geochemical cycles, land surface characteristics, and ecosystems so much that the Earth system itself has changed significantly.

Human activity, especially fossil fuel consumption since the Industrial Revolution, is also responsible for substantial increases in atmospheric concentrations of such gases as carbon dioxide and methane. These increases (see Table 7.2) are mostly associated with the per capita consumption of fossil fuels and growth of the human population; deforestation and the production of cement, livestock, and rice for human consumption; the disposal of wastes from human settlement in landfalls; and increased use of fertilizers and industrial and agricultural chemicals. The likely consequences of these gas emissions include a warming of the global climate and a reduction in stratospheric ozone.

Such human activities have accelerated rapidly in recent decades. Between 1950 and 2000 the world's population will have increased from 2.5 billion to more than 6 billion people. Total energy consumption increased from 188,000 petajoules annually in 1970 to almost 300,000 petajoules in 1990, and per capita energy consumption increased from about 50 to 57 gigajoules.¹ Between 1970 and 1990, global forest area decreased by 6 percent, irrigated area increased by almost 40 percent, number of cattle increased by 25 percent, and use of chemical fertilizers doubled.²