Last December, representatives from 120 nations and the Intergovernmental Panel on Climate Change agreed for the first time that while many uncertainties remain, "the balance of evidence... suggests a discernible human influence in global climate."
As humans become ever more numerous and consumptive, our emissions and pollution begin to rival the natural processes of nature's cycles. At first this occurs locally, then at regional scales, and finally globally.
Globally, the clearing of land for agriculture and commerce amounts to ~50,000 mi2 per year or roughly the area of the State of Iowa.
[The disparity in resource use among nations] really hit home for me when one gentleman from a developing country asked me, "Professor Schnoor, how many children do you have?" And I said quite proudly that I had only two. "Well, that would be equivalent to 18 children in resource equivalents for my country," he replied good-naturedly.
This much is clear: Western countries must decrease their consumption patterns, and developing countries must limit their population growth if we are to limit pollution and enhance the quality of life.