There are a lot of reasons not to get too worked up over the impact we're having on our planet. Environmental changes seem nebulous and distant in time and place. Politicians don't appear concerned. Business people say calls to action are alarmist. And -- the ultimate justification for complacency -- even science seems divided.

No wonder most Americans, while professing environmentalism, remain unmoved by the looming but shadowy questions. It's enough to make active environmentalists want to grab us by the lapels -- as a group of them assembled by the nonprofit organization Environmental Media Services attempted to do here recently.

"The story of the last decade" said Bill McKibben, an environmental writer, is that "human beings crossed a certain threshold: We became big enough as a species to affect everything around us. The story of the next decade is increasingly going to be the story of the effects of crossing that threshold."

True, we don't know precisely what these effects will be, said Jane Lubchenco, professor at Oregon State University and past president of the American Association for the Advancement of Science. But scientists agree -- overwhelmingly -- that "we are disrupting the function of the systems on which we depend."

It's no accident that the U.S. public is confused about what science thinks. People who are heavily invested in our current patterns of behavior put money into assuring us that we shouldn't be worried enough to change those patterns. Lubchenco says that thanks to groups such as the Global Climate Coalition -- which calls itself "a voice for business in the global warming debate" -- "We've seen junk science, ignorance and misinformation drown out good, credible scientific information and public understanding."

In fact, the evidence that we're changing the world around us, and changing it dramatically, is unmistakable. Take a recent international study combining 46 years of data on the shrinking of sea ice in Arctic waters. Results indicated a less than 2 percent probability that the melting of the past 20 years was caused by normal climate variation, according to a Dec. 3 news story in The Post. "This strongly suggests that the observed decrease in Northern Hemisphere sea ice is related to [human-caused] global warming," the team of nine scientists wrote in the current issue of the journal Science.

Indeed, the business world increasingly is parting ranks with the do-nothings. European companies -- whose leaders traditionally have been more environmentally driven -- have led the way. But U.S. firms are now following. Ford Motor Co. announced recently that it is resigning from the Global Climate Coalition because it believes there is enough evidence of global warming that companies must begin working together more effectively to reduce carbon emissions. Dow Chemical Co. already had resigned from the coalition.

Whatever the level of certainty, as Lubchenco sees it, our current method is simply "to ignore the possibility of a problem until it hits us in the face." Instead, we should conclude that "uncertainty warrants precaution."

Also at the Washington gathering was Denis Hayes, coordinator of the first Earth Day, 30 years ago. His current project is Earth Day 2000, whose aim is "to forge a global majority around environmental values." Americans should be leading the way toward international engagement with the environment, he said. "But our credibility elsewhere depends on our succeeding at home."

While Americans brought about huge environmental improvements after the first Earth Day, we have been backsliding lately. Over the past 10 years, said Hayes, we've had 12 percent growth in the harmful emissions -- mostly from automobiles -- that contribute to global warming. That 12 percent is more than the total amount of emissions that Latin America produces in a year.

"We must transform the latent support of the American people to force change," said Philip Shabecoff, publisher of Greenwire, a daily environmental news service. The challenge for Americans is to move out of the complacency we're lulled into by the seeming distance and uncertainty of environmental challenges and to act on the values we profess.