Global efforts to address climate change will be dealt a severe blow if U.S. President-elect Donald Trump again pulls the country out of the Paris Agreement, the EU's head of climate change policy has warned.

Trump's transition team has prepared executive orders to withdraw the United States - currently the world's second-biggest polluter, after China - from the main global treaty on climate change, according to sources in the team.

"If that were to happen, that would be a serious blow for international climate diplomacy," EU climate commissioner Wopke Hoekstra told Reuters in an interview.

Another U.S. exit from the Paris Agreement would require other countries to "double down on climate diplomacy" in response, he said.

"There's no alternative to make sure that, in the end, everyone chips in, because climate change is indiscriminate," Hoekstra said of the U.N. climate talks. "This truly is a problem that the world needs to solve together."

The Paris Agreement is the centrepiece of United Nations climate negotiations in which nearly 200 countries discuss steps to curb emissions and funding to pay for these efforts.

The U.S. has played a central role in the talks, including by working with China - the world's biggest polluter and second-biggest economy - to lay the groundwork for recent global climate deals.

A turnaround is expected under Trump, who returns to the White House on Jan. 20. He has called climate change a hoax, and withdrew from the Paris Accord during his first term from 2017 to 2021. Last month he warned the EU it must buy more U.S. oil and gas or face tariffs.

Hoekstra said the EU will "constructively engage" with the new U.S. administration on issues including climate change. He said the Commission is reaching out to U.S. contacts across the political spectrum, including at the non-federal level.

"Making sure that our American friends, as much as is possible, are actually staying on board and are working on this together with us, is clearly something I will strive for," he said.

But even as Brussels faces pressure to step up its climate leadership to fill a potential U.S. vacuum, the EU is set to miss a February deadline for all countries to send new national climate plans to the U.N. The outgoing Biden administration already published the U.S.'s contribution.

Hoekstra said the timings of the EU's political cycle did not line up with the U.N. deadline but that Europe would have its 2035 climate plan ready by this year's U.N. climate summit in November in Belem, Brazil.

"The important thing here is to make sure we have an ambitious number before we walk into Belem," he said. "I can promise you that we will have."