'Can we just fill the gap? No because we will be too fragmented and too inward looking,' one EU official, involved in climate talks, told Reuters. 'Europe will now be looking to China to make sure that it is not alone.'

The EU's top climate diplomat Miguel Arias Canete will travel to Beijing at the end of March, EU sources said. Offering EU expertise on its plans to build a 'cap-and-trade' system is one area officials see for expanded cooperation.

Enticed by huge investments in solar and wind power in economies such as China and India, Germany, Britain and France are seeking closer ties to gain a share of the business.

But hurdles stand in the way of an EU clean energy alliance with China after the two sides narrowly averted a trade war in 2013 over EU allegations of solar panel dumping by China. 'We need to embrace the fact that China has invested very heavily in clean energy,' Gregory Barker, climate change minister to former British Prime Minister David Cameron, told Reuters on the sidelines the environment conference in Brussels organized by conservative politicians.

'If America won't lead then it's clear that China will.'

Climate change pact

China's partnership with former U.S. President Barack Obama's administration helped get nearly 200 countries to support the Paris climate change pact in 2015.

That agreement, which looks to limit the rise in average global temperature to 'well below' 2 degrees Celsius compared with preindustrial levels, entered into force late last year, binding nations that ratified to draft national plans to reduce greenhouse gas emissions.

But despite Beijing's green policy drive, propelled by domestic anger over smog and the environmental devastation wrought by rapid economic growth, some EU officials are skeptical it can pull as much weight as the United States on climate issues.

'We will make a lot of noises [about allying with China], but let's be honest we lost an ally - a major one,' a senior EU energy diplomat said, speaking on condition of anonymity. 'China's biggest issues are domestic. ... It's clean water, air and food.'

When the United States last took a step back on climate diplomacy, giving up on the 1997 Kyoto protocol on CO2 emissions under former U.S. President George W. Bush, Europe assumed leadership of global negotiations to cap planet warming.

It is among the first now to legislate on how to spread the burden among its member nations of its promise to cut emissions by 40 percent by 2030.

Talks are tough, though, particularly for coal-dependent nations such as Poland, and EU officials fear climate skepticism in the Trump administration may slow efforts.

'This may give the perfect excuse to a number of countries like Poland,' another EU official said. 'The deal has always been that we move when the big players [the United States and China] move.'

Dent, not destroy

Others are more sanguine, saying a U.S. retreat would dent, but not destroy, the current global momentum in tackling climate change - not least because cities, businesses and civil society are driving for change as much as governments.

'If the U.S. doesn't play the game, that's a problem. But it's a trade problem,' an EU diplomat said. 'Maybe European business will win out.'

To date, there has been no sign that any other country is preparing to pull out of the Paris agreement. Days after Trump's election, almost 200 nations at the Marrakesh annual U.N. talks agreed a declaration saying that tackling climate change was an 'urgent duty.'