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

CO 2 released from combustion of fossil fuels equilibrates between the various carbon reservoirs of the atmosphere, the ocean, and the terrestrial biosphere on time scales of a few centuries. However, a sizeable fraction of the CO 2 remains in the atmosphere, awaiting a return to the solid earth by much slower weathering processes and deposition of CaCO 3 . Common measures of atmospheric CO 2 lifetime, including the e-folding time scale, disregard the long tail. Its neglect in the calculation of global warming potentials leads many to underestimate the longevity of anthropogenic global warming. Here we review the past literature on the atmospheric lifetime of fossil fuel CO 2 and its impact on climate, and we present initial results from a model intercomparison project on this topic.The models agree that 20-35% of the CO 2 remains in the atmosphere after equilibration with the ocean (2-20 centuries). Neutralization by CaCO 3 draws the airborne fraction down further on time scales of 3-7 kyr.

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

[1] Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon-cycle Model Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse-resolution ocean circulation models. The model mean (±1s) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr À1 . Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation-based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep-ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep-ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross-model means of several biogeochemical metrics show better agreement with observation-based estimates when restricted to those models that best simulate deep-ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.

Using a simple box model, we investigate the effects of a reduced Si:N uptake ratio by Antarctic phytoplankton on the marine silica cycle and atmospheric pCO2. Recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added [Hutchins and Bruland, 1998; Takeda, 1998; Franck et al., 2000]. The Southern Ocean may have supported diatoms with reduced Si:N uptake ratios compared to today during the dustier glacial times [Petit et al., 1999]. A similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as Phaeocystis. Our model shows that reduced Si:N export ratios in the Southern Ocean create excess silicic acid, which may then be leaked out to lower latitudes. Any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes CaCO3 production and therefore diminishes the carbonate pump. In our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric CO2 from the interglacial 277 to 230–242 ppm, depending on where the excess silicic acid is consumed. By comparison, the atmospheric pCO2 sensitivity of general circulation models to carbonate pump forcing is ∼3.5–fold greater, which, combined with carbonate compensation, can account for peak glacial atmospheric pCO2. We discuss the importance of the initial rain ratio of CaCO3 to organic carbon on atmospheric pCO2 and relevant sedimentary records that support and constrain this “silicic acid leakage” scenario.