Phil, as I noted before, simple regressions of the same ice core data tell us that with the current levels of methane the globe should be 40°C warmer than it is. Do you believe this is the case? If so, then diddling around with CO2 is a fool's errand.The short answer: no.You can't just extrapolate the current position in (CO2, dT) parameter space vertically upward to the equilibrium line. Yes, a (stable) system out of equilibrium will tend to revert to equilibrium, but the climate system will do so by both reducing atmospheric CO2 and increasing global temperature over time (for a position with CO2 > equilibrium, and dT < equilibrium, as is the present case). The exact path back to equilibrium depends on the relative timescales for removal of CO2 from the atmosphere, and for the full effect of the warming due to the extra radiative forcings from that CO2 to be felt (i.e., for additional melting of ice sheets and the adjustment of the ice albedo to occur). Both of these processes have long timescales. If all fossil fuel burning ceased today, the CO2 concentration in the atmosphere would start to decrease fairly quickly at first. Gillett (2011; Nature Geoscience, 4, 83) show that the excess (from equilibrium) CO2 would drop by about 30% over a century, but then the rate slows so that CO2 concentrations would decrease by only another 15% over the next 1000 years. It would take 10 kyr or so for all of the excess CO2 to be removed from the atmosphere. The ice albedo feedback, from the paleoclimate record, operates on timescales of 500 to 1000 years. So -- the net result (for CO2 excesses) would be a fairly rapid initial reduction of CO2, followed by a much slower reduction of CO2, and a steady increase in temperature over 1000 years. The situation with regard to methane is different, because of short lifetime of methane in the atmosphere (< 10 years). If only atmospheric methane was discrepant, the the result would be a rapid return to equilibrium by removal of methane on a 7 (or so) year timescale. The methane excess would be gone in 2-3 decades, and the climate would be back on the equilibrium line before there was time for temperatures (the long ice albedo feedbacks) to respond. The +40 C warming would not happen.For example: if all further fossil fuel burning ceased as of today, with CO2= 400 ppm (an excess of 125 ppm over the pre-industrial equilibrium value of 275 ppm) and dT= +0.6 C (relative to the pre-industrial equilibrium), I estimate that CO2 would fall to 360 ppm within a century, and to about 340 ppm within a millenium, with a warming of dT= 3.0 C after 1000 years. The result: conditions somewhat warmer than the Eemian, with substantial polar ice melt.A far more realistic scenario would be to assume that fossil fuel burning continues apace for another century before being brought under control. This would result in atmospheric levels of CO2 of about 700 ppm by 2200. If all fossil fuel burning ceased at that time, after a further millenium, we would have atmospheric CO2 = 500 ppm, and a warming of dT= 8 C. This is probably an overestimate, because the slope of the equilibrium curve decreases after all polar ice melts (at about CO2= 370 ppm, dT= 4.5 C) and the ice albedo feedbacks go away. Assuming a climate sensitivity of 3.0 C for the warm Earth gives dT= 6 C after 1000 years. Even so, this would be Hansen's "different, practically uninhabitable, planet", with no polar ice remaining and 220 foot higher sea levels, almost uninhabitable tropics -- and we won't even have to burn all our fossil fuels to get there.Phil
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