IPCC Sea Ice Forecasts

There has been a lot of discussion in recent years of the large drop in Arctic sea ice. People who are particularly concerned about CO2 induced global warming have pointed to this drop as clear current evidence that the effects of warming are accelerating. People who are generally skeptical about CO2 induced global warming have brought up the fact that Antarctic sea ice levels appear to be increasing as a counter argument. It has always seemed logical to me that global sea ice level was probably the most complete measure in this area, but I had read several informal documents that suggested that the Arctic was the true signature test for CO2 induced global warming.

In particular earlier this year a blogger wrote a post noting that global sea ice was at the same level as 1979. This generated a response from "Cryosphere Today." The response was that while the information was correct it was referring to the global trend and not on the Arctic trend. They point out that the reduced sea ice in the Arctic was being compensated for by increased ice in the Antarctic. The response includes the following. "In the context of climate change, GLOBAL sea ice area may not be the most relevant indicator. Almost all global climate models project a decrease in the Northern Hemisphere sea ice area over the next several decades under increasing greenhouse gas scenarios. But, the same model responses of the Southern Hemisphere sea ice are less certain."

I had heard statements like this before, but I had never looked to see what the basis of the statements might be. So I went to the AR4 WG1 report. On page 770 you can find chapter 10.3.3.1 "Changes in Sea Ice Cover." In that section you can find the following. "In 20th and 21st-century simulations, antarctic sea ice cover is projected to decrease more slowly than in the Arctic (Figures 10.13c,d and 10.14)." I have copied figure 10.13 below.

The top two projections are for the Arctic, and the bottom two are for the Antarctic. The black line represents the ensemble mean for the various scenarios and models. What is obvious from looking at this picture is that while the Arctic sea ice is projected to decrease more rapidly, Antarctic sea ice is projected to decrease as well. Not only that but the decrease should have been occurring fairly steadily in the last few decades, and increasing at this point.

In that section, which it is true is quite short, I found no reference to higher uncertainty regarding Antarctic sea ice. The final paragraph of the section just refers to the general uncertainty of projections including the amount of climate change in the polar regions in general.

So what evidence did CT have for their assertion? They link to a single 2005 study discussing a simulation where increased snow fall in the Antarctic tends to preserve sea ice.

So the consensus view as established by the IPCC is that while the Arctic is expected to decline more rapidly we should see declining sea ice at both poles. This means that increased Antarctic sea ice over the last 30 years is not in line with these projections. Does this prove that increasing CO2 does not cause global warming? Not even a little. But it does mean that looking at global sea ice level is relevant, and that focusing exclusively on Arctic sea ice might reasonably be considered cherry picking.