Guest Post by Willis Eschenbach
I wrote a post called I Used To Be Snow White (… but then I drifted) a week or so ago about a study titled “Impact of Declining Arctic Sea Ice on Winter Snowfall” (PDF) that claimed to link low arctic ice levels with high snow levels. To recap, their specific claims were:
While the Arctic region has been warming strongly in recent decades, anomalously large snowfall in recent winters has affected large parts of North America, Europe, and East Asia. Here we demonstrate that the decrease in autumn Arctic sea ice area is linked to changes in the winter Northern Hemisphere atmospheric circulation that have some resemblance to the negative phase of the winter Arctic Oscillation. However, the atmospheric circulation change linked to the reduction of sea ice shows much broader meridional meanders in mid-latitudes and clearly different interannual variability than the classical Arctic Oscillation. This circulation change results in more frequent episodes of blocking patterns that lead to increased cold surges over large parts of northern continents. Moreover, the increase in atmospheric water vapor content in the Arctic region during late autumn and winter driven locally by the reduction of sea ice provides enhanced moisture sources, supporting increased heavy snowfall in Europe during early winter, and the northeastern and mid-west United States during winter. We conclude that the recent decline of Arctic sea ice has played a critical role in recent cold and snowy winters.
I showed that if there is such an effect, it is not visible using the snow data for the whole US. I thought this would settle it. But folks said, and fairly, that I wasn’t really dealing with their claim. They said I needed to deal with a) the regional nature of their claim, involving northeastern US and Europe, and b) the temporal nature of the claim, comparing only winter snowfall to autumn sea ice. So I decided to take a look at the winter snow data for the northeastern US compared to autumn sea ice.
Unfortunately, I couldn’t find area-wide data for the northeastern US, but I did find something better. This is one of the longest continuous records of snowfall in the northeastern US—the century and a half long record of the snowfall in Central Park in New York City.
So … how well does this correlate with the arctic ice levels? Well, not to put too fine a point on it … no better than my first look at the question.
Here’s the comparison of the snow and ice. I have standardized both of them so that we can compare them directly.
As you can see, there is little correlation, and the numbers bear that out. There is a weak statistical relationship (r^2 = 0.13) which is not significant at the p<0.05 level.
I thought that because there is no trend in the snowfall data, perhaps I might get better significance if I detrended the ice data. This would highlight the year by year variations that are theoretically responsible for the year-by-year variations in snowfall. Figure 3 shows that relationship, with the ice data inverted to better illustrate the relationship.
Now, this is a very interesting figure, because it illustrates the way that our eyes find patterns when none are there. At first glance, this looks like it is a pretty good relationship. But in fact, that is an illusion. The mathematical analysis says that the r^2 is even worse, only 0.02, and like the previous graph, it is also not statistically significant, in fact the significance is worse (p ≈ 0.4).
Upon closer examination, we can see why that is so. For example, from about 1990 to 1995 when ice decreased, snow generally increased … but not proportionally. When the ice was extremely low there was a little more snow, and when the ice was only a little low, there was a lot more snow. For there to be a relationship, it needs to be proportional. Also, although in general the snow seems to change with the ice, in fact on a year by year basis, there are huge excursions. Look at 2011, for example, very low ice, but in contradiction to their claim, there’s also very low snow.
So I have found the same thing on a regional level using their autumn ice/winter snow claim, that I found when I looked at the data for the entire US for the full year. If there is any association between winter snowfall in the northeastern US and the autumn ice levels, it is very, very weak. There certainly is no sign of it in the Central Park records.
All the best,