The National Oceanic and Atmospheric Administration, the nation’s premier agency for weather and climate science, is set to lose another 1,000 workers under the Trump administration’s downsizing of the federal government.

The new dismissals would come in addition to the roughly 1,300 NOAA staff members who have already resigned or been laid off in recent weeks. The moves have sparked concern that the agency, which issues lifesaving forecasts from the National Weather Service, would be stretched thin as hurricane and disaster season approaches.

Together, the reductions would represent nearly 20 percent of NOAA’s approximately 13,000-member work force.

Managers within NOAA have been told to draw up proposals for layoffs and reorganizations to trim the agency’s staff by at least 1,000 people, according to eight people who requested anonymity because they weren’t authorized to discuss the plans publicly. The effort is part of the “reductions in force” that President Trump required as part of an executive order last month, as he and the billionaire Elon Musk make rapid, large-scale cuts to the federal bureaucracy.

NOAA managers have been asked to complete their proposals by Tuesday, one of the people said. The proposals are likely to involve eliminating some of the agency’s functions, though managers have received little guidance about which programs to prioritize for cutting.

Representatives for NOAA didn’t immediately respond to a request for comment on Saturday.

The recent employee departures have already affected NOAA’s operations in many realms: predicting hurricanes and tornadoes, overseeing fisheries and endangered species, monitoring the changes that humans are bringing about to Earth’s climate and ecosystems. The National Weather Service has suspended launches of weather balloons in some places because its local offices don’t have enough staff members.

NOAA, a $6.8 billion agency within the Commerce Department, has been singled out for cuts by some of Mr. Trump’s allies. Project 2025, the policy blueprint published by the Heritage Foundation that is echoed in many of the Trump administration’s actions, calls NOAA “one of the main drivers of the climate change alarm industry.” The document calls for the agency to be dismantled and some of its functions eliminated or privatized.

Organizations including the American Geophysical Union, which represents earth and space researchers, have called on Congress to oppose the administration’s actions.

“Undermining NOAA’s operations could risk the safety of millions of Americans and destabilize countless industries, from farming and fisheries to energy and finance, threatening job losses and economic downturn,” the organizations wrote in a letter. They pointed out that, as the planet warms, extreme weather is becoming more frequent and more damaging, making NOAA’s work more critical.

The idea that private companies could replace NOAA in forecasting the weather is a “gross misunderstanding,” said Keith Seitter, a distinguished visiting lecturer in meteorology and climate science at College of the Holy Cross in Worcester, Mass.

“The app on your phone or what you’re watching on TV, those are private-sector companies, but those private-sector companies depend critically on NOAA for all the information that they’re using to create those forecasts,” Dr. Seitter said. “It’s a coordinated effort.”

Employees who are still working at NOAA describe feelings of deep anxiety. Their colleagues have been let go unannounced, meaning they have no idea who might simply not show up for work. With their government-issued credit cards frozen, they can’t buy supplies for research projects or travel to retrieve instruments that have been installed at sea. They are scrambling to back up their scientific data, fearful that programs might be shuttered or leases on buildings canceled.

At least three NOAA facilities were on a list of federal properties that the Trump administration flagged last week for possible sale. The list was later taken down, replaced by a web page that said a new inventory was “coming soon.”

The firings of scientists at NOAA and other agencies, plus potential cuts to federal funding for research at universities and hospitals, have fed worries that the administration is undermining the foundations of America’s modern scientific leadership.

On Friday, crowds gathered at “Stand Up for Science” rallies in cities around the nation including Austin, Birmingham, Ala., Boston, Chicago, Denver, Nashville and Washington.

“This is the most challenging moment I can recall for science,” Michael Mann, a climate researcher at the University of Pennsylvania, told the rally in Washington, where the crowd peaked at 5,000 people, according to the organizers. “Science is under siege,” Dr. Mann said.

The National Weather Service has faced budget cuts, hiring freezes and calls for privatization before, Dr. Seitter said. “But nothing where you’ve just arbitrarily whacked whole chunks out of the work force, or potentially taken away whole chunks of budget that support mission-critical things,” he said.