Washington has seen its share of big protests over the years, and most of them center on the White House, the Mall or the Capitol. That will change tomorrow, when the first big protest of the Obama era -- and the first mass civil disobedience against global warming in this country -- will take place against the not-very-scenic backdrop of the Capitol Hill Power Plant, a dirty symbol of the dirtiest business on Earth, the combustion of coal.

In that one plant -- owned and operated by our senators and representatives -- you can see all the filth that comes with coal. There are the particulates it spews into the air and hence the lungs of those Washington residents who enjoy breathing. There are the profits it hands to the coal industry, which is literally willing to level mountains across West Virginia and Kentucky to increase its fat margins. And most of all there is the invisible carbon dioxide it spews each day into the atmosphere, drying our forests, melting our glaciers and acidifying our oceans.

The power plant is only a symbol, of course -- a lunch counter or a bus station in the fight for environmental justice. We'll sit down at its gates for a single afternoon, but the message is much larger: It's time to start figuring out how to shut down every coal-fired plant on the planet. Success won't come right away because we're up against some of the world's richest corporations, but we have to start turning this tanker around someday, and tomorrow is that day.

This may seem like an odd time to take to the streets -- after all, the new administration has done more in a month to fight global warming than all the presidents of the past 20 years. But in fact, it's the perfect moment. For one thing, our leaders may actually listen -- in the anti-science years of the Bush administration, global warming activists concentrated their work on state capitols, knowing that the federal government would never budge. Now, if we demonstrate that there's real public pressure, we may give the Democratic Congress and the White House some room to act.

More to the point, the time not to act is running out. Climate science has grown steadily darker in the past 18 months, ever since the rapid melt of Arctic sea ice in the summer of 2007 showed scientists that change was coming faster than they'd reckoned. That message was underlined recently at the Washington meetings of the American Association for the Advancement of Science, when Stanford researcher Christopher Field said: "We are basically looking now at a future climate that's beyond anything we've considered seriously in climate model simulations." Our foremost climatologist, NASA's James Hansen, has given that future a number -- any level of carbon dioxide in the atmosphere beyond 350 parts per million, his team has demonstrated, is "incompatible with the planet on which civilization developed."

Since we're already past that number -- the carbon dioxide level is at 387 parts per million -- the fight is on. Indeed, by Hansen's calculation, the world will need to be out of the coal-burning business by 2030, and the West much sooner than that, if we're ever going to get back to 350. It's no accident that he's announced he'll be on hand to get arrested. So will Gus Speth, who ran the United Nations Development Programme, and the farmer and author Wendell Berry who has seen the devastation of his native Kentucky, and many more.

Getting the planet off coal -- getting the planet back to 350 -- will be the main political and economic challenge for the lifetimes of those college students. Those of us who are older won't live long enough to see the final victory, but we can help get it started, by lobbying, by writing e-mails -- and by sitting down in the street on an afternoon in March.