ABSTRACT
The Grand Canyon is in a region once noted for its clear air, but in my many visits to it over the years – the fi rst 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, 640 km (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, China, 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.
