Beijing (AFP) - The United States and China on Wednesday announced ambitious targets on greenhouse gas emissions as part of a "historic" pact that climate scientists acclaimed but US Republicans denounced as a job-killer.
At a Beijing summit, the leaders of the world's two biggest polluters put their stamp on attempts to breathe new life into action against global warming ahead of international talks in Paris next year.
US President Barack Obama said the joint announcement on the two countries' emissions targets was a "historic agreement" and a "major milestone in the US-China relationship".
Chinese President Xi Jinping said the two had "agreed to make sure that international climate change negotiations will reach an agreement in Paris".
Attempts to deal with climate change, which scientists warn is approaching a potentially catastrophic point of no return, have long been stymied by the unwillingness of the United States and China to work together on the problem.
But China set a target for its greenhouse gas output to peak "around 2030", which Obama commended as a commitment to "slow, peak and reverse the course" of its emissions.
And Obama, who faces scepticism as well as outright denial about climate change in the US Congress, set a goal for the United States to cut its own emissions of greenhouse gases by 26-28 percent from 2005 levels by 2025.
"We have a special responsibility to lead the worldwide effort against climate change," Obama said at a joint news conference with Xi.
- 'A new day' -
China and the US, which together produce around 45 percent 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.
Christiana Figueres, head of the UN Framework Convention on Climate Change (UNFCCC) organising the Paris negotiations, welcomed the announcement as providing "both practical and political momentum".
UN Secretary General Ban Ki-moon said the announcement provided "ambitious" models for other nations to follow.
The International Energy Agency branded the deal "a giant leap for mankind".
Environmental advocates also hailed it as a "breakthrough", with the US-based World Resources Institute president Andrew Steer saying: "It's a new day to have the leaders of the US and China stand shoulder-to-shoulder and make significant commitments to curb their countries' emissions."
But while it was the first time China agreed to an approximate target date for emissions to peak -- officials have previously only spoken of doing so "as soon as possible" -- the commitment was qualified, leaving considerable room for manoeuvre.
China has trumpeted its efforts to reduce dependence on coal and oil in the past, and is the world's largest hydropower producer, with a growing nuclear sector.
But economic growth remains a vital priority and has seen demand for energy soar, with coal use a significant source of Beijing's notorious pollution.
- Opposition in Congress -
Much of Obama's action on climate change meanwhile has been carried out with executive orders rather than cooperation from an often confrontational legislature.
The deadline for Obama's new pledge is in more than a decade's time, but he only has two years left in his presidency. He faces a Congress now set to be controlled by opposition Republicans in both houses after this month's mid-term elections, making passing environmental legislation even more difficult.
In an early portent of the battles to come, the US Senate's new Republican leader was quick to slam Obama's proposed greenhouse gas reductions.
"This unrealistic plan, that the president would dump on his successor, would ensure higher utility rates and far fewer jobs," Senator Mitch McConnell said.
This year is the 35th anniversary of diplomatic relations between Washington and the People's Republic, and the joint announcement was a rare moment of common purpose from the leaders of the world's two largest economies, which regularly clash on issues from trade to rights and are increasingly seen as competing on the world stage.
But Xi said: "The Pacific Ocean is broad enough to accommodate the development of both China and the United States", while Obama responded that a "strong, cooperative relationship" with China was "at the heart of our pivot to Asia".
The European Union pledged last month to reduce emissions by at least 40 percent by 2030 compared with 1990 levels.
The EU accounts for 11 percent of the world's greenhouse gas emissions, compared to 16 percent for the United States and 29 percent for China.