Europe must cover America's share of the Intergovernmental Panel on Climate Change's budget, Emmanuel Macron said at UN climate talks in Bonn, adding that France would "meet that challenge".
"I can guarantee that starting in 2018 the IPCC will not be short a single euro," he said to loud applause.
A collaboration of thousands of experts worldwide, the IPCC issues reports every few years to synthesise and update the prevailing science about global warming caused by atmosphere-fouling emissions from burning fossil fuels.
President Donald Trump, who has vowed to pull the United States out of the climate-rescue Paris Agreement, has also cut funding to the IPCC, the UNFCCC climate secretariat, the Green Climate Fund and other multinational bodies.
Washington has traditionally contributed about $2 million to the IPCC's budget, which was about $5 million last year.
Sharing a stage with German Chancellor Angela Merkel and UN chief Antonio Guterres, Macron described climate change as "the most significant struggle of our time".
"The point of no return has now been crossed."
'Long way to go'
Envoys are gathered in Germany to negotiate a nuts-and-bolts "rule book" for executing the Paris Agreement.
The hard-fought pact commits countries to limiting average global warming to under two degrees Celsius (3.6 degrees Fahrenheit) over Industrial Revolution levels, and to 1.5 C if possible, to avert calamitous climate change-induced storms, drought and sea-level rises.
To bolster the agreement, nations submitted voluntary commitments to curb greenhouse gas emissions from fossil fuels.
But the 1 C mark has already been passed, and scientists say that based on current country pledges, the world is headed for a 3 C warmer future, or more.
The IPCC is due to bring out a highly anticipated report next year on the feasibility of the 1.5 C target.
Merkel agreed Wednesday that climate change was "a, if not the, central challenge of mankind."
But she conceded the move away from fossil fuel was not always "that easy".
Coal still provides about 40 percent of Germany's electricity needs, and the country is set to miss its own goal of cutting emissions by 40 percent by 2020 from 1990 levels.
jpegMpeg4-1280x720"We have a long way to go yet," the chancellor told conference delegates, adding that job retention, affordability and economic viability were all aspects to be considered when weighing energy projects.
The issue has dogged Merkel's efforts to form a coalition government consisting of her conservative allies, the anti-coal Greens, and pro-industry Free Democrats.
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