3.41 million kilometers, which is 18% less than the previous record for lowest ice extent, which was set in 2007. It is also 49% less than the median for 1979-2000, which is outlined in the orange. This is the fifth consecutive year, and the fifth time in recorded history, that ice-free navigation was possible from the north coast of Canada to the north coast of Siberia, and, as you can see, this navigation window was massive.
Retrieved from Chapman & Walsh (2001), and updated by the University of Illinois Cryosphere Today. In addition, Wunderblog blogger Jeff Masters updated the graph to include 2011 and the first nine months of 2012.
As the above graph shows, this event, while record-setting, is by no means an outlier. The loss of sea ice has been accelerating since the 50s, and the sea ice has drastically declined in the past five years or so. An important measurement to point out is that the winter sea ice cover has changed relatively little – from ~15.5 million square kilometers to 14.5 square kilometers. However, the summer ice extent has dropped drastically – from ~11 million square kilometers to ~ 6 million square kilometers.
This graph begs the question: why has the change in summer ice been so much more drastic than winter ice? Well, this graph measures ice extent
, and not the thickness of ice. As the Earth becomes warmer due to global warming (which is very real and very serious – see my post here
), the ice thins, so it takes less heat to expose a certain amount of water. Add this with the positive feedback of the albedo of water being much lower than the albedo of sea ice, and you have a recipe for disaster… a polar ice cap that is dying before our very eyes. In the mid 2000s, the scientific consensus was that the arctic would be ice-free in the summer by 2040 or 2050. But as there has been a drastic decline in sea ice in the past five years, most scientists predict that arctic summer sea ice will be completely gone by 2030, and some radical estimates show it being completely gone by as soon as 2016.
This general sequence of editing previous estimates to become more dire is pretty common, especially when it comes to ice caps and sheets, both on land and on the ocean. The summer sea ice extent has declined at ~12% per decade, which is far more than the 2007 IPCC’s “worst case scenario.”
Retrieved from Kinnard et al.‘s 2011 paper: Reconstructed changes in Arctic Sea Ice over the past 1,450 years.
Here’s a longer term graph of average sea ice extent. Needless to say, this type of warming is unprecedented in this graph, and according to climate proxies such as ocean sediments and ice cores, perhaps much longer.
Interestingly enough, in the article I got this information from showed that the Antarctic sea ice extent was actually growing instead of shrinking, even as the air temperature by the southern oceans has begun to rise.
Why is this? Bear with me here, because the reasons are complex. First off, the water at the surface of the Antarctic Ocean (specifically the Weddell Sea) is actually colder than the deeper water because of the cold air temperatures at the surface. This warm water upwells and melts sea ice. However, there has been an increase in precipitation in recent years as the air temperatures have warmed. This does two things: 1.) the snow acts to expand ice caps, and 2.) the surface water freshens because of the increased precipitation, which leads to a more stratified ocean where the water mixes less easily, and since the decreased salinity in the surface ocean decreases the density, the water from the surface ocean does not sink as easily, and the warmer water from the ocean bottom, which acts to melt the ice, rises more slowly, so less ice is melted.
It is very important to look at everything that is changing in our climate, whether there is a decrease or increase in temperature and/or ice caps. This can go both ways… global warming deniers will specifically point out places where ice is expanding (like the Antarctic) and say that global warming is nonsense, and some global warming activists only point out places where the Earth is warming and ice cover is decreasing. Both of these methods are flawed. The only way we can definitively prove that the Earth, as a whole, as warming, is by looking at everything that is happening to our climate and taking an unbiased, scientific approach to an environmental change unlike anything the Earth has experienced for a very long time.
We have to go back 55 million years ago to the Paleoeocene – Eocene Thermal Maximum (PETM) to find an analog for the climate change the Earth will undergo in the future. But we have to be logical and scientific in our analysis of the problem, and we cannot specifically pick out data that supports global cooling or global warming. We have to look at all the data.