Guest Post by Willis Eschenbach
Inspired by this thread over at Bishop Hill’s excellent blog, I thought I’d write about sea ice. Among the many catastrophic things claimed to be the result of “global warming”, declining sea ice is one of the most popular. We see scary graphics of this all the time, things that look like this:
FIgure 1. Terrifying computer projections showing that we may not have any Arctic sea ice before the end of this century. Clearly, the implication is that we should be very concerned … SOURCE
Now, what’s wrong with this picture?
The problem with the picture is that the earth has two poles. And for reasons which are not well understood, when one pole warms, the other pole cools.
Looking at just the Arctic sea ice is like looking at someone who is pouring water from one glass to another and back again. If we want to see how much water there is, it is useless to observe just one of the person’s hands. We need to look at both hands to see what is happening with the water.
Similarly, to see what is happening in the frozen parts of the ocean, we need to look at global sea ice. There are several records of the area of sea ice. One is the Reynolds Optimally Interpolated dataset (Reynolds OI V2). A second is the National Snow and Ice Data Center (NSIDC) record. Finally, we have the Hadcrut Ice and Sea Surface Temperature dataset (HadISST1). All of them are available from that most marvellous resource, the KNMI data portal .
It turns out that the NSIDC and the HadISST1 records are nearly identical. The correlation between the two in the Arctic is 0.995 (1.0 is perfect agreement), and in the Antarctic it is 0.999. So in Fig. 2, I have not shown the NSIDC dataset, but you can imagine that there is a third record almost identical to the HadISST1 dataset. Here is what has happened to the global sea ice area from 1982 to the present:
FIgure 2. Global Sea Ice Area 1982-present. Data from satellite observations.
As you can see, while it is certainly true that the Arctic has been losing ice, the Antarctic has been gaining ice. And the total global sea ice has barely changed at all over the period of the record. It goes up a little, it goes down a little, it goes nowhere …
Why should the Antarctic warm when the Arctic cools? The short answer is that we don’t know, although it happens at both short and long time scales. A recent article in Science Magazine Online (subscription required) says:
Eddies and the Seesaw
A series of warm episodes, each lasting several thousand years, occurred in Antarctica between 90,000 and 30,000 years ago. These events correlated with rapid climate oscillations in the Arctic, with Antarctica warming while the Arctic was cooling or already cold. This bipolar seesaw is thought to have been driven by changes in the strength of the deep overturning circulation in the North Atlantic Ocean, but some have questioned how completely that process can account for the fine details of Antarctic warming events.
Keeling and Visbeck offer an explanation that builds upon earlier suggestions that include the effects of shallow-water processes as well as deep ones. They suggest that changes in the surface salinity gradient across the Antarctic Circumpolar Current were caused by the melting of icebergs discharged from the Arctic, which allowed increased heat transport to Antarctica by ocean eddies. This mechanism produces Antarctic warming of the magnitude observed in ice core records.
However, not everyone agrees that this is the full explanation. Henrik Svensmark adds another factor to what may be happening:
The cosmic-ray and cloud-forcing hypothesis therefore predicts that temperature changes in Antarctica should be opposite in sign to changes in temperature in the rest of the world. This is exactly what is observed, in a well-known phenomenon that some geophysicists have called the polar see-saw, but for which “the Antarctic climate anomaly” seems a better name (Svensmark 2007).
To account for evidence spanning many thousands of years from drilling sites in Antarctica and Greenland, which show many episodes of climate change going in opposite directions, ad hoc hypotheses on offer involve major reorganization of ocean currents. While they might be possible explanations for low-resolution climate records, with error-bars of centuries, they cannot begin to explain the rapid operation of the Antarctic climate anomaly from decade to decade as seen in the 20th century (figure 6). Cloud forcing is by far the most economical explanation of the anomaly on all timescales.
Regardless of why the polar see-saw is happening, it is a real phenomenon. Ignoring it by looking just at the Arctic leads to unwarranted conclusions about what is happening to sea ice on our most amazing planet. We have to look at both hands, we have to include the other side of the ice, to see the full situation. The real answer to what is happening to global sea ice is …