The Grand Canyon is in a region once noted for its clear air, but in my many visits to it over the years—the first one in 1948-I have noticed a grayish haze that increases in frequency and turbidity. Photographs from space reveal one of its sources: smog drifting eastward across the desert from the Los Angeles basin, 640km (400 mi) away. But there are other sources even further away. Air over the Arctic Ocean has a layer of pollution that can be traced to Europe, Russia, Canada, and the United States. In the late twentieth century, it became clear that environmental problems affect the whole Earth. In former decades, it seemed to most people that problems affecting the natural environment were locally caused, with local impacts. A city’s industries and transport polluted its own air, logging threatened a particular park or wilderness area, and sewage seemed a worry for those downstream in a single watershed. But in this period environmental impacts crossed boundaries and became international or worldwide in scope. As the magnitude of the effects of human actions increased, the size and number of the ecosystems affected by them increased. Radioactive particles, chlorine compounds that react with the ozone shield in the stratosphere, greenhouse gases such as carbon dioxide and methane, and pollutants in the sea, spread worldwide and affected the largest of ecosystems, the biosphere itself.
The images of rapid environmental destruction in the late twentieth century were numerous, and information technology made possible a degree of accuracy in gathering them and an extent of dissemination that made an unprecedented impression on human consciousness. The last half of the twentieth century saw a remarkable expansion of knowledge about the workings of the biosphere, but at the same time activities that damaged the biosphere accelerated faster than ever before. Although the period covered by this chapter is shorter than any of the previous ones, it is the one in which the most rapid impacts of humans were made on the earth, including depletion of resources and impairment of natural systems of life in the land, sea, and atmosphere. Investigation of the structure and dynamics of these communities and the damage being done to them also reached a scale unmatched before.
In 1950, many of the Earth’s ecosystems had been altered by human intervention, but by the end of the century, almost every ecosystem was either degraded or seriously threatened. There were few corners of the globe without evidence of human presence and change caused by humans. Antarctica was dotted with research stations which generated waste and had to arrange for its disposal. Globules of oil and pieces of plastic foam floated throughout the oceans. Passing jet planes and their vapor trails were often visible in the sky from every part of Earth. The pressure of human numbers was pushing settlements into forests and grasslands where natural functions once were dominant. In cities, suburbs,