Climate change is a global phenomenon, but the institutions needed to implement effective policy reside mainly with national governments. This mismatch explains why serious efforts to control emissions of greenhouse gases, such as markets for emission credits, are fragmented across national and regional lines. Climate policy is emerging from the bottom up rather than through globally orchestrated treaties such as the Kyoto Protocol. The authors of this Policy Forum state that fuller efforts to control emissions will require serious engagement of the United States and developing countries, as well as viable schemes for integrating the many fragmented policies that are arising as governments grapple with the climate challenge.

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.

When addressing an externality such as air pollution, regulators can control policy inputs (e.g., pollution taxes and technology standards) or outputs (e.g., emission caps). Economists are familiar with this debate, known broadly as "prices vs. quantities," but analysts of international environmental agreements have rarely focused sustained attention to such questions. Using an inventory of all international air pollution agreements, we document the historical patterns in instrument choice. Those agreements that require little effort beyond the status quo are usually codified in terms of effort, but agreements that require substantial actions by the parties nearly always deploy a cap on emission quantities as the central regulatory instrument.

We suggest that this concentration of experience with emission caps and paucity of serious efforts to coordinate policy inputs may explain why the architects of international environmental agreements appear to believe that emission caps work best. We illustrate what's at stake with the example of international efforts to control the emissions that cause global climate change. We also show that the conventional history of the agreement that is most symbolic of the superiority of emission caps - the Montreal Protocol on Substances that Deplete the Ozone Layer - has wrongly overlooked a little-known provision that operates akin to a "price" instrument.