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 warmingthe 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.
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.
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.
The world's major sources of greenhouse emissionsthe U.S., China, India, Europe, and Japanremain 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."