The observed increase in global mean surface temperature (GMST) over the industrial era, about 0.8 K, is less than 40% of that expected from observed increases in long-lived greenhouse gases together with the best-estimate equilibrium climate sensitivity given by the 2007 Assessment Report of the Intergovernmental Panel on Climate Change. This warming discrepancy is due mainly to some combination of two factors: too high a climate sensitivity and/or partial offset of the greenhouse gas forcing by increased concentrations of atmospheric aerosols; thermal disequilibrium accounts for less than 25% of the discrepancy, and cooling by natural temperature variation might account for 15%. Current uncertainty in climate sensitivity precludes determining the amount of future fossil fuel CO2 emissions that would be compatible with any chosen maximum allowable increase in GMST; even the sign of such allowable future emissions is unconstrained. Resolving this situation requires substantial reduction in the uncertainty of aerosol forcing.