How difficult is it to recover from dangerous levels of global warming?

Abstract

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