Although multinational agreements on global warming try to spread the burden among all nations, data from the MILAGRO project in Mexico City suggest that the major responsibility for excess production of greenhouse gases and other pollutants lies with the megacities, which constitute a rather small number of culprits and ones that not all nations possess. Perhaps global, and indeed national, initiatives to control air pollution should focus on compelling these megacities to curb their waste.

James M. Bryant
Riverside, Calif.