At question time I asked what was the effective global radiative forcing of this ice loss, due to the ice-albedo feedback -- ice reflects sunlight (that's why it's white), and dark sea water absorbs it. So as ice melts the region absorbs more heat, a positive feedback on AGW.
The speakers thought someone must have calculated this, but didn't know the number themselves. A few scientists who were in the crowd sideled up and gave me their business cards and offered to talk about it. One of them pointed me to this paper that came out earlier this year:
K. Pistone, I. Eisenman, and V. Ramanathan (2014). Observational determination of albedo decrease caused by vanishing Arctic sea ice. Proc Natl Acad Sci USA 111, 3322-3326.This paper calculates the effective global radiative forcing due to melting Arctic sea ice,and their result is astonishing, to me at least:
We find that the Arctic planetary albedo has decreased from 0.52 to 0.48 between 1979 and 2011, corresponding to an additional 6.4 ± 0.9 W/m2 of solar energy input into the Arctic Ocean region since 1979. Averaged over the globe, this albedo decrease corresponds to a forcing that is 25% as large as that due to the change in CO2 during this period, considerably larger than expectations from models and other less direct recent estimates. Changes in cloudiness appear to play a negligible role in observed Arctic darkening, thus reducing the possibility of Arctic cloud albedo feedbacks mitigating future Arctic warming.25% as large as CO2's extra forcing! I find that remarkable -- I never would have guessed the Arctic ice-albedo feedback could be that large.
Of course, Antarctic sea ice is increasing (probably due to changes in winds, some rushing down off the heights of Antarctica -- no many people realize that the elevation of the South Pole is quite high: 9,300 feet.) Eric Steig just wrote about this on RealClimate, and a separate paper just came out.
Using the NSIDC's daily data on Arctic sea ice extent, I calculate the linear trend from 1979 to 2011 is -60.1 Kkm2/yr.
Integrating that from 1979-2011 givea a loss of -1,980 Kkm2.
The albedo difference between sea ice (0.5, according to Pierrehumbert's textbook, pg 154)) and open ocean water (0.1) is 0.4.
However, the calculation of the equivalent global radiative forcing is much more complicated by the angle of sunlight in the Arctic, and the fact that it disappears completely for about six months of the year. (WUWT's chief pretend-scientist Willis Eschenbach just badly barfed on that fact.)
So really the calculation requires knowing where the ice is (its range of latitude) for all individual times, so you can do the integral. That's pretty difficult, which is why this Pistone paper is a good one.