Stephen Budiansky in his Correspondence1 “How affluence could be good for the environment” calculates that, thanks to increased crop yields, the 'ecological footprint' of North Americans is much smaller than that calculated by the World Wide Fund for Nature. He touches on the inclusion of per capita greenhouse-gas emissions, but he overlooks the impact of intensive crop production on emissions of the non-CO2 greenhouse gases methane (CH4) and nitrous oxide (N2O).

Agricultural soils and livestock farming already emit around 7 million tonnes of nitrogen as N2O to the atmosphere each year2; worldwide nitrogen-fertilizer applications at the rate Budiansky suggests as typical for the United States (115 kg ha−1) would lead to further increases. Similarly, agriculture around the world constitutes a source of more than 100 million tonnes of methane each year — more intensive farming is likely to increase this methane source, while at the same time increased global nitrogen application and deposition may greatly reduce the soil methane sink (about 30 Tg CH4 yr−1; ref. 3).

Budiansky argues that, with more intensive farming methods and as countries become wealthier, the world's population could significantly increase per capita consumption without increased land requirements.

I doubt whether he has included in his calculations the loss of usable land owing to sea-level rise, as predicted by the Intergovernmental Panel on Climate Change, and to desertification resulting from global warming4.