Is our planet in peril? Many scientists fear that global warming is slowly threatening Earth's future.

• Author: Revkin, Andrew C.
• Position: ENVIRONMENT

Throughout much of the 20th century, environmental problems were clear and present dangers. It was possible to see and smell the results: Factories and cars spewed dark clouds over cities like Los Angeles. Raw sewage poured into rivers. Eagles' eggs were cracking because of pesticides in their prey. This may help explain why Republicans and Democrats, acting together in the 1960s and 1970s, passed laws like the Clean Air Act, Clean Water Act, and Endangered Species Act, all of which helped cut pollution and protect wildlife.

Today, some of the most challenging environmental threats confronting the United States and the rest of the world are less obvious, hiding in plain sight and taking their toll very gradually. The loss of rain forests and other wildlife habitat is a daily nibble. The millions of dribbling nozzles at gas stations are polluting America's coastal marshes with one-and-a-half supertanker loads of petroleum every year.

But there is no "slow drip" as potentially serious as global warming--the decades-long rise in the average temperature of the Earth. Global temperature is calculated by tracking thousands of readings from around the world, year after year, and distilling them down to a single number, which is now about 59 degrees Fahrenheit.

Earth's temperature rose about 1 degree Fahrenheit over the 20th century, but the rate of warming in the last 30 years is three times the average rate of warming for the last hundred years. And 2005 appears to have beaten 1998 as the warmest year on Earth in at least a century of measuring.

In 2001, a group of more than 1,000 scientists from around the world (the Intergovernmental Panel on Climate Change) concluded that most of the warming since 1950 was probably caused by a buildup in the atmosphere of carbon dioxide and other gases that trap heat in the atmosphere.

GREENHOUSE EFFECT

These invisible gases let sunlight through, but they prevent some of the resulting heat from radiating back out to space. Because they behave like the panes in a greenhouse, they are called greenhouse gases, and their influence on Earth's temperature is called the greenhouse effect. All other things being equal, the higher the concentration of such gases in the atmosphere, the warmer the planet gets.

Carbon dioxide in particular poses a challenge because it is a by-product of burning fossil fuels, mainly coal and oil, that are the foundation of every modern economy; carbon dioxide can also remain in the air 100 years or more. This means that to limit future warming, actions would...