Everybody in the United States could trade their cars for bicycles. The Chinese could close all their factories. Europe could give up electricity and return to the age of the lantern.

But all those steps together would not come close to stopping global warming.

A landmark report from the United Nations' Intergovernmental Panel on Climate Change released Friday warned that there is so much carbon dioxide and other greenhouse gases in the atmosphere that even if concentrations could be held at their current levels, the effects of global warming would continue for centuries.

There is still hope. The report notes that a concerted world effort could stave off the direst consequences of global warming, such as widespread flooding, drought and extreme weather.

But to reach the ultimate goal of eliminating the threat of global warming would require radical action. Stabilizing atmospheric levels of carbon dioxide - the primary contributor to global warming - would require reducing CO2 emissions by 70 percent to 80 percent, said Richard Somerville, a theoretical meteorologist at the Scripps Institution of Oceanography in San Diego.

Such a reduction would bring emissions into equilibrium with the planet's natural ability to absorb carbon dioxide. The last time the planet was in balance was more than 150 years ago before the widespread use of coal and steam engines.

What would it take to bring that kind of reduction?

"All truck, all trains, all airplanes, cars, motorcycles and boats in the United States - that's 7.3 percent of global emissions," said Gregg Marland, a fossil fuel pollution expert at the Oak Ridge National Laboratory in Tennessee.

Scrapping all fossil fuel-powered electricity plants worldwide and replacing them with windmills, solar panels and nuclear power plants would make a serious dent in the problem - "a 39 percent reduction globally," Marland said.

Carbon does not dissipate rapidly from the environment. 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, a calculation based on federal statistics.

Given the scale of the problem, experts said there is 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, it would take several hundred years for carbon levels to approach those found before the Industrial Revolution.

The only hope now is to slow the buildup of carbon. If emissions could be reduced enough, the gradual process of warming could be stretched into centuries.

From this perspective, there is some hope. While the savings from any one measure might look small, in combination, they could add up to something significant, experts said.

EPA Administrator Stephen Johnson said high-efficiency products and appliances in the Energy Star program last year eliminated greenhouse gas emissions equal to the pollution from 23 million cars. 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.

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