The announcement capped a trip that also resulted in steps to cut tariffs on technology products, adopt warning measures to reduce the chance of accidental military conflict, and ease visas. The two countries together account for about 40 per cent of the world's greenhouse gas emissions, and their commitments are likely to energize talks underway to set new post-2020 targets when climate negotiators meet in Paris in December next year. "The announcement provides a real shot of momentum for international climate negotiations," said one administration official who like others interviewed was not authorised to talk on the record ahead of the joint announcement from Obama and Xi. "Historically, the US and China have often seemed like antagonists in these negotiations." Meeting the goals will be difficult for both countries. China completes a new coal plant every eight to 10 days, and while its economic growth has slowed, it is still expanding at a brisk rate of more than 7 per cent. In the United States, Obama faces stiff opposition on climate issues from Republicans, who want to use their control of Congress to curb the power of the Environmental Protection Agency to limit carbon emissions from power plants, a key part of Obama's "climate action plan." But Xi and Obama have both made climate measures a priority.

China's announcement is the culmination of years of change in attitudes among Chinese now fed up with dire levels of pollution that a study in the British medical journal The Lancet blamed for 1.2 million premature deaths in 2010 alone. China has cap-and-trade pilot programs in five provinces and eight cities. It is also the world's largest investor in solar and wind energy. Moreover, it has barred coal-plant construction in some regions. Such construction has dropped from more than 90 gigawatts in 2006 to 36.5 gigawatts in 2013, according to the World Resources Institute. The joint climate announcement could undercut US critics who have said that limiting greenhouse gases is pointless while China refuses to join such efforts. And it could quiet those in China who have argued that carbon emissions should be measured on a per-capita basis or by improvements in energy intensity. "It is imperative that these two countries - the world's largest emitters of greenhouse gases - show real leadership. This is an important start," said Phil Sharp, president of Resources for the Future. "Agreements like this are more important than they might appear at first glance, because in both countries there are political factions that justify inaction by pointing at the failures of the other country." Obama administration officials called the climate agreement a "historic step" that came together after months of negotiations, beginning when Secretary of State John Kerry raised the subject on a trip to Beijing in February. Obama followed up with a letter to Xi in the spring and met with Vice Premier Zhang Gaoli during the U.N. General Assembly in September. Chinese officials expressed interest in pursuing a deal that could be completed ahead of the Asia-Pacific Economic Cooperation meetings here this week, administration aides told reporters in a background briefing Wednesday.

Whether Obama's target can be set now and achieved technologically - or enforced years after he leaves office - remains unknown. US officials said they were confident the president possesses the authority to set the new climate targets without additional authorisation from Congress. They acknowledged that the Republican takeover of the Senate makes it unlikely that lawmakers will pass laws furthering carbon reductions and that the GOP might attempt to roll back executive actions Obama already has taken to do so. A second administration official said that it is uncertain whether other countries will seek to join the United States and China in pursuing new standards. "Climate negotiations have been fraught for 20 years," this official said. "There are 190-plus countries in these tough negotiations, and it's not like they will all fall into place in five minutes. But this a really important step." Earlier, the United States and China reached agreements designed to defuse tensions over international trade and military maneuvers, even as both leaders courted other Asian nations attending the APEC forum in Beijing to join separate trade pacts.

Chinese and American trade negotiators agreed Tuesday to eliminate tariffs on $US1 trillion ($1.15 trillion) a year of global sales of information and communications technology, including dozens of high-tech products such as GPS devices, medical equipment and game consoles. US Trade Representative Michael Froman called the deal a "breakthrough" that could boost trade and create jobs. The agreement now goes to the World Trade Organisation, where 52 nations must approve. If they do, it would be the first major WTO agreement since 1996. The deal would benefit US companies seeking greater access to China as well as US and Chinese companies with plants in China aimed at making products for the American market. The APEC summit also has provided a venue for high-level encounters between heads of state. Obama held his first face-to-face conservations with Russian President Vladimir Putin since June, seeking to make headway on the conflict in Ukraine, the civil war in Syria and negotiations over Iran's nuclear program. The two leaders spoke three times, for a total 15 to 20 minutes, on the sidelines of the APEC meetings. The Washington Post