During the recent discussion of ice albedo and how strongly the warming influence of northern hemisphere sea ice loss outweighs the cooling influence of southern hemisphere sea ice gain, it was mentioned by several readers that snow loss in the northern hemisphere is also a major warming influence, a potent amplifying feedback of global warming.
The Rutgers Univ. Global Snow Lab has data on snow cover during the satellite era. Like most climate-related variables, snow cover shows a strong seasonal cycle, with more snow in winter and less in summer. When looking for trends, it’s useful to compute anomaly, the difference between a given month’s value and the average for the same month throughout the data record. Here’s the anomaly data for northern hemisphere snow cover:
There is a clear, and statistically significant, overall decline in snow cover. In spite of what we often hear from the fake skeptics — who love to bellow about any big snowfall (even when it hasn’t happened yet) as though it were disproof of global warming — the actual trend in snow cover is one of decline.
More important, the declining trend is strongest when it really counts — when incoming sunlight is strongest during summer. Snow is very highly reflective, and when present tends to reflect much of the incoming solar energy back to space, which has a cooling influence on the climate. As snow cover declines, so does this cooling influence.
In fact the downward trend in snow cover is strongest during the month of June, when solar input is also strongest:
The trend accounts for a net loss of over 5 million km^2 June snow cover since 1979. That’s considerably larger than the loss of Arctic sea ice over the same time span.
In fact the snow loss has been greatest during all the months when it counts most, i.e., when solar input is strongest:
The graph shows the net snow cover change due to trend since 1979. For those (like me) who prefer a time series plot, here it is:
A few months (most notably December) have shown snow cover increase, albeit not statistically significant. But the increases have all been in winter when solar input is weakest so in terms of climate feedback, it counts least. There’s no doubt that the albedo feedback due to snow cover change has been sizeable, and has tended to warm the planet.
It has tended to warm the Arctic more than any other region of the planet. Which is probably one of the reasons that the Arctic is warming so much faster than the globe as a whole.
The cryosphere — the frozen parts of earth — is the “canary in a coal mine” of earth’s climate. Every part of it, snow cover, the sea ice, the great ice sheets in Greenland and Antartica, the mountain glaciers, ice cover on lakes and rivers, even the permafrost, is sending a message loud and clear. Our entire planet is changing. Heed its warning.