BONN, Germany — So much for the Sino-European climate bromance.

The EU and China were supposed to become the tag team of international climate diplomacy, buddying up after President Donald Trump announced his intention to pull the U.S. out of the Paris climate agreement. Brussels and Beijing, the world's other big polluters, are publicly committed to tackling global warming.

Now this relationship is under strain. The main culprit? With the U.S. marginalized on this scene, the EU is left to carry the standard for rich countries — and China for the developing world.

The strain created by this dynamic has emerged behind closed doors at the COP23 climate summit here. The EU is pushing to level out the share of responsibility between rich and poor countries for man-made climate change and for reducing emissions. For its part, Beijing wants developing countries to have greater leeway to meet targets set out in the 2015 treaty.

The differences are likely to grow more stark at next year's COP24, when the complex set of rules for meeting the goals of the Paris climate agreement is due to be finalized.

At the technical talks in Bonn that ended Tuesday, China pushed hard to demarcate responsibilities between developed and developing countries — called differentiation — throughout the text of the Paris climate agreement in a draft handed to ministers on Wednesday, according to negotiators and observers following the discussions. The political negotiations involving ministers run through Friday.

"China was putting differentiation in anywhere and weren't agreeing to the text," Mark Lutes, head of the World Wildlife Foundation's delegation, said of last week's technical talks.

China is particularly emboldened on the issue now that the U.S. — which has always been staunchly opposed to differentiation — has stepped to the side in the wake of Trump's call to quit the Paris accord.

Bifurcation makes a comeback

The idea of treating rich and poor countries differently is called "bifurcation." The Paris deal managed to paper over the issue by calling on everyone to pitch in and help slash greenhouse gases to the extent that they could.

Now developing countries are returning to the idea that wealthy countries should bear more of the emissions cuts. More than a century of their pollution, after all, is what many scientists say caused a lot of the problem of global warming. In response, developed nations point to burgeoning greenhouse gas emissions from countries like China, India and Brazil to say that everyone should shoulder the weight.

"We thought we put it away in Paris, that we went beyond that. But bifurcation is coming back," said a European diplomat in Bonn, asking not to be named. "It's quite a big fight and it's a very important issue for the EU."

The call to differentiate burdens and standards is especially loud in talks over the transparency and accounting system that will be used to keep track of emissions reductions, financial aid and other steps intended to tackle climate change. It is part of a broader set of rules that countries are negotiating at the COP23 to help them meet the Paris climate agreement's voluntary goals.

'We argue as good friends'

The EU and U.S. have long argued for a single, robust system to make sure everyone is measuring and reporting their emissions and aid in the same way. Without it, there's no way to know, for instance, that one ton of CO2 cut in one country isn't then emitted somewhere else. The poorest and most vulnerable countries are also keen to adopt a tough system, particularly to verify that developed countries fulfill their promise to spend $100 billion a year in public and private aid on climate change from 2020.

Emerging economies, however, are wary of public scrutiny of their record books, and argue they need time and support to catch up to the developed side's experience. They're balancing the need to show that they're acting to curb global warming, without wanting to upset their growing industries.

“Developing countries will need both flexibility and support to implement the framework,” said J. Antonio Marcondes, Brazil’s chief negotiator. “Brazil also believes that developed countries should increase the level of ambition in respect of transparency, and there should be no backtracking from current commitments on transparency.”

The tussle is likely to intensify over 2018, with the Paris rulebook due at the end of next December's COP24 meeting.

Lax transparency and accounting rules, however, could further weaken a landmark climate accord. It is already under criticism for so far failing to do much other than promise to keep global warming well below 2 degrees Celsius and eventually 1.5 degrees by 2100.

The key will be to find a framework that sets all countries on the path to strengthening their data, while recognizing that they're starting from different points, said Yamide Dagnet, a senior associate at the World Resources Institute. Flexibility remains the most contentious issue, but negotiators made good progress on the issue in the first week of the COP23, she added.

EU Climate Action and Energy Commissioner Miguel Arias Cañete stressed this week that the EU would not accept divisions in the Paris deal's transparency and accounting rules — but said Brussels may allow some room for developing countries to catch up.

"Some countries opted for a bifurcated approach; we think that with enhanced transparency there is a single [system]," he said. "There is lots of flexibility to accommodate the different capabilities of the countries."

Any compromise could follow the example set in international agreements to reduce emissions from aviation and refrigerants and coolants, Arias Cañete said. Both give developing countries more time to comply with rules that will eventually apply to everyone.

The trick, however, will be to decide how far to go, as some worry too much wiggle room would make it harder to assess what a developing country is doing to cut emissions, and to pressure it to do more.

The tussle is likely to intensify over 2018, with the Paris rulebook due at the end of next December's COP24 meeting.

While there is tension in the negotiating room, neither Brussels not Beijing want to undermine the narrative of a China-EU climate alliance.

"We do argue quite a lot, but we argue as good friends," Xie Zhenhua, China's special representative for climate change, said Tuesday of Arias Cañete.