How Much Warming are we Committed to and How Much Can be Avoided?
B. Hare, M. Meinshausen (October 2004)
This paper examines different concepts of a ‘warming commitment’ which is often used in various ways to describe or imply that a certain level of warming is irrevocably committed to over time frames such as the next 50 to 100 years, or longer. We review and quantify four different concepts, namely (1) a ‘constant emission warming commitment’, (2) a ‘present forcing warming commitment’, (3) a ‘zero emission (geophysical) warming commitment’ and (4) a ‘feasible scenario warming commitment’. While a ‘feasible scenario warming commitment’ is probably the most relevant one for policy making, it depends centrally on key assumptions as to the technical, economic and political feasibility of future greenhouse gas emission reductions. This issue is of direct policy relevance when one considers that the 2003 global mean temperatures were 0.8°C above the pre-industrial mean and the European Union has a stated goal of limiting warming to 2°C above the pre-industrial mean: What is the risk that we are committed to overshoot 2°C? Based on the conventional IPCC uncertainty range for climate sensitivity (1.5°C to 4.5°C) and more recent estimates, we found that a (1) constant emission scenario is virtually certain to overshoot 2°C with a central estimate of 2.0°C by 2100 (4.2°C by 2400). (2) While for the present radiative forcing levels it seems unlikely (risk between 0% and 30%, central estimate 1.1°C by 2100 and 1.2°C by 2400), the risk of overshooting is increasing rapidly if radiative forcing is stabilized much above today’s levels (roughly 400ppm CO2 equivalence) in the long-term. (3) From a geophysical point of view, if all human-induced emissions were ceased tomorrow, it seems ‘exceptionally unlikely’ that 2°C will be overshoot (central estimate: 0.7°C by 2100; 0.4°C by 2400). (4) Assuming future emissions according to the lower end of published mitigation scenarios provides (350ppm CO2eq to 450ppm CO2eq) the central temperature projections are 1.5°C to 2.1°C by 2100 (1.5°C to 2.0°C by 2400) with a risk to overshoot of 10% to 50% by 2100 and 1%-32% in equilibrium. Furthermore, we quantify the ‘avoidable warming’ to be 0.16-0.26°C for every 100GtC of avoided CO2 emissions - based on a range of published mitigation scenarios.
