In a hallway beneath the UN climate change headquarters in Bonn, Germany, Sue Biniaz leans on a table, scribbling some thoughts on a piece of paper.

It’s May 2018, three years after representatives from nearly 200 countries convened in France in an extraordinary display of international unity and agreed to keep global warming below 2C and to pursue a tougher target of 1.5C.

How the Paris climate agreement will achieve that remains an open question. The rules to govern the deal are due to be agreed at the next United Nations climate change conference in the coal-mining town of Katowice in south-west Poland in December. With the clock ticking, diplomats have gathered in the former West German capital for mid-year talks. Things are not going well.

Negotiators rush in and out of adjoining rooms. In years gone by, Biniaz would have been at the centre of these meetings. From 1989 to early 2017, she was a deputy legal adviser at the US state department and its leading climate lawyer. She personally drafted key passages of the 2015 Paris accord. In global warming diplomacy, few are more respected. She still attends UN climate meetings, but only as an observer, with limited access to the behind-doors wrangling. So in Bonn she stands in a visible spot and the meetings come to her.

“Hi Sue!” calls out Chinese negotiator Xiang Gao, who is standing alongside Andrew Rakestraw, a Trump state department official and Biniaz’s former colleague. Beside these sharp-suited hotshots, Biniaz is an elder: rimless oval spectacles, grey hair loosely pulled back. They fill her in, cracking jokes and sounding hopeful about the stream of talks they are co-chairing. Biniaz writes something down.

Biniaz left the department in 2017 and took up teaching in the law schools of Columbia and Yale as well as a fellowship with the UN Foundation, a Washington DC philanthropic institution. She did so partly because she thought nothing as professionally enthralling as the Paris agreement could ever happen to her again. She also predicted, correctly, that the new president, Donald Trump, would renege on the deal. “I just did not want to be part of an administration that was going to take the position that this was a terrible agreement, because I don’t agree with that,” says Biniaz.

Sue Biniaz, the State Department’s lead attorney in climate negotiations, in Chicago. Photograph: University of Chicago

At the UN Foundation she shares an office with two former colleagues from the Obama state department climate team: Julie Cerqueira and Jen Austin.

Between them, Biniaz, Cerqueira and Austin cover the three arms of climate politics outside federal policy: international talks, local and state government, and the business sector. Austin is policy director for the We Mean Business coalition. Cerqueira leads the US Climate Alliance, an umbrella office for “subnational” governments who want to coordinate efforts to combat climate change.

Turnover between administrations is a great Washington tradition, as party devotees slip into the private sector, thinktanks or lobbying firms. But the exodus from the state department climate team has been different, says Biniaz.

“The number of people who have left has been kind of extraordinary and it’s definitely been not just because the administration changed, but it changed into an administration that was very hostile to both the Paris agreement and climate change action more generally,” she says.

Cerqueira says: “A lot of civil servants gave up their nice state department or [Environmental Protection Agency] job or whatever because they are so committed to addressing climate change as fast as possible.” She loved her state job and regrets deeply the change in policy that drove her out.

“You stay in the federal government as long as you feel like you can make a difference and as long as you feel like you can act in good faith and uphold your oath to the constitution,” she says, “and at some point you realise that you can’t do that any more.”

As a result, the most effective diplomatic and legal outfit ever assembled in global climate politics is now spread through the thinktanks and environmental organisations of Washington DC.

“Some people have been calling it climate diaspora,” says Biniaz. “It’s not an organised group. But to varying degrees we keep in touch because our work is related. Well, we keep in touch socially too of course, just because we are friends.”

What now for the Paris agreement?

Trump makes a US Paris climate agreement statement in the Rose Garden of the White House. Photograph: Joyce N Boghosian/White House Press Office

The 2015 Paris accord drew up the global system to bring down greenhouse gas emissions in only the broadest strokes. At just 27 pages long, it represents what could be agreed at the time. But interpreting the agreement into a concrete set of rules is proving an immensely complicated task. For example, the Paris deal asks all countries to submit their plans to fight climate change. But much is left open as to what those plans should cover, or how often they should be updated, using which metrics and baselines, and how much leeway should be given to poor countries with bad data. These questions can all be split into increasingly finer details, with such eventual complexity that in many negotiating parties, the lead diplomat has only a rudimentary understanding of what their staff are working on. Consensus is proving difficult to find.

