摘要：Climate models provide compelling evidence that if greenhouse gas emissions continue at
present rates, then key global temperature thresholds (such as the European Union
limit of two degrees of warming since pre-industrial times) are very likely to be
crossed in the next few decades. However, there is relatively little attention paid
to whether, should a dangerous temperature level be exceeded, it is feasible for
the global temperature to then return to safer levels in a usefully short time.
We focus on the timescales needed to reduce atmospheric greenhouse gases and
associated temperatures back below potentially dangerous thresholds, using a
state-of-the-art general circulation model. This analysis is extended with a simple
climate model to provide uncertainty bounds. We find that even for very large
reductions in emissions, temperature reduction is likely to occur at a low rate.
Policy-makers need to consider such very long recovery timescales implicit in the
Earth system when formulating future emission pathways that have the potential
to 'overshoot' particular atmospheric concentrations of greenhouse gases and,
more importantly, related temperature levels that might be considered dangerous.
