Carbon dioxide and climate impulse response functions for the computation of greenhouse gas metrics: a multi-model analysis

Abstract

The responses of carbon dioxide (CO2) and other climate variables to an emissionpulse of CO2into the atmosphere are often used to compute the Global WarmingPotential (GWP) and Global Temperature change Potential (GTP), to characterizethe response time scales of Earth System models, and to build reduced-form mod-5els. In this carbon cycle-climate model intercomparison project, which spans the fullmodel hierarchy, we quantify responses to emission pulses of different magnitudes in-jected under different conditions. The CO2response shows the known rapid declinein the first few decades followed by a millennium-scale tail. For a 100 GtC emissionpulse, 24±10 % is still found in the atmosphere after 1000 yr; the ocean has absorbed1060±18 % and the land the remainder. The response in global mean surface air tem-perature is an increase by 0.19±0.10◦C within the first twenty years; thereafter anduntil year 1000, temperature decreases only slightly, whereas ocean heat content andsea level continue to rise. Our best estimate for the Absolute Global Warming Po-tential, given by the time-integrated response in CO2at year 100 times its radiative15efficiency, is 92.7×10−15yr Wm−2per kg CO2. This value very likely (5 to 95% confi-dence) lies within the range of (70 to 115)×10−15yr Wm−2per kg CO2. Estimates fortime-integrated response in CO2published in the IPCC First, Second, and Fourth As-sessment and our multi-model best estimate all agree within 15%. The integrated CO2response is lower for pre-industrial conditions, compared to present day, and lower for20smaller pulses than larger pulses. In contrast, the response in temperature, sea leveland ocean heat content is less sensitive to these choices. Although, choices in pulsesize, background concentration, and model lead to uncertainties, the most importantand subjective choice to determine AGWP of CO2and GWP is the time horizon.