If it is ever agreed, the “Paris rulebook” will iron out these technical, in some cases toxic, details. Without it, the deal will collapse.

It is over these troubled waters that Biniaz keeps watch in Bonn. Using intelligence harvested from old contacts, she keeps track of which parts of the talks are in most trouble. When they become stuck, Biniaz arranges small, informal gatherings of key negotiators to try to unblock them.

One evening Biniaz held an after-dinner session. “People are in a better mood when they are fed,” she says. Diplomats sat behind nameplates bearing their own names, instead of the name of their country. These meetings are private and completely off the record. Biniaz believes the relaxed setting can help adversaries “inch toward a point of view”.

Biniaz has hosted at least five such meetings, held with the assistance of the Paris-based Institute for Sustainable Development and International Relations (IDDRI), on the margins of the busy 2018 negotiating calendar. These have usually focused on the area of the talks Rakestraw and Xiang oversee, which covers what information countries must provide as they report their efforts to cut carbon pollution. The role is neutral, but there is one ironclad principle under which she operates: the deal cut in Paris is not up for negotiation.

There is an undercurrent at the talks that is disturbing the former US diplomat, who says there are serious attempts afoot to write rules Biniaz thinks “ignore the language” agreed in 2015.

Former secretary of state John Kerry and a team of advisers including Sue Biniaz discuss the final draft language of the Paris agreement. Photograph: State Department photo

Based on legal language Biniaz helped to draft, Barack Obama and Xi Jinping agreed to universal rules that gave flexibility for the poorest countries. That compromise is credited with making Paris possible. It was good for small, vulnerable nations that feared China’s gigantic future emissions. But for emerging, coal-dependent economies it was hard to swallow. Hence their efforts now to reverse the deal and the possibility that a dual deal could be reached in Poland under which China is subject to different rules than the US.

A group of developing countries, spurred by US betrayal and most likely coaxed on by China, are now trying to create one set of rules for the rich and one for the poor. This approach would govern China’s massive emissions more loosely, which would be unacceptable to the US and EU. If talks fail to produce a rulebook in Katowice over the coming weeks, it will be over this.

The collapse of influence

A logo at the American Pavilion during the UN climate change conference in Bonn, November 2017. Photograph: Ronald Wittek/EPA

It does not help that US diplomatic influence over climate change has collapsed under Trump. Just three years ago the Obama state department, with Biniaz as its lieutenant, ran the show. The Trump administration diplomats who still turn up to negotiate are tolerated, but their power has evaporated.

As an Obama diplomat, Cerqueira built alliances between countries with similar interests. In her new job, the international experience has come in handy. As cooperation and dialogue on climate change with the US federal government has dried up, other – usually European – nations have sought diplomatic connections with governors and mayors across the US. Cerqueira is often their first call.

When the UK and Canadian governments began gathering names for a global pact to end coal power, she says, “they wanted to coordinate on the recruitment of states into the alliance and so we are like a natural fit”. So far, seven US states and two cities have signed up.

A short ride across town from the UN Foundation is the Brookings Institution, where Todd Stern has taken up a fellowship. If Biniaz was the driving intellectual force behind US efforts in Paris, Stern, as Obama’s special envoy for climate change, was its voice and face. His relationship with China’s lead negotiator and government minister Xie Zhenhua was a crucial factor when talks hit the skids in the final hours. The final draft contained a single word – some claimed it was a typo – the US found unacceptable. As the talks threatened to unravel, Xie stepped in, backed the change the US demanded and the gavel came down.

Todd Stern (centre) with Su Wei (left) and Xie Zhenhua (right) in Doha in December 2012. Photograph: IISD

Unlike many of his staff, Stern had no intention of staying on after the Paris deal was struck. He had had enough of the constant travel, the time away from his three children. He began packing up his offices and in May 2016, after striking a final bargain with his old friend Xie for the US and China to ratify the agreement together, he departed. Like Biniaz, he had agreed to teach a course at Yale. He was thinking about writing a book: the story behind the global accord. He was not thinking much about the ongoing negotiation of the Paris rulebook; certainly he didn’t imagine getting involved. Then Trump won.

“I really had been assuming that Clinton was going to win,” he says. Initially, Stern thought “wiser voices in the room” would convince Trump to remain in the Paris agreement. “Obviously that changed over time.” But he knew immediately that “it wasn’t good and we weren’t going to see anything like a bona fide climate change effort internationally or domestically”.

