Long-term climate implications of twenty-first century options for carbon dioxide emission mitigation

Long-term future warming is primarily constrained by cumulative emissions of carbon dioxide. Previous studies have estimated that humankind has already emitted about 50% of the total amount allowed if warming, relative to pre-industrial, is to stay below 2C (refs,). Carbon dioxide emissions will thus need to decrease substantially in the future if this target is to be met. Here we show how links between near-term decisions, long-term behaviour and climate sensitivity uncertainties constrain options for emissions mitigation. Using a model of intermediate complexity, we explore the implications of non-zero long-term global emissions, combined with various near-term mitigation rates or delays in action. For a median climate sensitivity, a long-term 90% emission reduction relative to the present-day level is incompatible with a 2C target within the coming millennium. Zero or negative emissions can be compatible with the target if medium to high emission-reduction rates begin within the next two decades. For a high climate sensitivity, however, even negative emissions would require a global mitigation rate at least as great as the highest rate considered feasible by economic models to be implemented within the coming decade. Only a low climate sensitivity would allow for a longer delay in mitigation action and a more conservative mitigation rate, and would still require at least 90% phase-out of emissions thereafter.