The Trump administration has fired hundreds of workers at the National Oceanic and Atmospheric Administration (Noaa), the US’s pre-eminent climate research agency housed within the Department of Commerce, the Guardian has learned.

On Thursday afternoon, the commerce department sent emails to employees saying their jobs would be cut off at the end of the day. Other government agencies have also seen huge staffing cuts in recent days.

The firings specifically affected probationary employees, a categorization that applies to new hires or those moved or promoted into new positions, and which makes up roughly 10% of the agency’s workforce.

“The majority of probationary employees in my office have been with the agency for 10+ years and just got new positions,” said one worker who still had their job, and who spoke to the Guardian under the condition of anonymity for fear of reprisal. “If we lose them, we’re losing not just the world-class work they do day to day but also decades of expertise and institutional knowledge.”

Another anonymous staffer called the laid-off workers “dedicated, hard-working civil servants who came to Noaa to help protect lives and keep our blue planet healthy”.

“These indiscriminate cuts are cruel and thoughtless,” the second worker said.

It is not only laid-off employees who will be harmed by the cuts, the second worker said. Ordinary Americans who rely on Noaa’s extreme weather forecasts, climate data and sustainably monitored fisheries will also suffer.

“Words can’t describe the impact this will have, both on us at Noaa and on the country,” the employee said. “It’s just wrong all around.”

Andrew Rosenberg, former deputy director of Noaa’s National Marine Fisheries Service, said Thursday was a “sad day”.

“There is no plan or thought into how to continue to deliver science or service on weather, severe storms and events, conservation and management of our coasts and ocean life and much more,” he said. “Let’s not pretend this is about efficiency, quality of work or cost savings because none of those false justifications are remotely true.”

Among those who received the emails were workers hired through Schedule A, a hiring authority agencies can use to connect with diverse candidates including veterans and people with disabilities. Schedule A workers have a two-year probationary period in which they are at-will employees and enjoy fewer labor protections.

All of the probationary employees at Noaa’s Environmental Modeling Center, which improves weather, marine and climate predictions, lost their jobs, one employee posted on social media.

The Maryland senator Chris Van Hollen called mass layoffs within the commerce department “flatly illegal” in a letter on Wednesday to the commerce secretary, Howard Lutnick.

That same day, the climate non-profit Union of Concerned Scientists sent a separate letter to Lutnick, signed by more than 2,500 scientific experts calling for Noaa to maintain its funding and staff.

“This will cost American lives,” said the congresswoman and ranking member of the House science, space and technology committee, Zoe Lofgren, in a written statement. Her comments were issued alongside Congressman Gabe Amo’s, the ranking member of the subcommittee on environment, after news of the firings broke.

“By firing essential staff who work tirelessly on behalf of the American people, President Trump and Elon Musk are playing politics with our national security and public safety,” Amo said. “Leaving Noaa understaffed will inevitably lead to additional chaos and confusion – I call on them to rehire these public servants immediately before preventable tragedy strikes.”

Lutnick assured Congress during his confirmation hearing that Noaa would not be dismantled under his watch. “It seems either Lutnick willingly lied to Congress and the American people or that he has caved in record-breaking time to the destructive agenda of the Trump-Musk regime,” said Dr Juan Declet-Barreto, a senior social scientist at the Union of Concerned Scientists.

Rick Spinrad, who served as Noaa administrator until January, said the cuts were “at best misguided and ill-informed”. At worst, he said, they would be “dangerously risky to the lives and property of Americans all around the country”.

Craig McLean, the former director of Noaa research who served the agency for four decades before retiring in 2022 agreed, calling the firings “callous, insulting, vengeful and offensive”.

“The nation will be compromised in safety, science and international standing by these reckless, blind acts,” he said. “This is not my America.”

The Trump administration has not commented on the firings.