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

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 need to be taken soon to slow the buildup of greenhouse gases.

The international climate panel concluded that somewhere between 2 and 10 degrees of additional warming is possible in this century, given the projected growth in greenhouse-gas emissions both in countries that are already wealthy and in those, like China and India, where the economy is growing at a blazingly fast pace.

One place where the impact of global warming can be seen is in the enormous ice sheets covering Antarctica and Greenland. New evidence suggests they could melt and crumble into the sea at a quicker pace than once thought. The melted ice can have far-ranging effects, eventually raising sea levels to a point where flooding of coastal areas could occur in the U.S., Asia, and other parts of the world.

As sea ice on the Arctic Ocean dwindles in the summer, it may be difficult for polar bears to find enough food, and some of the bears are drowning as the ice floes shrink.

'CLIMATE ALARMISM'?

None of this resembles what we would normally think of as a crisis, because the most serious consequences are all decades away and will differ around the world. And while many scientists say the biggest impact on human affairs could be the rise in sea levels, at a rate of a foot or two over the next 100 years or so, this will still be a slow-motion disaster.

While most climate experts agree that humans are warming the planet, debate continues about how much of an impact it is having. Richard S. Lindzen, a climatologist at the Massachusetts Institute of Technology, cautioned against "climate alarmism" in an op-ed piece in The Wall Street Journal, characterizing the 1-degree increase in global temperature since the late 19th century as "barely discernible."

But James E. Hansen, NASA's top climate expert, says that if actions are not taken to curb greenhouse gases within the next 10 years, Earth could undergo such big changes in temperatures, sea levels, and polar ice that it would "constitute practically a different planet" by the end of the 21st century.

Two treaties are in effect right now. A 1992 treaty on climate change called for voluntary measures to cut greenhouse emissions, and was signed by almost all the world's countries, including the U.S. A few years later, countries agreed that a treaty with binding restrictions was needed and so, in tumultuous talks in Kyoto, Japan, in 1997, they drafted an addendum to the original treaty called the Kyoto Protocol, which requires cuts in emissions between 2008 and 2012 by about three dozen industrial powers.

Vice President Al Gore signed the Kyoto Protocol for the U.S. in 1998, but the treaty faced so much opposition that President Clinton never sent it to the Senate for approval. President Bush rejected it outright in 2001: He argues that its limits on using coal and oil would harm the economy by raising energy costs. He also objects that the treaty requires no actions by poorer, fast-growing countries like China.

Last December, 10,000 diplomats, environmentalists, industry lobbyists, protesters, journalists, and people from communities that feel threatened by global warming gathered in Montreal for the United Nations Climate Change Conference. Also in attendance were about 600 students and recent college graduates from as far away as Australia.

NO CONSENSUS

The world's major sources of greenhouse emissions--the U.S., China, India, Europe, and Japan--remain divided over how to proceed. In the meantime, the world is poised to build about 800 new coal-burning power plants in the next several decades, millions of cars and S.U.V.'s are being bought by the growing middle class in Asia, and experts say there is inadequate research under way to find new sources of energy that would not heat the planet.

In a speech near the end of the Montreal meeting, former President Clinton gently chided the countries resisting binding steps to control greenhouse gases, like the U.S. and China, and those squabbling over what to do after the Kyoto treaty expires in 2012. Clinton said the world might be better off if everyone agreed to specific, smaller initiatives to develop and spread technologies that could greatly reduce emissions in both rich and poor countries.

"If you can't agree on a target, agree on a set of projects so everyone has something to do when they get up in the morning," he said. "I think it's crazy for us to play games with our children's future."

BACKGROUND

In 1896, Swedish scientist Svante Arrhenius linked the burning of fossil fuels to global warming. But Arrhenius thought warming would take many centuries and was probably beneficial. Today, most scientists think global warming is a threat, but some think it is a natural part of Earth's endless climate cycles.

CRITICAL THINKING

* Focus on the impact of "gradualism" on how people respond to the threat of global warming. Ask students why they think that if change is gradual, people pay less attention than if the change is more rapid.

Ask students why, if so many scientists have concluded that global warming is a problem, there are still some scientists who caution against "climate alarmism." Is it possible that different scientists demand different standards of evidence?

DEBATE

* Tell students to assume that President Bush is correct when he says adherence to the Kyoto Protocol could hurt the U.S. economy. Then have them take sides on this question: Should the U.S. act to curb greenhouse gases, even if that would weaken the U.S. economy?

DISCUSSION QUESTIONS

* Would you be witting to use less electricity and gas if that would help the U.S. curb emissions that cause global warming?

* Why do you suppose the Kyoto Protocol exempted poorer but fast-growing nations like China?

WRITING PROMPT

* Have students write five-paragraph essays in which they explain why they agree with one side or the other in the global warming debate.

FAST FACT

* Global warming has melted 265,000 square miles of Arctic sea ice over the last 30 years, according to a study by Arctic nations.

WEB WATCH

www.nsidc.org/data/glacier_photo Before-and-after photos of Alaskan glaciers. http://yosemite.epa.gov/oar/globalwarming.nsf/content/ImpactsStateImpacts.html An EPA study of global warming's impact on each state in the U.S.

IS OUR PLANET IN PERIL? > Pages 20-23

1. The Clean--Act and the Clean--Act were two federal, laws passed in the 1960s and 1970s that helped cut pollution.

2. Technically speaking, global, warming is calculated by

a readings from the space satellites of various countries.

b tracking thousands of temperature readings from around the world.

c interviewing scientists who live in tropical climates.

d measuring the difference between summer and winter temperatures.

3. Scientists predict growing greenhouse gas emissions in both wealthy countries and in two rapidly developing countries, China and

a Venezuela.

b Bangladesh.

c Japan.

d India.

4. Briefly explain the Greenhouse Effect.

5. Which of the following does the article identify as a possible future impact of global warming on Antarctica and Greenland?

a the development of plant life in those areas.

b flooding of coastal areas in parts of the U.S. and Asia.

c a warming of the oceans.

d increased air pollution.

6. President Bush has rejected the Kyoto Protocol (named after Kyoto, Japan), which requires nations to

a cut greenhouse gas emissions.

b build more electric and hybrid cars.

c build more nuclear-powered electric plants.

d cut electricity generation by 10 percent.

IN-DEPTH QUESTIONS

1. Should the United States take steps to sharply cut greenhouse gases now, or wait until there is irrefutable proof that global warming will seriously impact the health of the planet?

2. Some scientists who are concerned about global warming charge that those who say there is no problem are motivated more by economics than by science. What do you think they mean?

ANSWER KEY

1. Air; Water

2. (b) tracking thousands of temperature readings from around the world.

3. (d) India.

4. Gases let sunlight through but prevent much of the heat from escaping the Earth. [Similar wording is acceptable.]

5. (b) flooding of coastal, areas in parts of the U.S. and Asia.

6. (a) cut greenhouse gas emissions.

Andrew C. Revkin writes about the environment for The New York Times. His new book, "The North Pole Was Here" (Kingfisher, 2006), examines floating sea ice in the Arctic.