Insensitivity of global warming potentials to carbon dioxide emissions scenarios
Ken Caldeira & James F. Kasting
Since it is used as a metric for the global warming potential of most gasses, determining the per unit climate forcing of carbon dioxide is important. Studies done previous to this one have generally determined climate forcing by observing the effects of rising CO2 on a steady state model. These models do not take into account the fact that climate forcing may change as CO2 levels rise. Here, it is shown that most of the effects that rising CO2 has on its own climate forcing cancel each other out, making its per unit warming potential nearly independent of how much is released.
Global warming potentials for radiatively active trace gases (such as methane and chlorofluorocarbons) have generally been expressed 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 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 concentrations of dissolved carbon in the surface ocean will reduce the ocean's ability 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 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 CO2 emission scenario. Thus, the concept of the global warming potential remains useful, despite the nonlinearities in the climate system and uncertainties in future emissions.