Global warming and marine carbon cycle feedbacks on future atmospheric CO₂

Fortunat Joos, Gian-Kasper Plattner, Thomas F. Stocker, Olivier Marchal, and Andreas Schmittner, Climate and Environmental Physics, Physics Insitute, University of Bern, Bern, Switzerland

A low-order physical-biogeochemical climate model was used to project atmospheric carbon dioxide and global warming for scenarios developed by the Intergovernmental Panel on Climate Change. The North Atlantic thermohaline circulation weakens in all global warming simulations and collapses at high levels of carbon dioxide. Projected changes in the marine carbon cycle have a modest impact on atmospheric carbon dioxide. Compared with the control, atmospheric CO₂ increased by 4% at year 2100 and 20% at year 2500. The reduction in ocean carbon uptake can be mainly explained by sea surface warming. The projected changes of the marine biological cycle compensate the reduction in downward mixing of anthropogenic carbon, except when the North Atlantic thermohaline circulation collapses.

Figures:

Fig. 1.(A) Carbon emissions in 10¹² kg of carbon per year (Gt of C year^-1) for the IPCC IS92a, IS92c, and IS92e scenarios. The scenarios depict potential evolutions of carbon emissions based on estimates of future population growth and economic development. (ps-file) (B) NADW formation when atmospheric CO ₂ and radiative forcing were calculated using the coupled model forced with the emissions shown in (A) and a climate sensitivity of 3.7^oC for a doubling of CO₂ (Delta-T_2x = 3.7^oC). (ps-file) (C) Projected atmospheric CO₂ for the emissions shown in (A) for the baseline (Delta-T_2x = 0^oC) (dashed lines) and the standard (Delta-T_2x = 3.7^oC) (solid lines) simulations. (ps-file)












Fig. 2. (A) Profiles resulting in the stabilization of atmospheric CO₂ at levels between 350 and 1000 ppmv (WRE350, WRE450, WRE550, WRE650, WRE750, and WRE1000) considered by IPCC. The profiles follow the observed concentration history until the present and then approximately follow the IS92a concentration path for the next decades to eventually approach a stabilization level. (ps-file) (B) NADW formation for the CO₂ profiles shown in (A) and Delta-T _2x = 3.7^oC. (ps-file) (C) Global anthropogenic carbon emissions that are consistent with the WRE1000 and WRE550 CO₂ profiles shown in (A) for the baseline (Delta-T_2x = 0^oC) (dashed lines) and the standard (Delta-T_2x = 3.7^oC) (solid lines) simulations. Anthropogenic emissions as obtained by the Bern model (dashed-dotted lines) are given for comparison. (ps-file)










Fig. 3. Ocean carbon uptake and reduction in carbon uptake for simulations in which selected feedbacks are operating for the WRE1000 (A) (ps-file) and the WRE550 (B) (ps-file) CO₂ stabilization profiles. The reduction in CO₂ uptake by all feedbacks is the difference between simulation A and B (A - B) (thin solid line). The SST feedback is the difference between simulation A and C (A - C) (thin dotted-dashed line). The SST feedback simulated by the Bern model is shown by the thin dotted line. The circulation feedback is the difference between simulation C (dotted-dashed line) and D (short dashed line). The biota feedback is the difference between simulation D (short-dashed line) and B (solid line).







Fig. 4. Dependence of the NADW formation on the climate sensitivity, Delta-T_2x, varied between 0 and 4.5^oC for the WRE1000 CO₂ profile (Fig. 2A). The collapse in NADW formation is irreversible for Delta-T_2x values that are higher than 3^oC. (ps-file)