ALBERT

Description: Eight earth system models of intermediate complexity (EMICs) are used to project climate change commitments for the recent Intergovernmental Panel on Climate Change’s (IPCC’s) Fourth Assessment Report (AR4). Simulations are run until the year 3000 a.d. and extend substantially farther into the future than conceptually similar simulations with atmosphere–ocean general circulation models (AOGCMs) coupled to carbon cycle models. In this paper the following are investigated: 1) the climate change commitment in response to stabilized greenhouse gases and stabilized total radiative forcing, 2) the climate change commitment in response to earlier CO2 emissions, and 3) emission trajectories for profiles leading to the stabilization of atmospheric CO2 and their uncertainties due to carbon cycle processes. Results over the twenty-first century compare reasonably well with results from AOGCMs, and the suite of EMICs proves well suited to complement more complex models. Substantial climate change commitments for sea level rise and global mean surface temperature increase after a stabilization of atmospheric greenhouse gases and radiative forcing in the year 2100 are identified. The additional warming by the year 3000 is 0.6–1.6 K for the low-CO2 IPCC Special Report on Emissions Scenarios (SRES) B1 scenario and 1.3–2.2 K for the high-CO2 SRES A2 scenario. Correspondingly, the post-2100 thermal expansion commitment is 0.3–1.1 m for SRES B1 and 0.5–2.2 m for SRES A2. Sea level continues to rise due to thermal expansion for several centuries after CO2 stabilization. In contrast, surface temperature changes slow down after a century. The meridional overturning circulation is weakened in all EMICs, but recovers to nearly initial values in all but one of the models after centuries for the scenarios considered. Emissions during the twenty-first century continue to impact atmospheric CO2 and climate even at year 3000. All models find that most of the anthropogenic carbon emissions are eventually taken up by the ocean (49%–62%) in year 3000, and that a substantial fraction (15%–28%) is still airborne even 900 yr after carbon emissions have ceased. Future stabilization of atmospheric CO2 and climate change requires a substantial reduction of CO2 emissions below present levels in all EMICs. This reduction needs to be substantially larger if carbon cycle–climate feedbacks are accounted for or if terrestrial CO2 fertilization is not operating. Large differences among EMICs are identified in both the response to increasing atmospheric CO2 and the response to climate change. This highlights the need for improved representations of carbon cycle processes in these models apart from the sensitivity to climate change. Sensitivity simulations with one single EMIC indicate that both carbon cycle and climate sensitivity related uncertainties on projected allowable emissions are substantial.

Description: A cost-efficient, seasonally forced three-dimensional frictional geostrophic balance ocean model (Bern3D) has been developed that features isopycnal diffusion and Gent–McWilliams transport parameterization, 32 depth layers, and an implicit numerical scheme for the vertical diffusion. It has been tuned toward observed chlorofluorocarbon (CFC-11) inventories and deep ocean radiocarbon signatures to reproduce the ventilation time scales of the thermocline and the deep ocean. Model results are consistent with the observed large-scale distributions of temperature, salinity, natural and bomb-produced radiocarbon, CFC-11, anthropogenic carbon, 39Ar/Ar, and estimates of the meridional heat transport. Root-mean-square errors for the temperature and salinity fields are 1 K and 0.2 psu, comparable to results from the Ocean Carbon-Cycle Model Intercomparison Project. Global inventories of CFC-11 and anthropogenic carbon agree closely with observation-based estimates. Model weaknesses include a too-weak formation and propagation of Antarctic Intermediate Water and of North Atlantic Deep Water. The model has been applied to quantify the recent carbon balance, surface-to-deep transport mechanisms, and the importance of vertical resolution for deep equatorial upwelling. Advection is a dominant surface-to-deep transport mechanism, whereas explicit diapycnal mixing is of little importance for passive tracers and contributes less than 3% to the modeled CFC-11 inventory in the Indo-Pacific. Decreasing the vertical resolution from 32 to 8 layers causes deep equatorial upwelling to increase by more than a factor of 4. Modeled ocean uptake of anthropogenic carbon is 19.7 GtonC over the decade from 1993 to 2003, comparable to an estimate from atmospheric oxygen data of 22.4 ± 6.1 GtonC.