The major part of world migration is constituted by economic migrants moving from poor countries to the richer countries of the world in order to improve their standard of living. It is an unfortunate reality that if such migrants remained in their own - less-consuming - countries, this would be better for the environment in the longer term. By moving to countries with higher per-capita consumption rates, such as the USA, migrants, as they develop their circumstances, contribute adversely to the global problem of over-consumption of natural resources.
The United States of America, as the world's most voracious consumer society, contributes far more per capita to the world's environmental problems. The USA also has the world's highest level of immigration, something like 800,000 legal immigrants each year, with the possible addition of a further 300,000 illegal immigrants. Natural increase and immigration, the US Census Bureau projects, may combine to swell the population by as much as 50 percent in the next 50 years, bringing it to nearly 400 million. An extra hundred million Americans means, for instance, a staggering amount of carbon dioxide entering the atmosphere and warming the climate.