Among other factors contributing to smaller households are more single young adults setting up on their own, a trend toward postponing or forgoing marriage, an increase in life expectancy and in the number of elderly people living on their own, and a growing taste for privacy. Because this move toward more and smaller households is most prevalent in the nations that already use the most resources, it could have substantial global impacts. The environmental impacts of providing food for people exhibit a more direct link to population. Finding substitutes for the lost acreage accounts for 60 to 80 percent of the world's deforestation, he reports. Most of this land is less well suited for agriculture than the farmland it is replacing, and the supply of potential substitutes is increasingly limited.
Agriculture also takes a toll on the world's water.
In addition to using surface water, farmers are mining underground aquifers. Although currently there is enough land and water, in theory, to feed Earth's population, Pimentel reports that per capita availability of cereal grains, the source of 80 percent of the world's food, has been declining for 15 years. Most nations today rely on imports of surplus grains. Only a handful of countries account for 80 percent of all cereal exports, with the United States providing the largest share. Acknowledging these daunting numbers does little to answer the question of how many people Earth can sustain.