Climate Changes, American Agriculture, and Natural Resources: Discussion

Rosenzweig's paper deals with material in which I have no expertise, so my comments are brief. The paper provides essential scientific background for this session, and she does a good job in presenting it. My only caveat is that she could have given us a bit more that would have been especially relevant for agricultural economists beginning to think about climate change. I have particularly in mind two aspects of the general circulation models (GCMs) on which projections of future climates depend. Rosenzweig notes the many limitations of these models, including their "inability to simulate current regional climates accurately." She could usefully have made more of this. Most analyses of climate change impacts on agriculture (e.g., Adams) measure the impacts, for example on crop yields, against the present situation, including implicitly the present climate. However, if the present climate had the characteristics the GCMs say it has, the present structure of agriculture likely would be different in some significant ways from what it actually is. This likelihood imparts some ambiguity to comparisons of present yields with those representing impacts of the climate changes given to us by the GCMs. (I owe this observation to my colleague, Norman Rosenberg). In her account of GCM limitations, Rosenzweig also notes the lack of agreement among the models with respect to regional climate effects of global warming. I wish she had told us more about this, and if she had more space in these pages she doubtless would have. The issue is important because the lack of agreement among the GCMs with respect to regional climates, especially precipitation, is considerable, and it is a major problem for those of us seeking to say something sensible about regional impacts of global warming. Rosenzweig notes that largely because of the poorly understood thermal inertia of the oceans, the time required for the atmosphere to warm by the 2.50C to 5.50C predicted for an equivalent doubling of CO2 is estimated to range from several decades to a century. The size of this range is a major problem for analysis of climate change impacts and of policies for dealing with them. If the earth warms by 2.50C to 5.50C in several (three or four?) decades, the ability of natural and social systems to mitigate the impacts almost surely will b much more limited than if the warming occurs over a century. This probably is more true for some sectors, e.g., public utilities, than it is for agriculture, where, as Adams notes, the time scale of most adjustments can b measured in a few decades or less. Nonetheless, agricultural adjustments occur in the context of the total economy, so the great uncertainty about the time scale for adjustments of nonagricultural sectors is relevant for agriculture also. Adams' paper is a most useful complement to Rosenzweig's, which no doubt is what the planners of this session intended. Adams' treatment is a thoroughly professional piece of work. It advances our understanding of the complex problem of climate change impacts while simultaneously revealing the crucial limitations of this kind of analysis. If I here emphasize the limitations rather than the increased understanding, it is because we must overcome the former if we are to have more of the latter.