Climate diplomats beginning work on the next report from the Intergovernmental Panel on Climate Change (IPCC)— the crucial assessment on global warming that helps shape policy for governments to companies — are meeting in China this week without U.S. officials.

U.S. government scientists participating in the IPCC’s global assessments were issued a stop-work order from the Trump administration, according to media reports late last week. NASA’s chief scientist Kate Calvin, who holds a leadership role in the new report cycle, is not attending the meeting, a spokesperson from the space agency said on Monday.

NASA denied a request to interview Calvin. The White House did not immediately respond to requests for comment.

Up until recently, it was assumed the U.S. was funding a team of roughly 5 to 10 U.S. experts for a team known as the Working Group III Technical Support Unit, which is critical for completing a key part of the IPCC’s next climate report, according to a person directly familiar with the matter. The team’s contracts, which were provided through NASA, were terminated and the U.S. experts were not able to travel to Hangzhou as a result, the person said, who asked not to be named discussing private matters.

The undoing of the commitment is unprecedented in IPCC history and will cause delays to its work, the person said, but a government or philanthropic organization could possibly step in to fill the funding hole.

Earlier on Monday, an IPCC spokesperson said the panel’s secretariat hasn’t received any official communication in relation to any change in the status of the U.S.-based section of the organization’s Working Group III Technical Support Unit. "While awaiting more clarity, we refrain from speculating or commenting on this matter,” the spokesperson said in an email.

The U.S. absence comes amid broader cuts to research funding and a retreat from climate diplomacy under the Trump administration, raising new questions on what the IPCC’s future might look like without U.S. leadership. The group’s assessments are widely viewed as the world’s most trusted source of information on climate change.

"Without the U.S., the IPCC fails,” said Benjamin Horton, director of the Earth Observatory of Singapore, who has contributed to past assessment reports. "The U.S. puts more money, more personnel, collects more data and runs more models for climate science than the rest of the world combined.”

About 18% of IPCC authors have been from the U.S., more than twice the next biggest national contributor, the U.K., according to a 2023 analysis by Carbon Brief.

The meeting, which began Monday and runs through Friday in Hangzhou, will revolve around outlines and budgets for components of the IPCC’s seventh assessment report, which is expected to be released in 2029, and carbon removal and capture technologies. The reports summarize scientific consensus on the state of climate change, guiding policy decisions and negotiations.

At the the opening of the meeting in Hangzhou on Monday, Inger Andersen, executive director of the United Nations Environment Program, said in a video message the goals of the landmark Paris Agreement — which sets to keep global warming well below 2 degrees Celsius — are in danger and "every fraction of a degree avoided” matters in terms of deadly and damaging impacts.

"Science is physics, not politics,” she said. "Science cannot be politicized because science will always remain science. The IPCC has the clout to take this message to the world and show us what must be done, starting with the decisions taken at this session.”

The IPCC was formed in 1988 under the auspices of UNEP and the World Meteorological Organization.

It would be difficult to overstate the confidence the IPCC has in the vast basics of climate science and the influence of its findings in shaping global policy, business and investment. The world began a global charge towards net zero in 2018 after the panel published a special report on global temperatures.

The authoritative climate science body has now produced six assessments since its founding, each thousands of pages long. Over time the IPCC’s reports have become more confident and detailed on humankind’s contribution to the warming planet. In 1995 the IPCC agreed there was evidence of "a discernible human influence on global climate,” while in 2007 it found "warming of the climate system is unequivocal.” In 2021 they wrote: "It is unequivocal that human influence has warmed the atmosphere, ocean and land.”

Not every scientist thinks the IPCC's existence depends on the U.S. Detlef van Vuuren, a Utrecht University professor, climate researcher at PBL Netherlands Environmental Assessment Agency and a past IPCC contributor, said the group would survive if the U.S. decides to leave the organization for good, despite the country’s monumental role over the decades.

"It’s obviously highly problematic that a country that has contributed so much to IPCC, but also to global emissions, would decide that facts are not a good basis to inform climate policy,” he said.