Long-Term Climate Commitments Projected with Climate–Carbon Cycle Models

Abstract

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 CO 2 emissions, and 3) emission trajectories for profiles leading to the stabilization of atmospheric CO 2 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-CO 2 IPCC Special Report on Emissions Scenarios (SRES) B1 scenario and 1.3-2.2 K for the high-CO 2 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 CO 2 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 CO 2 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 CO 2 and climate change requires a substantial reduction of CO 2 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 CO 2 fertilization is not operating. Large differences among EMICs are identified in both the response to increasing atmospheric CO 2 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.n Current affiliation: IBP, ETH Zürich, Zürich, Switzerland.

Historical and idealized climate model experiments: an intercomparison of Earth system models of intermediate complexity

“…There is, therefore, still some controversy as to the response of the meridional overturning circulation over the 20th century (Latif et al, 2006). However, this moderate response to a warming climate is similar to what has been seen in previous studies (Plattner et al, 2008).…”

“…Pethoukhov et al, 2005;Rahmstorf et al, 2005;Brovkin et al, 2006;Plattner et al, 2008). More typically, however, EMICs have been included in model intercomparisons with coupled AOGCMs (e.g.…”

The large contribution of projected HFC emissions to future climate forcing

“…Projected HFC emissions are 9-19% of these CO 2 values. Instead, if the scenarios are chosen to be those of long-term CO 2 stabilization at atmospheric mixing ratios of 450 and 550 ppm (16,17), the projected HFC emissions in 2050 are 28-45% and 14-23%, respectively, of CO 2 emissions (Fig. 2B).…”

“…We use the new emission scenarios and GWPs of HFCs to calculate their CO 2 -equivalent emissions and RF contributions to global climate forcing. The results are compared with ''business-as-usual'' SRES CO 2 emissions and those required to stabilize CO 2 concentrations at 450 and 550 parts per million (ppm) (16,17). We also consider a range of hypothetical mitigation options, some of which reflect current policy proposals, to demonstrate how projected consumption and, hence, RF in 2050 could be reduced.…”

Geoengineering climate by stratospheric sulfur injections: Earth system vulnerability to technological failure

“…EMICs usually have a coarser spatial and temporal resolution than general circulation models, but they are computationally efficient and often include more climate system components than GCMs . That makes EMICs ideally suited for the fast calculation of the long-term consequences of CO 2 mitigation measures (Lenton et al 2006;Plattner et al 2008).…”