Global change is perhaps the greatest environmental issue facing humanity. The Earth's atmosphere and biosphere are undergoing profound change, due not simply to terrestrial or extraterrestrial geophysical forces, but to the numbers and activities of people. Over the last 100 years the human population soared from little more than one billion to six billion, and over the last 50 years economic activity increased nearly tenfold. Half the Earth's land surface has been modified for direct human use, and nearly all is affected by humans in one way or another. Over two thirds of ocean fisheries are fully or overexploited, and scant pristine coastline exists outside of the high latitudes. The composition of the atmosphere-particularly greenhouse gases, reactive gases, and aerosol particles-is significantly different from a century ago.