But these modest efforts are being swamped by rising energy use in developing countries like China, India and Brazil. In those lands, economic growth is not simply desirable — it is a moral imperative, to lift more than a third of the human race out of poverty. A recent scientific paper referred to China’s surge as “the biggest transformation of human well-being the earth has ever seen.”

China’s citizens, on average, still use less than a third of the energy per person as Americans. But with 1.3 billion people, four times as many as the United States, China is so large and is growing so quickly that it has surpassed the United States to become the world’s largest overall user of energy.

Barring some big breakthrough in clean-energy technology, this rapid growth in developing countries threatens to make the emissions problem unsolvable.

Emissions dropped sharply in Western nations in 2009, during the recession that followed the financial crisis, but that decrease was largely offset by continued growth in the East. And for 2010, global emissions are projected to return to the rapid growth of the past decade, rising more than 3 percent a year.

Many countries have, in principle, embraced the idea of trying to limit global warming to two degrees Celsius, or 3.6 degrees Fahrenheit, feeling that any greater warming would pose unacceptable risks. As best scientists can calculate, that means about one trillion tons of carbon can be burned and the gases released into the atmosphere before emissions need to fall to nearly zero.

“It took 250 years to burn the first half-trillion tons,” Myles R. Allen, a leading British climate scientist, said in a briefing. “On current trends, we’ll burn the next half-trillion in less than 40.”

Unless more serious efforts to convert to a new energy system begin soon, scientists argue, it will be impossible to hit the 3.6-degree target, and the risk will increase that global warming could spiral out of control by century’s end.