Attention au rythme du changement climatique!

Abstract

Global mean temper ature rise is a better indicator of climate risks than are atmospheric greenhouse gas concentration ceilings because it is a synthetic index of the on-going climate change, because every assessment of regional impacts refers to this parameter and because it allows to take into account the rate of climate change, a key determinant of vulnerability. To assess climate policies in a cost-efficiency framework with constraints on the magnitude and rate of global climate change, we developed RESPONSE_Θ, an optimal control integrated assessment model. Our results show that the uncertainty about climate sensitivity leads to significant short-term mitigation efforts, a fact amplified by the tardy arrival of the information regarding this parameter. Given this uncertainty, a +2°C objective could lead to rather stringent policy recommendations for the coming decades and might prove unacceptable. We finally show that for short-term decision, solving the uncertainty about the rate constraint is even more important than solving that about climate sensitivity or magnitude of warming. This means that the critical rate of climate change, i.e. a transient characteristic of climate risks, matters much more than the long-term objective of climate policy, i.e. the critical magnitude of climate change. Therefore, research should be aimed at better characterizing climate change risks in order to help decision makers to agree on a safe guardrail to limit the rate of global warming. © NSS Dialogues, EDP Sciences 2006.