Allowable CO2 concentrations under the United Nations Framework Convention on Climate Change as a function of the climate sensitivity probability distribution function

Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC) calls for stabilization of greenhouse gas (GHG) concentrations at levels that prevent dangerous anthropogenic interference (DAI) in the climate system. Until recently, the consensus viewpoint was that the climate sensitivity ( the global mean equilibrium warming for a doubling of atmospheric CO2 concentration) was 'likely' to fall between 1.5 and 4.5 K. However, a number of recent studies have generated probability distribution functions (pdfs) for climate sensitivity with the 95th percentile of the expected climate sensitivity as large as 10 K, while some studies suggest that the climate sensitivity is likely to fall in the lower half of the long-standing 1.5-4.5 K range. This paper examines the allowable CO2 concentration as a function of the 95th percentile of the climate sensitivity pdf ( ranging from 2 to 8 K) and for the following additional assumptions: (i) the 50th percentile for the pdf of the minimum sustained global mean warming that causes unacceptable harm equal to 1.5 or 2.5 K; and (ii) 1%, 5% or 10% allowable risks of unacceptable harm. For a 1% risk tolerance and the more stringent harm-threshold pdf, the allowable CO2 concentration ranges from 323 to 268 ppmv as the 95th percentile of the climate sensitivity pdf increases from 2 to 8 K, while for a 10% risk tolerance and the less stringent harm-threshold pdf, the allowable CO2 concentration ranges from 531 to 305 ppmv. In both cases it is assumed that non-CO2 GHG radiative forcing can be reduced to half of its present value, otherwise; the allowable CO2 concentration is even smaller. Accounting for the fact that the CO2 concentration will gradually fall if emissions are reduced to zero, and that peak realized warming will then be less than the peak equilibrium warming ( related to peak radiative forcing) allows the CO2 concentration to peak at 10-40 ppmv higher than the limiting values given above for a climate sensitivity 95th percentile at 4.5 K. Even allowing for the difference between peak realized and peak equilibrium warming, and assuming that present non-CO2 GHG forcing can be cut in half, a CO2 concentration of 410 ppmv or less constitutes DAI for every combination of harm-threshold pdf and risk tolerance considered here if the 95th percentile of the climate sensitivity pdf is 4.5 K or greater.