The latest RealClimate post, which describes the latest ice melt data from Greenland, features this really important figure. It illustrates an issue that arises every semester in my climate change course, and is, in a sense, fundamental to understanding to biogeochemical cycles and issues like why carbon is accumulated in the atmosphere.
The figure shows model-based annual anomalies (thanks ED) of snowfall (reddish-orange), water loss through surface melt and runoff (yellow) and net accumulation of mass (blue), all in Gt/yr. The key point is that an ice sheet shrinks not simply because it is melting (yellow), but because the loss of water through melt (yellow) is greater than the gain through snowfall (red). The difference is the change in mass (blue). It is an accounting problem; like your bank account, you need to look at the debits and the credits to know whether the balance is changing.
This is the same fundamental concept that underlies carbon dioxide accumulation in the atmosphere, and as MIT management expert John Sterman has shown, befuddles most people. Carbon dioxide is accumulating in the atmosphere not simply because we are burning fossil fuels and clearing land, but because the flux in to the atmosphere from those sources is greater than the flux out (to land, and the oceans).
As an aside, in my climate change course, one of the many ways we discuss these points is by watching the infamous "CO2 is life" advertisements created a few years back by the Competitive Enterprise Institute. The "Glaciers" video cites scientific evidence for snowfall-driven growth of ice sheets in the interior of Antarctica to suggest that ice sheets around the planet are not shrinking. The mistake in the ad is that in order calculating whether an ice sheet is shrinking, on net, you need to do the full accounting of all inputs (from snow) and all the outputs (from melt), not just cherry pick one part of the ice sheet, or one flux in or out.
The CEI ads, by the way, are hilarious. If you've not seen them, you really must. They are well worth two minutes out of your day; the Onion News Network couldn't have come up with fake ads that funny.
Thursday, September 22, 2011
Climate change: An accounting problem
Friday, September 19, 2008
Watching the cryosphere
The Arctic sea ice appears to have reached the minimum extent for this season, short of last year's record. If you just can't wait another eight or nine months for more news about the shrinking cryosphere, never fear. There a several sites monitoring the movement of mountain glaciers and the Greenland Ice Sheet.
The Extreme Ice Survey has some fantastic still (and time lapse photos) from cameras set up on a few mountain glaciers and in Greenland. And I just got a note from someone at Sermitsiaq, a Greenlandic newspaper about a new web cam that offers the opportunity to "watch as Greenland melts". Be sure to get yourself a comfortable seat, the melting may take a while.
Tuesday, September 11, 2007
Ice melt in Greenland
There's been plenty of discussion here and elsewhere about the current record lows for sea ice in the Arctic. A few days ago, the Guardian reported about evidence for rapid melting in Greenland as well this summer. The article reminded me of this picture I took in July of this meltwater pool on the slope of a east-flowing glacier in SE Greenland.
The IPCC's method for estimating of sea level rise do not account some of the processes that are now being observed and described in the article, particularly how meltwaters appear to be flowing to the bottom of the ice, effectively lubricating the path to the sea. There's a big push afoot to incorporate these more complex ice melt processes into global climate models. In a few years, the model predictions for sea level rise over the next century could be quite different.
