The responses of carbon dioxide (CO₂) and other climate variables to an emission pulse of CO₂ into the atmosphere are often used to compute the Global Warming Potential (GWP) and Global Temperature change Potential (GTP), to characterize the response timescales of Earth System models, and to build reduced-form models. In this carbon cycle-climate model intercomparison project, which spans the full model hierarchy, we quantify responses to emission pulses of different magnitudes injected under different conditions. The CO₂ response shows the known rapid decline in the first few decades followed by a millennium-scale tail. For a 100 Gt-C emission pulse added to a constant CO₂ concentration of 389 ppm, 25 ± 9% is still found in the atmosphere after 1000 yr; the ocean has absorbed 59 ± 12% and the land the remainder (16 ± 14%). The response in global mean surface air temperature is an increase by 0.20 ± 0.12 °C within the first twenty years; thereafter and until year 1000, temperature decreases only slightly, whereas ocean heat content and sea level continue to rise. Our best estimate for the Absolute Global Warming Potential, given by the time-integrated response in CO₂ at year 100 multiplied by its radiative efficiency, is 92.5 × 10⁻¹⁵ yr W m⁻² per kg-CO₂. This value very likely (5 to 95% confidence) lies within the range of (68 to 117) × 10⁻¹⁵ yr W m⁻² per kg-CO₂. Estimates for time-integrated response in CO₂ published in the IPCC First, Second, and Fourth Assessment and our multi-model best estimate all agree within 15% during the first 100 yr. The integrated CO₂ response, normalized by the pulse size, is lower for pre-industrial conditions, compared to present day, and lower for smaller pulses than larger pulses. In contrast, the response in temperature, sea level and ocean heat content is less sensitive to these choices. Although, choices in pulse size, background concentration, and model lead to uncertainties, the most important and subjective choice to determine AGWP of CO₂ and GWP is the time horizon

The responses of carbon dioxide (CO₂) and other climate variables to an emission pulse of CO₂ into the atmosphere are often used to compute the Global Warming Potential (GWP) and Global Temperature change Potential (GTP), to characterize the response time scales of Earth System models, and to build reduced-form models. In this carbon cycle-climate model intercomparison project, which spans the full model hierarchy, we quantify responses to emission pulses of different magnitudes injected under different conditions. The CO₂ response shows the known rapid decline in the first few decades followed by a millennium-scale tail. For a 100 Gt C emission pulse, 24 ± 10% is still found in the atmosphere after 1000 yr; the ocean has absorbed 60 ± 18% and the land the remainder. The response in global mean surface air temperature is an increase by 0.19 ± 0.10 °C within the first twenty years; thereafter and until year 1000, temperature decreases only slightly, whereas ocean heat content and sea level continue to rise. Our best estimate for the Absolute Global Warming Potential, given by the time-integrated response in CO₂ at year 100 times its radiative efficiency, is 92.7 × 10^−15 yr W m⁻² per kg CO₂. This value very likely (5 to 95% confidence) lies within the range of (70 to 115) × 10^−15 yr W m⁻² per kg CO₂. Estimates for time-integrated response in CO₂ published in the IPCC First, Second, and Fourth Assessment and our multi-model best estimate all agree within 15%. The integrated CO₂ response is lower for pre-industrial conditions, compared to present day, and lower for smaller pulses than larger pulses. In contrast, the response in temperature, sea level and ocean heat content is less sensitive to these choices. Although, choices in pulse size, background concentration, and model lead to uncertainties, the most important and subjective choice to determine AGWP of CO₂ and GWP is the time horizon

At present, the terrestrial biosphere is mitigating anthropogenic climate change by acting as a carbon (C) sink, compensating about 30% of global CO 2 emissions from fossil and land-use sources 2 . In contrast, 44-73% of global nitrous oxide (N 2 O) emissions 3,4 and 24-43% of global methane (CH 4 ) emissions 5 , both potent GHGs, originate from land ecosystems and partly offset the cooling effect of C uptake by the land. Terrestrial N 2 O and CH 4 emissions, henceforth termed eN 2 O and eCH 4 , are enhanced in a warm climate 6,7 and under high atmospheric CO 2 concentrations (cCO 2 ; ref. 8). The associated feedback loop amplifies anthropogenic climate change and is reflected in palaeo records on glacial-interglacial and centennial timescales 9,10 . However, despite its potential importance 6 there is yet a lack of studies investigating combined multiple GHG feedbacks between terrestrial ecosystems and climate.The strength of feedbacks between land and climate is determined by the sensitivity of the forcing agents (here: eN 2 O; eCH 4 ; terrestrial C storage, C; and Albedo change) to the drivers (climate and cCO 2 ), and the radiative efficiency of the respective forcing agent. Earlier quantifications of terrestrial GHG feedbacks have relied on observational data and land-only models to derive the sensitivities, multiplied by the radiative efficiency 6,7,9-11 . Here, we assess multiple feedbacks from terrestrial ecosystems in a coupled Earth system model of intermediate complexity and follow a quantification framework commonly applied to measure the strength of physical climate feedbacks 1,12 ( Fig. 1 and Methods). Applying future scenarios of N-deposition and Nfertilizer application in agriculture allows us to assess their impact on eN 2 O and related feedbacks.We start the discussion by exploring to which extent a processbased land biosphere model is able to reproduce the observationbased evolution of atmospheric N 2 O and CH 4 concentrations (cN 2 O, cCH 4 ) over the industrial period. Addressing the historical atmospheric GHG budgets serves as a test for the sensitivity of simulated GHG emissions to the combination of climate, cCO 2 and external forcings. LPX-Bern 13-18 is applied here to simulate the coupled cycling of carbon and nitrogen and the emissions of GHGs from agricultural and natural land and from peat. Sitescale evaluations of this model have been presented earlier 7,[15][16][17][18][19][20] . For this test, we force LPX-Bern with observational data for climate 21 , cCO 2 (ref. 22), Nr (N deposition 23 plus mineral N fertilizer inputs 24 ) and anthropogenic land-use area change and combine simulated emissions with independent emission data from remaining sources to assess atmospheric budgets (see Methods and Supplementary Fig. S1).For eN 2 O, we confirm earlier results 24 showing that the simulated emission increase in the second half of the twentieth century matches measured concentrations (Fig. 2a). Experiments with incomplete driving factors perform worse at reproducing the observed rate of i...