Sunday, September 2, 2012

The Antarctic carbon feedback: a worst case scenario

It's the single study to end all single studies:
The researchers estimate that 50 per cent of the West Antarctic Ice Sheet (1 million km2) and 25 per cent of the East Antarctic Ice Sheet (2.5 million km2) overlies preglacial sedimentary basins, containing about 21,000 billion tonnes of organic carbon.
If this carbon proves to be vulnerable in the way we now think the northern permafrost carbon is vulnerable, it will mean we could soon be literally incapable of stabilizing the climate without geoengineering.

The reason is very simple.  Based on the experience of the Eocene (above), at between 3 and 4 degrees above preindustrial, the Antarctica ice pack can be expected to melt completely (of the course question of exactly what temperature will lead to the deglaciation of Antarctica is a complex scientific question, while the above is an eyeballed guesstimate; see below). If these researchers are right, that will gradually release 21,000 gigatons of carbon.

So let's get down to brass tacks. The carbon is divided between the larger East Antarctic Ice Sheet and the more vulnerable West Antarctic Ice Sheet:
The researchers estimate that 50 per cent of the West Antarctic Ice Sheet (1 million km2) and 25 per cent of the East Antarctic Ice Sheet (2.5 million km2) overlies preglacial sedimentary basins, containing about 21,000 billion tonnes of organic carbon.
Experts place the chances of for the beginning of a total collapse of the WAIS in the next 200 years at upwards of 5%. Estimates of how long such a collapse would take to reach completion range from 300 to 1600 years. The temperature that would trigger that collapse is thought to be somewhere between +1C and +5C compared to 1990 temperatures (2).

Suppose that happened, and, since that's a fairly pessimistic assumption, let's throw in a couple of optimistic assumptions: the EAIS contributes nothing significant, and the northern permafrost and methyl hydrates contribute nothing.

The proportion of the trapped carbon under the WAIS is 4/9 of the total of 21,000 gigatons (about 9,300 gigatons) (1). If the WAIS melts, what proportion of that carbon will enter the atmosphere? Some will obviously remain locked a carbon sink that will develop as the continent warms. Suppose only 60% or so, 6,000 gigatons, ends up in the atmosphere.

A couple hundred billion tons of that will be methane; ignore it. The trapped methane has figured prominently in the news accounts about this research, but compared to the staggering amount of carbon under the ice, most of which would enter the atmosphere as carbon dioxide and the rest of which would be oxidized to carbon dioxide after a brief stint as methane, the news that we have a southern methane time bomb to go with the Arctic one is merely a footnote.

Suppose significant outgassing of the carbon under the WAIS starts in about a hundred years and takes two hundred years to run to completion. Six thousand gigatons over two hundred years is 30 gigatons of carbon per year (on average). By way of comparison, total fossil fuel emissions are running at about 7 gigatons per year:

Source
Thirty gigatons a year is about what the human race is expected to produce at the end of the century is the world economy grows rapidly despite our making no effort to rein in our emissions:

Source
The effect of 6,000 gigatons of carbon entering the atmosphere would be an extinction-level event. One part per million of atmospheric CO2 is equivalent to 2.13 gigatons of carbon. Assuming a 50% airborne fraction (which is incredibly optimistic) 6,000 tons of carbon entering the atmosphere would raise the CO2 concentration by about 1,400ppm. In a worst case in which the carbon sinks finally give up the ghost, the rise is 2,800ppm. Supposing we were at that point at 800ppm and +4C, that would quickly take us to +8C with the EAIS adding its share soon after.

You would be looking at centuries of further warming independent of human emissions -- leading to an increasingly inhospitable climate even if economic collapse reduced the anthropogenic contribution significantly.

There is a large distance between the publication of this one study and a solid scientific case for an Antarctic carbon bomb. The steps along that road are, roughly:

1. Are the authors correct about the scale of the buried carbon?
2. Are the authors right about the distribution of that carbon, specifically about a heavy concentration of it under the most vulnerable part of the ice sheet?
3. Will that ice sheet decay significantly over the next few centuries?
4. If the ice sheet decades, what proportion of the buried carbon will make its way into the atmosphere?
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1. "Potential methane reservoirs beneath Antarctica"  J. L. Wadham, S. Arndt, S. Tulaczyk, M. Stibal,  M. Tranter, J. Telling, G. P. Lis, E. Lawson, A. Ridgwell, A. Dubnick, M. J. Sharp, A. M. Anesio & C. E. H. Butler.  Nature 488, 633–637 (30 August 2012) -- abstract.
2. "The Future of the West Antarctic Ice Sheet: Observed and Predicted Changes, Tipping Points, and Policy Considerations" -- full text.
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UPDATE: Round up from around the interwebs.

 "Potential New Methane Risk in Antarctica" -- SustainableBusiness.com News

"Antarctica’s Hidden Carbon Stores Pose Warming Risk in Study" -- Bloomberg

“There’s a potentially large pool of methane hydrate in part of the Earth where we haven’t previously considered it,” Wadham said in a telephone interview. “Depending on where that hydrate is, and how much there is, if the ice thins in those regions, some of that hydrate could come out with a possible feedback on climate.”
"Large methane reservoirs suggested beneath Antarctic ice sheet" -- phys.org

 "Antarctic Methane: A New Factor in the Climate Equation" -- Climate Central
Wadham and her co-authors took soil samples from the margins of glaciers in both Antarctica and Greenland. “We spent a couple of years chain-sawing out sediments frozen into the bottom of the ice,” she said, adding with a connoisseur’s judgment, “They were very nicely preserved.”
They hauled the samples back to the lab and allowed them to thaw under carefully controlled, oxygen-deprived conditions where oxygen-hating, methane-belching bacteria known as methanogens could do their work — assuming they were there. And sure enough, the soil began producing methane.
The same thing should presumably be happening underneath Antarctica’s ice, where heat percolating from the depths of the Earth have prevented sediments from ever having frozen.

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