Several readers requested an approximation of the total climate forcing caused by the loss of ice and snow during the satellite era. OK
As before, this is a crude calculation but it should be correct to first order. The simplifying assumptions are that the combined snow+ice area is a circle around the pole, and that the insolation is that falling on the mid-latitude of the snow/ice pack on the mid-day of each given month. What’s computed is the total solar power falling on parts of the earth that are ice/snow covered. As more than one reader pointed out this isn’t really “insolation,” which is solar power per unit area — this is the total solar power (energy per unit time) hitting the top of the atmosphere over snow/ice regions. And the calculation is only for the northern hemisphere. I don’t have snow data for the southern hemisphere (but there’s far less snow-covered area in the south, since there’s far less land than in the north, especially at extreme latitudes, except for Antarctica).
First, here’s the total change in snow+ice covered area over the satellite era in millions of square kilometers, as estimated from the net change of the trend line:
The change has been greatest during exactly the months it really counts — during summer, when the incoming solar power is greatest. And the overall change has been a decline. An extreme decline. During June, the total snow/ice cover in the northern hemisphere has decreased by over 6 million km^2. The increasing trend during winter doesn’t reach statistical significance, but the decrease during summer does.
Here’s the annual average solar power falling on snow/ice covered areas, together with a linear trend (note this does not include the dramatic change in 2012, which isn’t over yet):
The net change estimated as the difference of the beginning and ending values of the trend line is about 880 TW. If spread over the entire surface of the earth, and if the difference in TOA albedo between snow/ice-covered and uncovered regions is 0.2, this accounts for a total climate forcing of about 0.34 W/m^2.
We can instead estimate the net change using a lowess smooth rather than a linear trend:
In this case the net change is about 1150 TW. If spread over the entire surface of the earth, and if the difference in TOA albedo between snow/ice-covered and uncovered regions is 0.2, this accounts for a total climate forcing of about 0.45 W/m^2.
These numbers are substantial, and as I say they’re only a first-order estimate based on some simplifying assumptions. But they’re in good agreement with other estimates from the literature, and certainly in the right ballpark.
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