[BEIJING] Barack Obama and Xi Jinping have few areas of common ground as they meet this week in Beijing, but could find themselves warming to each other on one longstanding sticking point: climate change.
The presidents of the US and China - the world's top two producers of greenhouse gases - appear willing to confront their differences and make commitments to fight the threat, negotiators say.
The rhetoric is raising hopes for a global agreement next year, when nearly 200 world leaders will meet in Paris.
Speaking at the largest ever gathering of leaders on climate change in September, Vice Premier Zhang Gaoli said China would "strive" for its emissions to peak in the not too distant future.
Obama told the same UN summit in New York: "As the two largest economies and emitters in the world, we have a special responsibility to lead. That's what big nations have to do." Scientists argue that drastic measures must be taken if the world is to limit global warming to the UN's target of two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels, and failing to do so could have disastrous results.
China and the US, which together produce around 45 per cent of the world's carbon dioxide, will be key to ensuring a global deal on reducing emissions after 2020 is reached next year.
The two countries have long been at loggerheads over global targets, with each saying the other should bear more responsibility for cutting emissions of gases blamed for heating up the atmosphere.
But after the 2009 Copenhagen Summit nearly ended in fiasco, salvaged only by a last-minute deal brokered by Obama and China's then premier, Washington and Beijing have started to move closer towards agreement.
The two leaders are due to have a private dinner on Tuesday evening on the sidelines of Asia-Pacific trade talks, and will meet again at Beijing's Great Hall of the People on Wednesday.
Laurence Tubiana, France's special representative at the Paris summit, said the spectre of another round of failed talks had spurred the countries into an "unprecedented" dialogue.
"This is a lesson learned from Copenhagen. It is also truly down to the progress of internal debate in China, linked in particular to the perception of climate risk," she added.
"The two leaders are determined to have an agreement, the hard part is to know where they will set the bar." For Obama, next year's meeting in Paris will be his last chance to live up to a 2008 pledge to reach an international deal on climate change before he leaves the White House.
On the Chinese side, the government is "under enormous popular pressure on environmental issues", said Kenneth Lieberthal, of the Brookings Institution, a Washington-based think tank.
"Certainly the approach of 2015 is a significant agenda item for the two presidents in Beijing," he added, but warned: "How far they will get is premature to say." Jennifer Morgan from US research group the World Resources Institute said that while their talks were good news, there was a risk that they could lead to a deal at the "lowest common denominator".
The European Union, which has pledged to cut greenhouse gas emissions by 40 per cent by 2030 compared to 1990, must pressure the two sides to raise their ambitions with new targets, she said.
China's Communist leaders regularly pledge to reduce the proportion of energy the country generates from fossil fuels, but have shied away from promises to cut the total amount used.
Beijing has so far not given a date for when it expects its emissions to peak, but US and EU negotiators predict it will be between 2025 and 2030.
Washington is expected to pledge to cut emissions by between 20 per cent and 30 per cent by 2025, compared to 2005 levels.
But Obama could find his ambitions scuppered after the Republicans - who count climate change sceptics among their ranks - snatched control of the Senate and tightened their grip on the House of Representatives in mid-term elections.
Republican Mitch McConnell, the next leader of the Senate, has frequently denounced Obama's "war on coal" and wants to cut funding to the US Environmental Protection Agency, a key watchdog.
Asked last month about climate change, McConnell responded: "I'm not a scientist."