BOE: Quantifying Catastrophe … Now with CH4!!!

The fact that the ice core records do not seem full of methane spikes due to high-latitude sources makes it seem like the real world is not as sensitive as we were able to set the model up to be. This is where my guess about a worst-case 1000 Gton from hydrates after 2000 Gton C from fossil fuels in the last paragraph comes from.

Much ado about methane (Real Climate, Archer, Jan 4, 2012)

Archer’s baseline ‘worst case’ of 2000 Gt C * 3.67 ~= 7340 Gt CO2 falls remarkably close to our previous guesstimate of a “most likely” release of 6100 Gt CO2 in future consumption plus 1000 Gt CO2 in previous consumption. So what would an additional 1000 Gt of C (3670 Gt CO2) arising from methane clathrates look like in the ECS/CO2 cumulative ‘PDF‘-omatic developed in the previous BOE posts?

Archer’s comment in the RC post seems to indicate that the airborne fraction has already accounted for. I believe this due to this comment in Archer’s Time-dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing (2005): The model thus constrained predicts methane fluxes of 200 Gton C or less on deglaciations, but eventual releases of 2000–4000 Gton C in response to a
2000 Gton C anthropogenic carbon release..

This ‘worst case’ is a bit of a rule of thumb. If clathrate response was much higher than this, Archer believes, the Earth’s climate would have likely tipped into run-away warming modes previously. So, using this 1:2 ratio, we can rerun the PDF-omatic developed in previous BOE posts. Given that our “most likely” future CO2 release resident in the atmosphere (ie … minus airborne fraction) is about 3000 Gt CO2, we initiate the methane scenario with an additional 1500 Gt CO2 or (roughly) an additional 200 ppm.

As seen in the chart above, the most likely warming increases 1.1 deg C from 3.7C to 4.8C. The PDF got rather choppy, probably due to a too coarse resolution. I’ve fitted them with a gamma distribution. Not only is the max temp at peak probability raised, but the high-end tail is fattened. The probability of seeing warming greater than 6C and over 1000ppm rises from 10% to 32%. The chance of more than 4C warming rises from 60% to 77%. And the chance of seeing CO2 concentrations > 1200 PPM rise from near zero to 24%.

Bottom line – given the IPCC AR4 ECS and a gaussian distribution for fossil fuel consumption centered on an additional 6000 Gt CO2, we are quite likely going to see a return to a Miocene climate with global temps running about 4C warmer and CO2 near 500 PPM. If there is strong methane feedback, we may see a return to early Cenozoic conditions with warming over 6C and CO2 concentrations above 1000 PPM.

This leads to questions about decay rates. Does CO2 have an “effective” half life driven by ocean cycling? Biosphere? Geology? Will the resulting changes in albedo, glacial and ice sheet melting, and changes in ocean chemistry put an effective end to the Quaternary? Or will glacial cycles reassert themselves ending even this interglacial now known as the Anthropocene?

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