摘要：Global policy targets for greenhouse gas emissions reductions are being negotiated. The
amount of emitted carbon dioxide remaining in the atmosphere is controlled by
carbon cycle processes in the ocean and on land. These processes are themselves
affected by climate. The resulting 'climate–carbon cycle feedback' has recently been
quantified, but the policy implications have not. Using a scheme to emulate the
range of state-of-the-art model results for climate feedback strength, including the
modelled range of climate sensitivity and other key uncertainties, we analyse
recent global targets. The G8 target of a 50% cut in emissions by 2050 leaves
CO2 concentrations rising rapidly, approaching 1000 ppm by 2300. The Stern Review's proposed
25% cut in emissions by 2050, continuing to an 80% cut, does in fact approach stabilization of
CO2 concentration on a policy-relevant (century) timescale, with most models
projecting concentrations between 500 and 600 ppm by 2100. However
concentrations continue to rise gradually. Long-term stabilization at 550 ppm
CO2 requires cuts in emissions of 81 to 90% by 2300, and more beyond as a portion of the
CO2 emitted persists for centuries to millennia. Reductions of other
greenhouse gases cannot compensate for the long-term effects of
emitting CO2.
