A senior Obama administration official said that the United States had pressed hard for a public commitment from China and was relieved that it had delivered. But the official, who spoke anonymously because of the delicacy of the matter, called the carbon intensity figure “disappointing,” and said that the administration hoped it represented a gambit that would be negotiated upward at Copenhagen or in subsequent talks.

The Chinese propose, by 2020, to reduce so-called carbon intensity  or the amount of carbon dioxide emitted per unit of economic output  by 40 to 45 percent compared with 2005 levels. By that measure, emissions would still increase, though the rate would slow. That falls far short of what many in Europe and other nations had hoped for  an increase in energy efficiency of at least 50 percent.

Analysts said the Chinese offer might take some of the pressure off the United States, which is offering to reduce the total tonnage of its greenhouse gas emissions “in the range of” 17 percent below 2005 levels by 2020 and 83 percent by 2050. But now China seems to be offering almost no deviation from its business-as-usual path, a more troubling development to some.

In a sense, the Chinese offer is less ambitious than the American proposal because China is already well on the way to its target with existing energy efficiency initiatives, while the American offer would require changes in many government policies. American efforts, though, have been mired in Congressional infighting.

Yet the offers by the United States and China both amount to politically safe opening bids in what is likely to be a long, tough process of negotiations on concrete steps that the two countries should take to address climate change.