Three Views of Two Degrees

Limiting global warming to 2° Celsius above global mean temperature in pre-indus- trial times has become a widely debated possible goal for climate policy. It has been supported by many scientists, the European Union, the G8 and larger international bodies. However, some claim that it is way too stringent, others that it is not sufficient to avoid major climate risks. We show how the limit emerged out of a marginal remark in an early paper about climate policy and distinguish three possible views of it. The catastrophe view sees it as the threshold separating a domain of safety from a domain of catastrophe. The cost-benefit view sees it as a strategy to optimize the relation between the costs and benefits of limiting greenhouse gas emissions. And the focal point view sees it as a solution to a complex coordination problem. We assess these three views, including a philosophical reflection on some conceptual confusion concerning value judgments, and conclude that the focal point view is the most appropriate. It leads to an emphasis on implementing effective steps towards a near-zero emissions economy and to accept that in the course of time the focal point may be redefined on the basis of practical experiences. [ECF Working Paper 2/2010]