Envoys from nearly 200 country signatories to the Paris Agreement kept a close eye on Washington throughout their 10-day huddle.
Envoys from nearly 200 country signatories to the Paris Agreement kept a close eye on Washington throughout their 10-day huddle for any signal about President Donald Trump's intentions.
On the campaign trail, Trump had threatened to "cancel" the hard-fought pact in which his predecessor, Barack Obama, played an instrumental role in dragging it over the finish line in 2015.
On the second day of the Bonn talks, the White House announced the postponement of a meeting to discuss America's future in the deal, compounding the uncertainty.
A historically small US delegation at the annual round of technical negotiations was thus also left in the dark.
"I personally have met with the head of the (US) delegation a couple of times and... he's just very open in repeating: 'Our position is under review'," UN climate chief Patricia Espinosa said on Thursday.
But delegates insisted that work continued on outlining a nuts-and-bolts "rule book" for implementing the agreement's goals, despite the ever-present "Sword of Damocles", as one put it.
Many commented that the mood was a positive one, and that the American delegation participated in the talks, though cautiously.
There is the fear, however, that whatever progress is made now could easily be swept off the table when the negotiators get together next, perhaps encountering a new US team with a different brief.
"The rest of the world must continue to work towards progress together," said Nazhat Shameem Khan of Fiji, who will preside over the next ministerial-level round of climate talks in November.
"We shouldn't give up because one of the community, one of the family, has decided that they will not walk with us."
Observers pointed to the importance of coming meetings of the G7 and G20, strategic country groupings of which the US is a member, in putting pressure on Trump, who has described climate change as a "hoax" perpetrated by China.
"We work very hard together with many other friends in the world to convince the US that staying in the Paris Agreement is the right way to go," Jochen Flasbarth, Germany's state secretary for the environment, told journalists in Bonn.
"Germany stays committed to the international UN climate process. We believe that it is irreversible and many, many countries indicate to us that nobody has the intention of thinking about another format, another track apart from the UN."
There are fears in some quarters that an American withdrawal may encourage others to follow suit, or at least harm the collective will, built up over two decades of tough negotiations, to act tougher on climate change.
The Paris Agreement commits signatories to limiting average global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.
This will be achieved by limiting emissions from burning coal, oil and gas. But the fossil fuel lobby in America exerts a strong influence over climate politics, both national and international.
"We don't know," what Trump will eventually decide, "but we won't stop our work even if the result is a negative one," Khan said.
"Progress must be pursued irrespective of the odds."
The "rule book" the Bonn participants started work on is meant to guide countries in implementing the Paris Agreement's goals -- what type of information to include in their emissions-curbing updates, for example.
The rules must be finalised by next year, leaving just over 18 months for what appears set to be a difficult negotiation.