Non-scientist here - I apologize if this is not the appropriate forum for newbie questions. I have been reading this site and others voraciously to educate myself on AGW, but I have been puzzled by the question of equilibrium temperature based upon a certain level of CO2 (or CO2-equivalent in terms of other GHG), as cited in the IPCC. It seems to me that if we stabilize atmospheric CO2 at a certain level, say 540 ppm (approx. double pre-industrial levels), that warming will continue to occur ad infinitum, at least until the earth is warm enough to cause a rebalancing of the amount of energy absorbed and reflected back into space. Is the IPCC projection of 2-4.5 deg C increase a statement that this is the long-term equilibrium at 540 ppm CO2, or is it just to a certain time horizon? Is it the consensus here (understanding that Arctic and Antarctic warming will be greater, with corresponding affects on sea ice and ice sheets) that 2-4.5 degrees is a reasonable projection? There are a number of feedback loops and tipping points which I would think should make projections a bit more non-linear (e.g. loss of albedo, release of methane due to melting of methane hydrates in the oceans, particularly after the Arctic sea ice vanishes during increasingly longer parts of the summer in coming years, release of methane from the permafrost, etc.).
The reason I ask, in addition to trying to gain a better understanding of the science, is that a number of projections in the IPCC seem wildly optimistic, particularly when looking at the impacts of AGW on human society, speaking in terms of moderate reductions of GDP, for example. I would think that crop failures, collapse of ecosystems, potable water scarcity, disease, and political stresses due to huge population centers going underwater would be much more likely to cause the collapse of civilization than merely cause a dent to GDP.
Thanks for any feedback.
