Delay of actions involves large risks in estimations of economic damages and reduction ratios of carbon dioxide emission for lower climate targets.

Because future projections by AOGCMs require huge computer and human resources, simple climate models are used under a wide range of emission scenarios. The observation obtained in the past cannot provide a strong constraint on equilibrium climate sensitivity (ECS) and thus the future projections by simple climate models. However, when observations are obtained more in future, the uncertainty of future projections is expected to reduce. There is a public debate over whether to start to reduce carbon dioxide emissions now or to delay implementing mitigation policy in future. If the observation obtained in future can provide substantive benefits to climate policy, a climate policy of “wait and see”, or a sequential-decision strategy for climate change would be useful.

We investigated how much the uncertainty in economic damage and reduction ratios of CO2 emission, by which a climate target can be achieved, will reduce in future after future observation can be obtained. To conduct this, we first produced hypothetical observations of different ECSs using a simple climate model, and then validated whether the sequential decision strategy is useful or not for the estimations of economic damages and reduction ratios of carbon dioxide emissions.

In low ECS, the magnitudes of the uncertainty for future projections in global mean SAT changes are small, and they reduce rapidly after observations are obtained in future. On the other hand, in high ECS, the magnitudes of the uncertainty for future projections in global mean SAT changes are large, and they still remain large in future. Because economic damages increase nonlinearly for the global mean SAT changes, the uncertainty of future projections in the economic damages is larger, and still remains larger after obtaining observations in future in high ECS. In particular, peaks of the pdfs of the economic damages shift to more serious values after obtaining observations in future in high ECS.