In September 2017, a few months after Trump had announced his intention to leave the Paris deal, Stern travelled to New York to talk to old contacts as they gathered for the UN general assembly. He wanted to find out what other countries were thinking. How would the negotiations on the Paris rulebook, so critical to its function, be conducted with US leadership stripped away?

Word of the binary system being pushed by China and its allies was circulating. This was something Stern and Biniaz believed they had buried in Paris. If US climate diplomacy has had a defining theme over years and across presidencies, it is that its great rival China must be bound by the same conditions. If December’s talks deliver a dual rulebook, it could stop the US rejoining the deal under a different president, says Biniaz. Neither Stern nor Biniaz denies that making sure this doesn’t happen is guiding their involvement. Six current and former non-US diplomats interviewed for this story said they thought this was at least partly their goal.

Certainly, Stern heard enough in New York to continue turning up. Over the past two years he has dropped in to most major meetings. He lunches and listens and tactfully gives his views. His book has made little progress.

His travels have included a visit to Beijing in May this year, where Xie hosted him at Tsinghua University. The two spent a lot of time together, says Stern, talking about the state of the negotiations.

“My lodestar is: what is the way in which we stay faithful to the Paris agreement?” he says. “Not what does the US want or not, because I can’t speak for the US, and it won’t come as a surprise to you that people aren’t all that interested about what the US likes or doesn’t right now.”

Delegates including Biniaz and Stern huddle to resolve outstanding issues at COP17 in Durban, South Africa, December 2011. Photograph: IISD

A senior diplomat from a developing country told the Guardian, speaking under condition of anonymity: “Some would say Todd and Sue are trying to keep the whole thing from crumbling. Others would say they are trying to keep some soft power for the US. Others could say they want to still feel relevant, or make sure that their vision of what the Paris agreement should be prevails.”

The US vision of the Paris deal is, broadly, what Obama’s state department got from Paris. They wanted voluntary targets, rather than ones that were in line with their historical responsibility for creating climate change. Once set, countries’ pledges could not be legally binding, or the US Senate would reject them. The world accepted the rule of the Senate. These priorities arguably weakened the power of the deal to cut emissions. Certainly the current paucity of these voluntary efforts (which are supposed to increase over time, at a pace determined by each government) means the world remains on a path to warming well beyond 2C.

Meanwhile, Trump has argued that Paris “demeaned” the US; that Obama was stitched up; that China will go on building coal plants (it will) while the US cannot (it can, but doesn’t need them).

Despite Trump’s radical reversals of domestic climate policies and threat to withdraw from the deal, his state department still turns up to the negotiations. The US cannot formally leave the Paris agreement until 2020, coincidentally on the day after the next US election.

The state department continues to advocate the same positions it has for years: common rules, non-binding, voluntary targets. A US department spokesperson said its priority was “to ensure a level playing field” – code for tying China to the same rules as all others. Of course, by flagging his intention to leave, Trump has significantly weakened his diplomats’ chances of protecting these goals.

Visitors chat in front of a giant screen featuring information related to global warning at the Bonn Zone in November 2017. Photograph: Patrik Stollarz/AFP/Getty Images

Because of this, Obama’s most senior climate advisers, who devised great chunks of the Paris accord, are now working in the shadows to defend a vision of the deal that is broadly in line with the efforts of the defanged Trump state department.

“It’s stupid to think that we would regard them [Stern and Biniaz] as the progressives,” says a former negotiator for a small island nation, noting that in Paris the US had been forceful in cutting off the most vulnerable countries on Earth from any form of reparation for damage they will incur from climate change.

When countries meet in Katowice this month, Biniaz and Stern will be there. In the final days of the two-week marathon, negotiators will hit a wall. Issues they cannot agree on will be handed to ministers. At that point Chinese and US intransigence could hamper the Paris agreement. “Who will China’s climate chief Xie turn to in that moment?” asks Greenpeace adviser Li Shuo. Stern’s old job at the state department no longer exists. The US delegation will not have anyone at Xie’s ministerial level who knows the deal like Stern.

For the most vulnerable nations on Earth, Trump’s election has had a little recognised, but insidious impact. In climate terms, it has shifted the emphasis away from improving the Paris agreement to defending it. Stern and Biniaz may well help rescue the deal, but will they want to make it more just?

Karl Mathiesen is editor of Climate Home News.