China and the US have unveiled new pledges on greenhouse gas emissions, as the leaders of the two countries met for talks in Beijing.
US President Barack Obama said the move was "historic", as he set a new goal of reducing US levels between 26%-28% by 2025, compared with 2005 levels.
China did not set a specific target, but said emissions would peak by 2030.
The two countries also agreed to reduce the possibility of military accidents in the air and sea.
The news came during a state visit by Mr Obama to Beijing, which followed a major Asian regional summit.
It is the first time China, the world's biggest polluter, has set an approximate date for emissions to peak.
The two countries together produce about 45% of the world's carbon dioxide.
The unexpected announcement is a bid to boost efforts to secure a global deal on reducing emissions after 2020, to be finalised next year in Paris.
"We agreed to make sure that international climate change negotiations will reach an agreement in Paris," said Mr Xi, speaking to reporters after the announcement.
Roger Harrabin, BBC environment analyst
This agreement between the great polluters is a landmark in the battle against one of the world's most intractable problems.
For years the US feared if it cut emissions, energy bills would rise - and divert jobs to China. Now the relationship is switching, from "we won't if you won't" towards "we will if you will".
President Obama's offer is based on cuts in carbon emissions from coal power (a policy the Republicans threaten to reverse).
China's offer to peak emissions is a long-awaited decision. Its emissions trajectory is now similar to Europe and the USA, just further behind because it still has so many people in poverty.
Scientists will fear this agreement is not yet strong enough. But it does show leadership - and it sends a powerful signal to financiers that investing in dirty fuels for the future is becoming a risk.
'Slow, peak then reverse'
The new goal from the US is up from a previous target to cut emissions by 17% by 2020, compared with 2005 levels.
In September, China told a United Nations summit on climate change that it would soon set a peak for carbon emissions and that it would make its economy more carbon efficient by 2020.
China had previously aimed to reduce its carbon intensity, which meant reducing the amount of emissions per dollar of economic output. This meant that with its rapidly growing economy, its emissions could still rise.
Wednesday's pledge is the first time it has agreed to set a ceiling, albeit an undefined one, on overall emissions.
Mr Obama called Wednesday's agreement "historic" and said the US would work with China to "slow, peak and then reverse the course of China's carbon emissions".
Republican leaders in the US reacted strongly to what they called an "unrealistic plan" proposed by Mr Obama.
"This unrealistic plan, that the president would dump on his successor, would ensure higher utility rates and far fewer jobs," Senate Republican Leader Mitch McConnell said.
The UN has previously warned that the impact of global warming is likely to be "severe, pervasive and irreversible", and would lead to problems including sea level rise, greater risk of flooding and changes to crop yields.
Hong Kong protests
In a rare joint press conference in which they answered questions from pre-selected journalists, Mr Obama and Mr Xi drew attention to the ways in which the two global super powers could work together.
They agreed to boost trade, investment and military co-operation to prevent the likelihood of military accidents.
But differences were also expressed - Mr Obama said he had pressed Mr Xi on China's human rights record and cyber security threats.
Pro-democracy protests in Hong Kong also provided a point of contention. Mr Obama said the US would encourage free and fair elections, while Mr Xi stated issues in the territory were an internal Chinese affair.
Mr Obama said the US would not intervene in South China Sea territorial disputes, but said it had an interest in freedom of navigation and that it hoped the conflicts would be resolved peacefully.
Source: Global Carbon Project