China and the United States, which together produce around 45 percent of the world's carbon dioxide, have set ambitious greenhouse gas emission targets, with Beijing setting a goal for its emissions to peak "around 2030".
It is the first time China, the world's biggest polluter, has set a date, even approximate, for its emissions to stop increasing, and the White House said China would "try to peak early".
At the same time the US set a goal to cut its own emissions of the gases blamed for climate change by 26-28 percent from 2005 levels by 2025.
The declaration came as President Barack Obama met his Chinese counterpart Xi Jinping for talks in Beijing.
Al Jazeera's Scott Heidler, reporting from Beijing, said the deal had been in the works for nine months.
"It's a landmark deal and this is new territory for China to actually set out some goals" for tackling greenhouse gas emissions, said our correspondent.
China will look to "increase the non-fossil fuel share of all energy to around 20 percent by 2030", the White House said in a statement. The proportion stood at 10 percent last year, Chinese officials have previously said.
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 will be key to ensuring that a global deal on reducing emissions after 2020 is reached in Paris 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.
Blaire Palese, CEO of 350.org, a movement that promotes awareness about climate change, welcomed the announcement, saying it "will bring a huge difference, and we will be watching their progress. We hope it will set the momentum for other countries to follow."
Noting that it is "unrealistic to expect an end to the fossil fuel addiction," Palese told Al Jazeera that announcing such goals "sends a message, especially right before the G20 summit, that this will not just be a change for and in the environment, but also in the business sector. It means that energy and fuel will now be looked at differently."
Australia will be hosting world leaders participating in the G20 summit this week.
However, Wednesday's promises are equally fraught with challenges.
While the US, which never ratified the Kyoto Protocol on climate change, has pledged to cut emissions in the past, goals have shifted or been missed altogether.
Its greenhouse gas emissions increased last year, despite Washington setting emissions reduction goals during a climate summit in 2009.
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 and faces a Congress controlled by opposition Republicans in both houses, which will make passing crucial environmental legislation more difficult.
While it was the first time China agreed to a 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.
Source: Al Jazeera and agencies