China vowed Thursday to cut its greenhouse gas emissions by nearly half over the next decade, a move that environmentalists and the Obama administration hailed as a major, and perhaps decisive, development toward agreement on a comprehensive climate treaty.

FOR THE RECORD

China’s climate promise: An article in Friday’s Section A incorrectly stated that China had agreed to reduce its overall carbon dioxide emissions by 40% to 45% from 2005 levels by 2020. China actually promised Thursday to reduce its “carbon intensity,” a measure of carbon dioxide emissions per unit of gross domestic product, by 40% to 45% by 2020, compared with 2005 levels. Also, an earlier headline on this online version of the story incorrectly said “China vows to cut greenhouse gas emissions 40% by 2020.”

The announcement came a day after President Obama unveiled a provisional target to reduce carbon emissions in the United States, and said he would attend climate negotiations in Copenhagen next month.

The promises by the two largest emitters of the gases that scientists blame for global warming dramatically raised expectations for the Copenhagen summit. Until this week, many climate activists considered the prospects for the Dec. 7-18 conference bleak.

The U.S. and Chinese announcements offer a “very much needed boost going into the final steps before Copenhagen,” said Jake Schmidt, international climate policy director for the Natural Resources Defense Council.

The Chinese announcement “is a pretty strong signal that China is ready to move forward aggressively on clean energy and global warming,” he added.

China’s pledge was met enthusiastically by leaders in Europe and at the United Nations, where climate chief Yvo de Boer said the vows of emission reductions by China and the United States could help “unlock” an international treaty to curb climate change.

The White House also praised the move by Beijing.

“We welcome China’s intention to cut the growth of their emissions by reducing the carbon intensity of their economy,” White House Press Secretary Robert Gibbs said.

“Building on the president’s productive talks in Beijing, the United States will continue to engage constructively with China on this and other elements of the negotiations going into Copenhagen.”

Jonathan Lash, president of the World Resources Institute, noted that the two countries will need to go further in their pledges to reach the levels of commitment of some other nations.

“But the road to an international agreement is now open more than ever,” he said.

Still, there were signs Thursday that negotiators still have work to do before completing even a preliminary climate deal in Copenhagen -- and that some countries, particularly those that scientists call most vulnerable to climate change, were unimpressed with this week’s announcements.

A group of small island nations Thursday criticized what it called a “lack of ambition” on the part of the United States and other wealthy nations, saying the world’s most developed countries must curb emissions more than they’ve pledged and offer billions of dollars in financial assistance to the developing world. Those island nations wield considerable power in international climate talks.

“These proposals are missing critical elements,” Grenada’s foreign affairs minister, Peter David, who chairs the 43-member Alliance of Small Island States, said in a statement. But he later added: “With clear, ambitious commitments and actions from the developed countries, individually and collectively, we know that we can succeed.”

Schmidt, of the Natural Resources Defense Council, predicted that the negotiators would eventually rally around the “critical” engagement of the United States and China during the conference in Denmark.

In the meantime, he said, “I’m sure there’ll be some grumbling in the first week, first week and a half at Copenhagen.”

In making its announcement about emission cuts, the Chinese government also said Premier Wen Jiabao would attend the Copenhagen summit.

“Wen’s presence at the meeting fully embodies the Chinese government’s great attention to the issue and its political willingness to address the issue with international cooperation,” Chinese Foreign Ministry spokesman Qin Gang said at a news conference in Beijing.

China’s State Council said that by 2020, the country would reduce its carbon dioxide emissions by 40% to 45% compared with levels in 2005.

This is “a voluntary action based on our own national conditions” and “is a major contribution to the global effort in tackling climate change,” the State Council said.

Although the cuts were welcomed, Greenpeace China said the targets did not go far enough, considering the Asian nation’s emissions are expected to continue rising. A pledge in the 45% to 50% range would have been better, it said.

And China’s gross domestic product is expected to grow, so its total emissions might not drop.

Despite this, the recent moves by China and the U.S. show a willingness to lead the globe toward a climate solution, said Yang Ailun, climate campaign manager for Greenpeace China.

“They’re definitely feeling the heat from Copenhagen,” Yang said. “The two big countries are setting up a good foundation. China will have to be more energy efficient, which means more renewable energy. They’ll have to tackle their over-dependency on coal.”

China and the U.S. have sparred over emission reduction commitments.

Beijing is reluctant to agree to any cuts that would jeopardize its economic growth and believes that developed nations, as the biggest polluters historically, should assume a larger share of overall reductions.

Washington has asserted that global warming cannot be stemmed unless China agrees to ambitious cuts. Some lawmakers are reluctant to enter binding agreements unless China and India do too, for fear it will make the U.S. less economically competitive.

On Wednesday, the White House said the U.S. would “put on the table” a commitment to reduce emissions by around 17% below 2005 levels by 2020, and 83% by 2050.

The pledge is consistent with language in a climate bill stalled in the Senate, but it is also different from the Chinese plan because it aims to lower total emissions from 2005 levels.

China’s total emissions could still go up compared with 2005 because its economic output is expected to climb. However, with the new pledge, China would do so at a slower rate.

david.pierson@latimes.com

jtankersley@latimes.com