On the Risk of Overshooting 2°C

This article explores different greenhouse gas stabilization levels and their implied risks of overshooting certain temperature targets, such as limiting global mean temperature rise to 2°C above pre-industrial levels. The probabilistic assessment is derived from a compilation of recent estimates of the uncertainties in climate sensitivity, which summarizes the key uncertainties in climate science for long-term temperature projections. The risk of overshooting 2°C equilibrium warming is found to lie between 68% and 99% for stabilization at 550ppm CO2 equivalence. Only at levels around 400ppm CO2 equivalence are the risks of overshooting low enough so that the achievement of a 2°C target can be termed “likely”. Based on characteristics of 54 IPCC SRES and post-SRES scenarios, multi-gas emission pathways are presented that lead to stabilization at 550, 450 and 400ppm CO2eq in order to assess the implications for global emission reductions. Sensitivity studies on delayed global action show that the next 5 to 15 years might determine whether the risk of overshooting 2°C can be limited to a reasonable range.

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