It is clear Mr. Obama’s immediate goal is not to solve the emissions problem, but to get the country moving faster in the right direction. The new rule alone offers little hope that the United States and other nations can achieve cuts on a scale required to meet the internationally agreed limit on global warming. But experts say that achieving the pledge Mr. Obama made in Copenhagen — a 17 percent reduction in the nation’s greenhouse gases by 2020, compared with the 2005 level — would be quite likely, if his plan survives.

Mr. Obama’s effort is aimed not just at charting a new course inside the United States, but at reclaiming for the country the mantle of international leadership in battling climate change. If the policy coaxes more ambitious goals from other countries, experts say it could be a turning point. The test of that will come soon, as world leaders meet in New York in September seeking to make headway on a new global climate treaty. The leaders are supposed to pledge ambitious new emissions targets for their countries by next spring, with a final treaty due in late 2015.

“He’s the first American president to permanently push down the U.S. emissions trajectory and get us within striking distance of the kind of global leadership that will be needed to tackle the problem,” said Paul Bledsoe, an energy analyst with the German Marshall Fund of the United States.

The world’s nations have set a goal to limit the warming of the planet to 3.6 degrees Fahrenheit, or 2 degrees Celsius, above the preindustrial level. If that is to happen, many studies suggest that global emissions need to peak no later than 2020 and then begin to fall. Today, emissions are not falling nearly fast enough in the West, and those reductions are being swamped by a rapid rise in the East. Experts say that a global peak in 2020 is exceedingly unlikely, if not impossible — and that will be true even if the United States and other nations manage to keep the pledges they made in 2009.

Well into the 2020s, it will still be technically possible to meet the global warming target, but the longer nations put off taking bold action, the more expensive and disruptive it will be to do so once they finally get serious.