Antarctica- when does it melt?

The short answer: at 1000 ppm CO2.

Global warming has many consequences, most far more momentous for other organisms than ourselves. Whole ecosystems will die and regress to more primitive networks and members. But real estate agents have cause for concern as well, as sea levels rise and innundate low-lying areas. When and how high?

Sea levels have already risen ~20 cm over the last century. But far more is coming, as CO2 levels continue to rise along what seems to be a "business as usual" trajectory. The IPCC has offered various optimistic scenarios of international cooperation which have all come to naught. Coal is being burned at record rates. The critical graph is readings of atmospheric CO2, courtesy of Wikipedia:

From a pre-industrial level of ~285 ppm, already high by the standards of the last few hundred thousand years of the ice ages, we have broken through to almost 400 ppm. (ppm is parts per million, or 0.0001%, so the current percentage of CO2 in the atmosphere is about 0.04%).

The atmosphere weighs about 5E18 kg, so each 0.01% or 100 ppm is about 5E14 kg, equivalent, in terms of wood, to 3.1E14 kg, which in terms of average forest density (~2400 kg per 100 sq m forest) corresponds to about 13 million sq kilometers of forest.

The land area of Earth is ten times that amount, which I hope offers some useful scale to the problem. About half our emissions are taken up annually by the oceans and forests, so the true scope is twice that size.

At any rate, where are we going? The IPCC graphs indicate that, barring action and assuming that CO2 emissions in 2050 are roughly double what they are now (scenario gray / VI), we would get to roughly 1000 ppm around 2100.

Click to see full size. The main point is the choice of remediation scenarios that get us (right graph) to various ultimate atmospheric concentrations of CO2. Brown and gray are the business as usual scenarios that we are currently following. 

The prospects of serious sea level rise come from the various frozen forms of water stored around the world. The IPCC consensus has sea levels rising only about 70 cm by 2100, but to me the dangers seem far more severe. To learn what a full melting scenario would mean, the USGS helpfully supplies the details:

• Greenland: 7m rise
• Antarctica west ice sheet- 8m
• Antarctica, rest-    65m
• Other glaciers, etc: 0.5m
• Thermal expansion- 1m

            Sum = 80 meters, or 262 feet.

So if everything were to melt, we would be in serious trouble. Whole states would practically disappear. This is quite aside from the many other brutal effects of such climate change all over the biosphere.

"A sea-level rise of 10 meters would flood about 25 percent of the U.S. population" 

Here is where a recent paper comes in, analyzing how Antarctica got so cold and snowy in the first place. We know that Antarctica iced over around 34 million years ago, but the precipitating circumstances (ouch!) have been in some dispute. Specifically, it is difficult to accurately estimate the atmospheric CO2 concentration from various fossil / chemical / geological traces. These authors focus mostly on better ways to deduce the CO2 record around this time, refining an estimate which indicates that atmospheric CO2 decline was the central driver of this process, and that falling below about 1000 ppm was the critical event.

This is the main graph, showing their inferred CO2 levels (colored circles) through the time at issue. The inset shows finer detail. Note that time goes backwards in the reverse direction, from recent to ancient. Obviously there is a noticeable decline around the time of Antarctic glaciation, and low atmospheric CO2 persists thereafter, lowering further (off the graph) going into our more recent epoch of ice ages. The gray lines are data from others, showing inferred CO2 levels from other analyses (∂18Oxygen in organic sediments, rather than the carbon isotope analysis the authors here focused on). The latter is more dramatic, but generally on the same trend.

Incidentally, Antarctica was pretty much in its current tectonic position by this time in Earth history. All this adds up to strong historical case that 1000 ppm CO2 is a plausible breakeven point for Antarctic melting. Such melting wouldn't happen overnight- it may take centuries, depending on how far over 1000 ppm CO2 we go. But clearly, among many other problems we are bequething to posterity is the likelihood that, if we continue along the business as usual trajectory, doing nothing about fossil fuel use, we will end up in hot water.

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