An American official will address the UN climate meeting in Bonn today, where envoys have battled to make progress in the shadow of President Donald Trump's rejection of a global action plan.
On the penultimate day of the annual climate huddle, most countries will be represented by heads of state or cabinet ministers at a "high-level segment", but Washington sent an acting assistant secretary of state, Judith Garber.
She replaces Thomas Shannon, number three at the State Department, who pulled out because of a "family emergency".
Garber will address delegates in the afternoon, just three days after White House officials drew the ire of conference-goers by hosting a sideline event defending the use of fossil fuels at a forum focused on reducing planet-warming emissions from burning coal, oil and gas.
"It will be very interesting to see both the content and the tone" of Thursday's speech," said Alden Meyer of the Union of Concerned Scientists.
Naomi Ages, a Greenpeace climate campaigner, said Garber would "likely reiterate Trump's decision to withdraw, or try to bargain for better terms."
Announcing Garber's participation, the State Department emphasised that the Trump administration's position on the climate-rescue Paris Agreement "remains unchanged".
"The United States intends to withdraw from the Paris Agreement as soon as it is eligible to do so, unless the president can identify terms for engagement that are more favourable to American businesses, workers, and taxpayers," it said in a statement.
The United States ratified the hard-fought global pact, championed by former president Barack Obama, just two months before Donald Trump, who has called climate change a "hoax", was elected to the White House.
Trump announced in June that America would abandon the pact, but the rules prescribe this cannot happen until November 2020.
The US, the State Department said, "is participating in ongoing negotiations... in order to ensure a level playing field that benefits and protects US interests."
The United States is the world's biggest historical greenhouse gas polluter, and second only to China for current-day emissions.
Its presence at the Bonn talks has not been universally welcomed, especially as it has taken a tough line on a demand from developing countries for a firmer commitment to climate finance.
The 2015 Paris Agreement, which took more than two decades to negotiate, commits countries to limiting average global warming to under two degrees Celsius (3.6 degrees Fahrenheit) over Industrial Revolution levels, and 1.5 C if possible, to avert calamitous climate change-induced storms, drought and sea-level rises.
Nations submitted voluntary emissions-cutting commitments to bolster the deal.
A report on Wednesday said America's withdrawal will boost global temperatures, calculated on current country pledges, by nearly half a degree Celsius by 2100, for a total of 3.2 C.
UN chief Antonio Guterres, French President Emmanuel Macron and German Chancellor Angela Merkel led a diplomatic push Wednesday to reinvigorate the Bonn talks.
Labelling climate change "the defining threat of our time", Guterres said continued investment in fossil fuel would mean an "unsustainable future".
Macron described climate change as "the most significant struggle of our time", while Merkel said it was "a, if not the, central challenge of mankind".