French Ecological Transition Minister Monique Barbut, whose country is hosting the five-day meeting in a Paris suburb, told the scientists their "extremely precious" work is crucial as multilateralism has weakened.

"There is also something that should concern us all: The rise of climate-related disinformation on our social media, in our newspapers and even at the heart of our policy political institutions," Barbut said.

"Too many people deny the results of your work," she told the experts from more than 100 countries gathered in a skyscraper in Saint-Denis for the opening of the Intergovernmental Panel on Climate Change (IPCC) meeting.

Their work faces hurdles in the face of a US administration whose president called climate change the "greatest con job ever" and a "hoax" during a speech at the United Nations in September.

One of the co-chairs of an IPCC working group is US climate expert Katherine Calvin, who was fired from her job as chief scientist at NASA following cuts ordered by the Trump administration.

"The statements, for example, from the American administration on the origin of climate change, the fact that it's a hoax, if you will, we still find that quite surprising," said an official at the French ecological transition ministry who requested anonymity.

- Veto power -

The previous IPCC report, released in 2023, had warned that the world was on track to exceed the 1.5C warming threshold by 2030.

The UN now says that safer limit will be breached earlier than feared, greatly increasing the risk of violent storms, floods and droughts and irreversible changes to nature.

The meeting in France launches a process that will culminate with the IPCC's Seventh Assessment Report (AR7), due to be published in 2028 or 2029.

It brings together lead authors of the report in a single venue for the first time, in an effort to tackle interdisciplinary climate questions.

The IPCC operates by consensus.

"If any country opposes the text, the report cannot be approved. Every country has a sort of veto," climate scientist Robert Vautard told reporters last week.

While the US government stays out of the climate fray, dozens of American scientists are among the experts working on the IPCC report.

"IPCC reports are going to continue to underpin climate policies and climate action at every level, including international negotiations," IPCC chairman Jim Skea told the gathering.

- 'IPCC not in crisis' -

There already appear to be disagreements over the timing of the next report's publication.

A group called the High Ambition Coalition, which includes European Union countries and developing nations vulnerable to climate change, wants the assessment to come out in 2028.

That would coincide with the global stocktake -- a review, required under the 2015 Paris Agreement, of the progress countries have made in limiting climate change and its impacts.

But a group of emerging economies and major fossil fuel-producing countries say more time is needed and are advocating for 2029.

The divide echoes the disagreements seen at the UN's recent COP30 climate summit in the Brazilian Amazon city of Belem, which concluded with a deal that left out an explicit call to phase out fossil fuels.

Despite the disagreements over when to publish the next report, Skea told AFP in March: "I don't think the IPCC is in crisis. We will resolve this issue about the timeline."