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Letters to Nature
Nature 366, 251 - 253 (18 November 1993); doi:10.1038/366251a0

Insensitivity of global warming potentials to carbon dioxide emission scenarios

Ken Caldeira* & James F. Kasting

*Present address: Global Climate Research Division, Lawrence Livermore National Laboratory, PO Box 808, L-256, Livermore, California 94550, USA.
Earth System Science Center, 248 Deike Building, Pennsylvania State University, University Park, Pennsylvania 16802, USA

GLOBAL warming potentials for radiatively active trace gases (such as methane and chlorofluorocarbons) have generally been expressed1–2 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.



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