Measuring the Marginal Impact of Co2 Emissions on Atmospheric Concentrations

The response of atmospheric CO2 concentrations to an incremental increase in emissions is usually measured using an impulse response function. These functions are then used in calculating Global Warming Potentials, marginal climate damages, and CO2 timescales. It is shown here that the impulse response method implicitly double counts the effects of emissions on concentration by not properly accounting for carbon cycle nonlinearities, leading to a systematic overestimation of the persistence of the effect of an emissions pulse. A new method of determining marginal response functions is derived which accurately proportions responsibility for CO2 concentration among emissions cohorts. Results show that the new functions indicate up to a 50% reduction in the essentially irreversible fraction of the effect of a present emission. This reduced long term response leads to a 20-30% reduction in 500-year Global Warming Potentials and a 10-15% reduction in climate damages per ton of emission. The new functions are also applied to historical emissions, and indicate that emissions from developed countries are responsible for approximately 75% of the present excess atmospheric CO2 concentration.