Divisions within Europe and tension between the United States and China pose major challenges to the next round of United Nations talks on climate change.

The most important U.N. climate conference since the Paris Agreement of 2015 opens on Sunday in Katowice, Poland, in one of the most polluted coal-mining regions in Europe. Success, according to the conference’s Polish host, will need a miracle.

Delegates from about 195 nations are on a deadline to produce a “rule book” to flesh out details of the pact, which the United States, at the behest of President Donald Trump, has announced it will quit.

The 2015 deal aims to shift the world economy away from fossil fuels and limit the rise in global temperatures to between 1.5 and 2 degrees Celsius to avert more extreme weather, rising sea levels and the loss of plant and animal species.

The pact was 20 years in the making and commits almost every country to fight global warming. But rules are needed to bring about real cuts in emissions and ensure they are closely monitored.

U.N. scientists in October added to pressure for a strong outcome from the talks, saying the world will hit the 1.5C threshold around 2040 and unprecedented changes are needed in the way people use energy.

Despite this call for unified action, there are divisions within the European Union and trade tensions between the United States and China, all of which have implications for climate policy.

The president of the talks, Poland’s former deputy energy minster Michał Kurtyka, has expressed hopes of a compromise while at same time acknowledging the scope of the task ahead.

“Only by a miracle can we realize success,” he said last month, although miracles were “a Polish speciality.”

Then earlier this month he added: “Undoubtedly, the geopolitical situation in 2015 was much easier for discussing global agreements than the one we have in 2018. However, my talks with representatives from the whole world show that there is a willingness to reach a compromise in Katowice.”

Political, Economic Rifts

Developing nations and those vulnerable to climate change, such as islands and low-lying states, are looking to big industrialized countries to deepen cuts in emissions.

Senior Chinese climate change official Li Gao told reporters last month that some industrialized countries more responsible for climate change were backsliding on the “polluter pays” principle, which “will make it hard to reach a consensus.”

In the European Union, European Commission climate chief Miguel Arias Canete’s call for more ambitious goals was rebuffed by Chancellor Angela Merkel, who said it would be counter-productive becaus some European countries, including Germany, were struggling to meet their targets.

The United States has a seat at the negotiating table until it can formally quit in November 2020 at the earliest.

It may use its remaining time to try to develop ammunition for its trade dispute with China. Former climate and energy adviser to President Trump, George David Banks, told Reuters the administration wants to make sure it “corrects as many of the flaws of the agreement that it can.”

The United States has always been interested in getting a robust framework for reporting and measuring emissions cuts, particularly for developing countries such as China.

“It helps inform your trade policymaking and negotiations vis-a-vis the Chinese and others,” Banks said.

The United States also plans to promote fossil and nuclear fuels at the talks, repeating a strategy that infuriated global-warming activists last year.

(Additional reporting by Timothy Gardner in Washington; Bate Felix in Paris; Barbara Lewis in London; David Stanway in Beijing and Agnieszka Barteczko in WARSAW.)