The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic
emissions is predicted to lead to significant changes in climate. About half of the
current emissions are being absorbed by the ocean and by land ecosystems, but this
absorption is sensitive to climate as well as to atmospheric carbon dioxide concentrations,
creating a feedback loop. General circulation models have generally excluded the feedback
between climate and the biosphere, using static vegetation distributions and CO2 concentrations
from simple carbon-cycle models that do not include climate change. Here we present
results from a fully coupled, three-dimensional carbon-climate model, indicating that
carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first
century. We find that under a 'business as usual' scenario, the terrestrial biosphere
acts as an overall carbon sink until about 2050, but turns into a source thereafter.
By 2100, the ocean uptake rate of 5 Gt C yr(-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 models, resulting in a global-mean warming of
5.5 K, as compared to 4 K without the carbon-cycle feedback.