Here´s why finding out the Gulfstream could be quite stable wouldn´t necessarily be such good news: under continued warming, the positive feedback of increased tundra peat soil CO2 and methane emissions far outweighs the negative feedback of ‘taiga creep’ and CO2 fertilisation.
Under continued climate change the Arctic tundra and boreal ecosystems turn from a net carbon sink into a net carbon source, new PNAS publication states. Both pictures show result for 2100. Left excludes carbon soil processes [and moves from sink to close to carbon neutral], right picture includes permafrost thaw and other soil processes, drastically increasing the total carbon disturbance.
Digging deeper into the tundra
A team of scientists from French, American, British and Canadian research institutions have conducted a model study into the carbon balance of the Arctic and boreal ecosystems, that was published last week in PNAS.
They think they have successfully incorporated organic soil processes to get a full picture of the carbon flux response to climate change. Previous model studies have largely ignored the huge subsurface carbon stores and may have put too much emphasis on vegetation-driven processes.
Quantifying the Arctic carbon feedback
The above-mentioned percentage of carbon that could be released is the model’s best estimate when including all permafrost soil processes. Under continued warming [IPCC SRES A2 scenario – that’s the bad boy of the four, but better still than actual emissions trends in reality] the Arctic would turn from a net sink to a net source of carbon – and eventually release some 62 gigatonnes of pure carbon over the course of this century.
To place this in the global carbon cycle perspective: if all the carbon would neatly turn to CO2 that would be the equivalent of adding 7 or 8 more years of unabated fossil fuel emissions, based on CO2 record year 2010. That would be bad enough.
If however the majority of these carbon molecules would decide to enter the atmosphere as methane, the associated climate forcing of tens of gigatonnes of carbon could well add a full degree to the global temperature rise – or even more.
From thaw to drought?
And with a thawing swamp the size of a small continent, there is little to be sure about what ratio to expect of the two Arctic greenhouse gases, the authors of the new study say. If we base our best guess on their model though, there is some reason to be a little less pessimistic:
“Whether carbon comes out as carbon dioxide or methane is dependent on hydrology and other fine-scale processes that models have a poor ability to resolve,” says Charles Koven, of the US Lawrence Berkeley National Laboratory and the French Laboratoire des Sciences du Climat et de l’Environnement, of the Centre National de la Recherche Scientifique.
“It’s possible that warming at high latitudes leads to drying in many regions, and thus less methane emissions, and in fact this is what we found.”
Although methane emissions would decline with a loss of Arctic wetlands, they would rise under CO2 fertilisation, permafrost thaw [simply releasing bubbles of methane, stuck under the ice] and warming-induced anaerobic biomass breakdown. These processes combined will according to the model lead to at most a doubling of Arctic methane emissions, from 34 megatonnes of methane per year, to an annual 70 Mt by the end of the century. [As a climate forcer that would be equivalent to a about 1.6 gigatonnes of extra CO2 per year.]
Downside of up
There would also be a downside to the reduced wetlands scenario though. If indeed the drought phenomenon would enter the frozen wetlands, we could witness the tundra wildfire climate feedback, as another PNAS study recently warned us. That would not just release yet more CO2, a lot faster – it would also further damage the permafrost, which could translate to the release of extra methane.
And even that would not be the end of the Arctic carbon story. Tundra wetland methane is not the only Arctic methane. On the margins of the American and Eurasian continental shelves there are clathrates too. But perhaps if we keep quite about these, we won’t wake them up either.
© Rolf Schuttenhelm | www.bitsofscience.org