Carbon does not dissipate rapidly. Some is eventually absorbed by oceans and plants, but about half stays in the atmosphere. And there is no easy way to get it out.
Maintaining current levels would require reducing worldwide carbon dioxide emissions by more than 20 billion tons a year, federal statistics suggest.
For some perspective on that number, consider an icon of the green movement: a 2007 Toyota Prius. Driving it 12,000 miles releases 4,200 pounds of carbon dioxide.
If hybrid cars replaced all 245 million cars in the United States -- more than a third of the cars in the world -- the carbon savings would be less than 3% of the needed reduction.
Rapid industrial development in some of the most populous nations has compounded the problem. Their burgeoning emissions could swamp environmental gains in other countries.
In India, carbon dioxide emissions increased 39% between 1993 and 2004 -- nearly double the global rate. The figure was 36% in Indonesia. China, which saw a 45% rise, now opens a coal-fired power plant every week to 10 days.
Given the scale of the problem, experts see no realistic way to lower the concentration of atmospheric carbon.
In fact, Robert Socolow, a carbon mitigation expert at Princeton University, said that even if the entire world stopped burning fossil fuels, carbon wouldn't approach pre-Industrial Revolution levels for several hundred years.
The only possibility now is to slow the buildup of carbon. If emissions can be reduced enough, the gradual process of warming can be stretched into centuries.
From this perspective, there is some hope. Though the savings from any one measure may look small, in combination, they could add up to something significant, experts said.
There is no shortage of ideas.
The Environmental Protection Agency's administrator, Stephen L. Johnson, said high-efficiency appliances and other products in the Energy Star program last year eliminated greenhouse gas emissions equal to the pollution from 23 million cars.
"As a citizen, each of us has an opportunity to make a difference," he said Friday after the release of the U.N. report.
He urged people to use compact fluorescent light bulbs, which provide the same light as a standard bulb on two-thirds of the energy.
Replacing one standard light bulb in every U.S. home would prevent greenhouse gases equivalent to the emissions of nearly 800,000 cars.
