Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model

The continued increase in the atmospheric concentration of carbon dioxide d ue to anthropogenic emissions is predicted to lead to significant changes i n climate(1). About half of the current emissions are being absorbed by the ocean and by land ecosystems(2), but this absorption is sensitive to clima te(3,4) as well as to atmospheric carbon dioxide concentrations(5), creatin g a feedback loop. General circulation models have generally excluded the f eedback between climate and the biosphere, using static vegetation distribu tions and CO2 concentrations from simple carbon-cycle models that do not in clude climate change(6). Here we present results from a fully coupled, thre e-dimensional carbon-climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century . We rnd that under a 'business as usual' scenario, the terrestrial biosphe re acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt Cyr(-1) is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p .p.m.v. higher in our fully coupled simulation than in uncoupled carbon mod els(2), resulting in a global-mean warming of 5.5 K, as compared to 4 K wit hout the carbon-cycle feedback.