Washington - China announced Thursday that it will cut its economy's carbon intensity by up to 45 percent by 2020, the state news agency Xinhua said, and that Premier Wen Jiabao will participate in international climate negotiations in Copenhagen next month.

The move by the world's largest greenhouse gas emitter to set a near-term target of a 40 to 45 percent reduction, coming a day after President Obama set U.S. climate goals for the talks, suggests a possible breakthrough in Demark next month in the long-stalled climate negotiations.

But the State Council's announcement that China will cut its carbon output relative to economic growth, using 2005 as a baseline, fell short of the 50 or 55 percent cut many world leaders had hoped Beijing would make.

Michael Levi, the David M. Rubenstein Senior Fellow for Energy and the Environment at the Council on Foreign Relations, called the announcement "disappointing," because the Energy Information Administration estimates that existing Chinese policies will already cut the nation's carbon intensity by 45 to 46 percent. Carbon intensity is a measure that captures the amount of carbon dioxide emitted per unit of gross domestic product.

"It does not move them beyond business as usual," Levi said. "The United States has put an ambitious path for emissions cuts through 2050 on the table. China needs to raise its level of ambition if it is going to match that. One can only hope that, now that China has made a proposal, negotiators are able to work out something better."

The European Commission, the executive body of the European Union, welcomed "the leadership China is bringing to this negotiation," while noting that it will be "disappointing to some" that the cuts did not go further. Others hailed China's commitment as a step that the country had not been willing to take before.

China is not obligated to cut its greenhouse gas emissions under the current framework for the United Nations-sponsored negotiations. But it is expected to account for 50 percent of the growth in global emissions over the next 20 years, making its output nearly 60 percent higher than U.S. output by 2020. Any future climate treaty will be ineffective unless China agrees to make deep cuts.

Given China's projected growth rates, its emissions are projected to rise even under the plan outlined by Xinhua Thursday. Still, any effort China makes to curb its carbon footprint has an enormous impact.

According to the Washington-based Center for Clean Air Policy, China's current plan to cut its energy intensity by 20 percent translates into a 1.6 billion-ton carbon cut.

Yvo de Boer, who is running the talks as executive secretary of the U.N. Framework Convention on Climate Change, welcomed both the U.S. and China's recent policy proposals. The White House said the U.S. would cut its emissions "in the range of 17 percent" by 2020, relative to 2005 levels.

"The U.S. commitment to specific, mid-term emission cut targets and China's commitment to specific action on energy efficiency can unlock two of the last doors to a comprehensive agreement," de Boer said.

At the same time, he said, "we need continued strong ambition and leadership. In particular, we still await clarity from industrialized nations on the provision of large-scale finance to developing countries for immediate and long-term climate action."

The European Commission voiced a mixed reaction to the U.S. climate targets, which would cut the nation's greenhouse gas emissions 83 percent below 2005 levels by 2050. The commission said the targets have "positive elements" but are "lower than we would like." European nations have called on industrialized nations to collectively cut their emissions by between 25 and 40 percent by 2020, using 1990 emission levels as a reference point.

In an interview Wednesday, Senate Foreign Relations Committee Chairman John Kerry, D-Mass., said he had urged Obama to attend the Copenhagen talks and set international climate goals in order to get "China and India and others to step up" and set their own emission targets.

"It seemed to me fairly straightforward the president ought to lead on this," Kerry said.