For the first time since Donald Trump's ascent to the White House, UN negotiators gather next week to draft rules to take forward the climate-rescue Paris Agreement he has threatened to abandon.
The mid-year round of haggling in Bonn is meant to begin work on a crucial rulebook for signatories of the pact.
But it risks being sidetracked by mounting uncertainty over the world's number two carbon polluter, with Trump at its helm.
"This was supposed to be a highly technical and uneventful meeting to flesh out some of the details in the Paris Agreement. But, obviously, the speculation coming out of Washington is now at the top of our minds," the Maldives environment and energy minister, Thoriq Ibrahim, told AFP.
He chairs the Alliance of Small Island States (AOSIS), a key negotiating bloc in the UN climate forum which will meet from May 8-18.
The deal was sealed at the 21st so-called "Conference of Parties" (COP 21) in the French capital in December 2015, after years of haggling.
A diplomatic push led by Trump's predecessor, Barack Obama, and China's Xi Jinping, saw 195 countries and the EU bloc -- 196 parties in total -- OK the deal to the popping of champagne corks. Palestine has also since joined.
The agreement sets the goal of limiting average global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels -- and 1.5 C if possible.
This will be done by curbing planet-warming greenhouse-gas emissions from burning oil, coal and gas -- an objective to which countries have pledged voluntary, nationally-determined "contributions".
Scientists project that on current pledges, Earth is on track for warming of around 3C -- a scenario that would doom the planet to potentially catastrophic droughts, floods, and rising seas.
Widely hailed as the last chance to stave off worst-case-scenario global warming, the Paris pact was savaged by Trump during his presidential campaign.
He called climate change a "hoax" perpetrated by China, and promised to "cancel" the deal as president.
With the rest of the world on tenterhooks ever since, Trump has said he will make his decision before the next G7 meeting on May 26-27 in Sicily.
"The question of whether this creates a difficult backdrop for the negotiations is clearly a 'yes'," said Paula Caballero, who heads the climate programme at the Washington-based World Resources Institute (WRI).
A State Department official confirmed a US delegation will travel to Bonn, though a "much smaller" one than in recent years.
"We are focused on ensuring that decisions are not taken at these meetings that would prejudice our future policy, undermine the competitiveness of US businesses, or hamper our broader objective of advancing US economic growth and prosperity," said the official, asked about the negotiators' brief.
Some fear a US withdrawal from the agreement would dampen enthusiasm for ramping up national emissions-cutting targets, required to bring them in line with the 2C target.
"I can see some countries... saying: 'Well, why should we do more if the US is doing less?'," said Alden Meyer of the Union of Concerned Scientists (UCS), a veteran observer of the climate negotiations.
The Trump administration has already proposed slashing funds for the UN's climate convention, which hosts the negotiations; for the UN climate science panel; and for the Green Climate Fund that helps poor countries combat global warming.
There has been a chorus of appeals from business leaders, politicians and NGOs for the US not to abandon the agreement.
Much of the pressure is at home, where businesses, majors and governors have pledged to pursue a clean energy track with or without Trump.
Observers say the momentum, politically at least, is unstoppable.
At the last COP, held in Marrakesh in November, news of Trump's election served to spur countries into reaffirming their commitment to the pact.
"International leadership on climate is more diffuse than before, and other countries are stepping up to lead both within and outside of negotiations," said Caballero -- pointing at major polluters China and India cutting back on coal.
In fact, the US may stand to lose the most -- in both political and economic influence.
"It would leave America behind while other countries are benefiting from the huge economic opportunities of a transition to cleaner economies," said Caballero.
Negotiators in Bonn, while attempting to take the pulse of the US delegation, must make progress on the "rulebook" which has an adoption deadline of end-2018.
The guide must clarify what kind of information countries include when they report on emissions, for example, and what counts as a contribution to climate finance.
The next COP, chaired by Fiji, will be held in Bonn in November.