INSENSITIVITY OF GLOBAL WARMING POTENTIALS TO CARBON-DIOXIDE EMISSIONSCENARIOS

GLOBAL warming potentials for radiatively active trace gases (such as methane and chlorofluorocarbons) have generally been expressed1,2 rela tive to the time-integrated climate forcing per unit emission of carbo n dioxide. Previous attempts to estimate the integrated climate forcin g per unit CO2 emitted have focused on perturbations to steady-state c onditions in carbon-cycle models. But for non-steady-state conditions, the integrated climate forcing from a CO2 perturbation depends both o n the initial conditions and on future atmospheric CO2 concentrations. As atmospheric CO2 concentrations increase, the radiative forcing per unit CO2 emitted will become smaller because the strongest absorption bands will already be saturated. At the same time, higher concentrati ons of dissolved carbon in the surface ocean will reduce the ocean's a bility to absorb excess CO2 from the atmosphere. Each of these effects taken alone would affect the climate forcing from a pulse of emitted CO2 by a factor of three or more; but here we show that, taken togethe r, they compensate for each other. The net result is that the global w arming potential of CO2 relative to other radiatively active trace gas es is nearly independent of the CO2 emission scenario. Thus, the conce pt of the global warming potential remains useful, despite the nonline arities in the climate system and uncertainties in future emissions.