Human impact on the Earth rose even faster than human numbers in the 20th century. Emissions of carbon to the atmosphere from human activities grew from a half billion tons to 7.3 billion tons per year, raising the carbon dioxide concentration in the atmosphere by about 20 percent. Today's level is higher than at any time in the last 150,000 years. Emissions of nitrogen from the combustion of fossil fuels grew 20-fold, to 25 million tons per year. World water withdrawals from all renewable freshwater sources grew 8-fold, to roughly 4,000 cubic kilometers per year currently.
Humans now withdraw annually roughly a quarter to half of all available renewable freshwater.
As the gross domestic product of the human economy increased 16-fold during this century, humans altered the habitats and populations of many other species, raising widespread concerns about extinctions. No one knows whether people can continue to transform carbon, nitrogen, water and other living species at present rates without severe damage to biogeochemical cycles and processes and systems that support human and all other life. Barring global catastrophes, most demographers expect that by the middle of the next century the world's population will be larger, growing more slowly, more elderly, and more urban than it is now. Putting precise numbers on any of these predictions is chancy. For example, in 1998 the U.N. estimated world population in 2050 at 8.9 billion, nearly three billion people more than we have now.