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.

[1] The evolution of the Atlantic Meridional Overturning Circulation (MOC) in 30 models of varying complexity is examined under four distinct Representative Concentration Pathways. The models include 25 Atmosphere-Ocean General Circulation Models (AOGCMs) or Earth System Models (ESMs) that submitted simulations in support of the 5th phase of the Coupled Model Intercomparison Project (CMIP5) and 5 Earth System Models of Intermediate Complexity (EMICs). While none of the models incorporated the additional effects of ice sheet melting, they all projected very similar behaviour during the 21st century. Over this period the strength of MOC reduced by a best estimate of 22% (18%-25%; 5%-95% confidence limits) for RCP2.6, 26% (23%-30%) for RCP4.5, 29% (23%-35%) for RCP6.0 and 40% (36%-44%) for RCP8.5. Two of the models eventually realized a slow shutdown of the MOC under RCP8.5, although no model exhibited an abrupt change of the MOC. Through analysis of the freshwater flux across 30 -32 S into the Atlantic, it was found that 40% of the CMIP5 models were in a bistable regime of the MOC for the duration of their RCP integrations. The results support previous assessments that it is very unlikely that the MOC will undergo an abrupt change to an off state as a consequence of global warming.

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A recent study found enhanced upwelling rates in the Southern Ocean during the last glacial termination that coincided with the deglacial warming in Antarctica and the rise in atmospheric CO2. They hypothesized that the intensification of Southern Hemisphere midlatitude westerlies, the presumed cause of the increased wind‐driven upwelling, was triggered by an initial cooling within the glacial North Atlantic whose influence was then communicated to the southern midlatitudes through an atmospheric teleconnection. In this study, we explore the viability of the above hypothesis using a modeling strategy, focusing on the atmospheric teleconnection. In simulations where North Atlantic cooling was applied, the model Intertropical Convergence Zone shifted southward, and westerlies and wind stress over Southern Ocean increased by as much as 25%. While the perennial westerly anomalies occur over the entire Southern Ocean, they are strongest over the South Pacific during the austral winter. When the wind stress anomalies were applied to an Earth system model incorporating interactive marine biogeochemistry, atmospheric CO2 rises between 20 and 60 ppm, depending on the biological response. We thus confirm the viability of the proposed atmospheric teleconnection hypothesis. The teleconnection appears to involves two distinct steps: first, the North Atlantic cooling shifts the Intertropical Convergence Zone southward, weakening the southern branch of the Hadley circulation, and second, how the altered Hadley circulation in turn modifies the structure of midlatitude westerlies in the South Pacific, via the former's influence on the Southern Hemisphere subtropical jet. This study underscores the control of the Northern Hemisphere has on southern midlatitude westerlies, mediating by tropical circulation, in contrast to past paleoclimate hypotheses that the magnitude and position of the southern midlatitude westerlies was controlled by global mean temperature. Our results do not preclude other potential mechanisms for affecting Southern Ocean ventilation, in particular through oceanic pathways.

Both historical and idealized climate model experiments are performed with a variety of Earth system models of intermediate complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth's orbital configuration, CO₂, additional greenhouse gases, land use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes appear to be slightly underestimated. It is possible that recent modelled climate trends or climate–carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO₂ and/or nitrogen fertilization is underestimated. Several one thousand year long, idealized, 2 × and 4 × CO₂ experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate–carbon feedbacks. The values from EMICs generally fall within the range given by general circulation models. Seven additional historical simulations, each including a single specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows a non-linear interaction between land-use change and CO₂ forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs to underestimate the drop in surface air temperature and CO₂ between the Medieval Climate Anomaly and the Little Ice Age estimated from palaeoclimate reconstructions. This in turn could be a result of unforced variability within the climate system, uncertainty in the reconstructions of temperature and CO₂, errors in the reconstructions of forcing used to drive the models, or the incomplete representation of certain processes within the models. Given the forcing datasets used in this study, the models calculate significant land-use emissions over the pre-industrial period. This implies that land-use emissions might need to be taken into account, when making estimates of climate–carbon feedbacks from palaeoclimate reconstructions