History's evaluation of this generation will surely depend to an important extent on its handling of the climate problem—not just on what gases we leave in the atmosphere but also on what durable climate policy architecture we leave to our heirs. This valuable collection sheds new light on what I believe to be the most difficult and important dimension of the climate policy problem. All who have had a hand in the creation of this volume deserve thanks and applause. In this brief essay I offer some thoughts on what makes the international dimension of the climate problem so difficult and important, on the history of climate policy debates, and on some key elements of policy architecture that those debates have so far produced.

The international dimension

Climate change would be a very difficult issue even without its international dimension, of course. Because much of the benefit of limiting greenhouse gas emissions would accrue to future generations, it would be both economically and politically hard to compare the costs and benefits of mitigation policies even if both were known. But the future benefits of reducing emissions are highly uncertain, both because we cannot confidently predict important regional-scale climate changes and because the adaptation technologies available to future generations are unknown. Similarly uncertain are the pace of technological innovation and the quality of future climate policy design and implementation, both critical factors in determining future costs of emissions reduction.