Anyone who follows climate change policy debates even casually knows that these debates are shot through with controversy about what ought to be done and who ought to be doing it. What sometimes get lost in these debates, however, are much deeper differences over the nature of the climate change problem itself. That is my focus in this chapter. I will take climate change as a prime example of broader debates over what constitutes "sustainable development'' and draw upon different strands of the sustainability literature to show how these disagreements play out in the climate change context.

What are the elements of the ''climate change problem?'' Some of them involve scientific facts or judgments: the implications of rising greenhouse gas (GHG) concentrations for climatic conditions, and the implications of changes in climatic conditions for attributes of the biosphere that affect the

*I am grateful for the wisdom and advice of many colleagues with whom I have discussed and written about climate change and sustainable development. I alone am responsible for the ideas expressed here.

Perspectives on Climate Change: Science, Economics, Politics, Ethics Advances in the Economics of Environmental Resources, Volume 5, 75-98 Copyright © 2005 by Elsevier Ltd. All rights of reproduction in any form reserved ISSN: 1569-3740/doi:10.1016/S1569-3740(05)05004-2

nature of human existence. Some of these involve social and behavioral scientific judgments: how changes in energy prices, or various market "barriers," might affect energy choices. But no less important are what Howarth (2000) has called the normative criteria for climate change policy analysis.

What makes climate change a "problem?" People approach this question with a variety of beliefs and sentiments and therefore with different emphases placed on various parts of the scientific and behavioral backdrop, including the large continuing uncertainties in that backdrop. If there were broad agreement on what makes climate change a problem, there would still be policy debate over the means for addressing it. For example, economists and technologists would still debate the extent to which regulations and energy prices can and should be relied upon to stimulate reductions in GHG emissions. But the debate would be more muted and more technocratic than today's debate over ends, which often remains loud and incoherent.

In this view of the issue, increased scientific understanding of the problem may be useful but it is hardly sufficient for achieving greater coherence. A fundamentally moral problem cannot be settled primarily through scientific investigation. Partisans on many sides of the debate over climate policy (the means) point to uncertainties about the risks and consequences of climate change to help support their positions. But perhaps what we do not yet understand well about ourselves is just as important as what we do not yet understand about the climate.

With these observations as backdrop, I turn first to a discussion of different ways in which the climate change mitigation problem has been characterized. The most familiar approach to economists is applied cost-benefit analysis. The economics literature is replete with such analysis, but the approach nonetheless is misunderstood on a number of occasions by both its detractors and its supporters. I then discuss what economics can tell us about the design of climate change policy and the linkages between means and ends.1

The conclusions I reach are inherently, and I believe unavoidably, somewhat contradictory. Consideration of costs and benefits of mitigating climate change, as well as of taking proactive steps to encourage adaptation to future climate change, is not just unavoidable but desirable. But how benefits of mitigation and adaptation are defined and measured (and, to a lesser extent, mitigation costs as well) is difficult and ambiguous. The moral dimension of climate change is unavoidable in the discussion of climate change policy. Economists and others who would rely on cost-benefit analysis as a tool for evaluating the climate change policy need to recognize this complication in the definition as well as measurement of preferences for more or less mitigation (or adaptation). The lessons of economics for the design of climate change policy are less cloudy, I believe, and need to be brought forward more forcefully in current policy debates - with due regard for the underlying moral ambiguity surrounding the ends.

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