Global society is seriously threatened by the environmental impacts of human activities. Although the Millennium Ecosystem Assessment and the Intergovernmental Panel on Climate Change have been established to analyze biophysical aspects of global change, there is no equivalent effort to assess the role of individual behavior in creating those environmental threats. The authors of this Policy Forum propose the institution of a Millennium Assessment of Human Behavior to establish serious dialogues about what can and should be done. It would draw heavily on social scientists and would engage groups of citizens globally in public forums to explore the moral elements and the consequences of choices about environmental change.

Uncertainty can hamper the stringency of commitments under cap and trade schemes. We assess how well intensity targets, where countries' permit allocations are indexed to future realised GDP, can cope with uncertainties in a post-Kyoto international greenhouse emissions trading scheme. We present some empirical foundations for intensity targets and derive a simple rule for the optimal degree of indexation to GDP. Using an 18-region simulation model of a 2020 global capand-trade treaty under multiple uncertainties and endogenous commitments, we estimate that optimal intensity targets could achieve global abatement as much as 20 per cent higher than under absolute targets, and even greater increases in welfare measures.

The optimal degree of indexation to GDP would vary greatly between countries, including super-indexation in some advanced countries, and partial indexation for most developing countries. Standard intensity targets (with one-to-one indexation) would also improve the overall outcome, but to a lesser degree and not in all cases. Although target indexation is no magic wand for a future global climate treaty, gains from reduced cost uncertainty might justify increased complexity, framing issues and other potential downsides of intensity targets.