MADRID (Reuters) - A future U.S. government will have to rebuild relations with China to revive international efforts to combat climate change, former New York mayor Michael Bloomberg, who is running for president, said on Tuesday.

Bloomberg was speaking at a U.N. climate conference in Madrid, where environment ministers are grappling with outstanding issues in the implementation of the 2015 Paris Agreement, which aims to avert catastrophic global warming.

The billionaire, who launched his Democratic presidential bid last month, has funded a delegation of U.S. states, cities and companies committed to honouring the Paris deal despite President Donald Trump’s moves to begin withdrawing.

A diplomatic partnership between the United States and China, the world’s two biggest emitters, was critical to laying the groundwork for the Paris accord under the administration of previous U.S. President Barack Obama.

Relations between Washington and Beijing have since been strained by a protracted trade dispute.

“You will never solve the problem unless you have China in the solution,” Bloomberg, who stepped down as a U.N. special envoy on climate change last month, told reporters.

China’s lead negotiator in Madrid, Zhao Yingmin, said that every country was needed.

“Responding to climate change is a joint global effort, no one should be absent,” Zhao told reporters in response to a question about U.S. moves to leave the Paris accord.

Members of the “We Are Still In” coalition - a group of U.S. states and cities still committed to the Paris goals - are on track to reduce greenhouse gas emissions 37% below 2005 levels by 2030, in line with the Obama administration’s goal, according to a report it released Tuesday.

Nevertheless, former U.S. Secretary of State John Kerry, who served under Obama and was a leading player in brokering the Paris Agreement, said the absence of U.S. leadership was undermining global climate action.

“We are at a critical stage in terms of being behind (our targets) and the absence of U.S. leadership is one component,” Kerry told Reuters at the talks. “The burden falls largely on the G20 to behave like adults, like great nations ought to.”

Negotiators worked late in the night on Monday to hash out rules guiding the use of carbon trading markets to achieve Paris emission reduction targets, a main focus this week.

Environmental groups say the slow pace of the talks does not reflect the anger of youth climate activists who have mobilised in their millions this year.[nL8N28J33D]

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“The gap (between the outside pressure and negotiations inside) is bigger than I have ever seen and it just makes my heart bleed,” Jennifer Morgan, director of Greenpeace International, told Reuters.