Tuesday, December 23, 2014

The (Strong) Ice-Albedo Feedback of Melting Arctic Sea Ice

There was a press conference at the AGU meeting on the Arctic sea ice report, which I wrote about here.

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.

1 comment:

Russell Seitz said...

Did you notice the AGU session devoted to addressing the problem:

Here's the abstract of the presentation co-authored by its chair :

Geoengineering Using Oceanic Microbubbles

Julia Anne Crook1, Lawrence S Jackson1, Annabel Ka Lai Jenkins1 and Piers Forster2, (1)University of Leeds, Leeds, LS2, United Kingdom, (2)University of Leeds, Leeds, United Kingdom

The most commonly studied solar radiation management schemes reduce the amount of solar radiation absorbed by reflecting more sunlight in the stratosphere (eg. stratospheric injection of SO2) or by making marine clouds more reflective (eg. injection of sea salt into the marine boundary layer). Both these schemes require technologies that do not currently exist and involve polluting the atmosphere. An alternative scheme which brightens the surface of the ocean without the use of chemicals has so far been largely overlooked. The technology already exists to efficiently create 1ยต radius bubbles in water. Such bubbles could enhance the albedo of open sea by 0.2 and have a lifetime of the order of days (Seitz, 2010). The top of atmosphere radiative forcing produced by the wakes of existing large ocean going vessels has been estimated to be 0.14 mWm-2 (Gatebe et al, 2011). However, existing ships are not optimised to produce such small bubbles; their wake lifetimes are in the order of minutes and their albedo gains are of the order of 0.02. If fitted with existing bubble generation technology, the current fleet of cargo and merchant ships could provide a surface radiative forcing of around -1 Wm-2.We present results from a climate model simulation implemented with this enhanced albedo in current shipping lanes. Currently ships are more abundant in the Northern Hemisphere but a larger and more uniform forcing could be produced by sending out extra ships with bubble generators in the more sparsely populated Southern Hemisphere oceans. Our climate model simulation with a uniform open sea albedo enhancement of 0.03 had an effective radiative forcing of -2.6 Wm-2 and reduced global mean temperature by 1.6 K, enough to offset global mean warming under RCP4.5 for at least 40 years.