Insensitivity of global warming potentials to carbon dioxide emission scenarios

Abstract

GLOBAL warming potentials for radiatively active trace gases (such as methane and chlorofluorocarbons) have generally been expressed^1–2 relative to the time-integrated climate forcing per unit emission of carbon dioxide. Previous attempts to estimate the integrated climate forcing per unit CO₂ emitted have focused on perturbations to steady-state conditions in carbon-cycle models. But for non-steady-state conditions, the integrated climate forcing from a CO₂ perturbation depends both on the initial conditions and on future atmospheric CO₂ concentrations. As atmospheric CO₂concentrations 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 concentrations of dissolved carbon in the surface ocean will reduce the ocean's ability to absorb excess CO₂from 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 together, they compensate for each other. The net result is that the global warming potential of CO₂ relative to other radiatively active trace gases is nearly independent of the CO₂emission scenario. Thus, the concept of the global warming potential remains useful, despite the nonlinearities in the climate system and uncertainties in future emissions.