President Trump on Monday signed an executive order to withdraw the United States from the Paris Agreement, the pact among almost all nations to fight climate change.

By withdrawing, the United States will join Iran, Libya and Yemen as the only four countries not party to the agreement, under which nations work together to keep global warming below levels that could lead to environmental catastrophe.

The move, one of several energy-related announcements in the hours following his inauguration, is yet another about-face in United States participation in global climate negotiations. During his first term Mr. Trump withdrew from the Paris accord, but then President Biden quickly rejoined in 2020 after winning the White House.

Scientists, activists and Democratic officials assailed the move as one that would deepen the climate crisis and backfire on American workers. Coupled with Mr. Trump’s other energy measures on Monday, withdrawal from the pact signals his administration’s determination to double down on fossil-fuel extraction and production, and to move away from clean-energy technologies like electric vehicles and power-generating wind turbines.

“If they want to be tough on China, don’t punish U.S. automakers and hard-working Americans by handing our clean-car keys to the Chinese,” said Gina McCarthy, former White House climate adviser and former head of the Environmental Protection Administration. “The United States must continue to show leadership on the international stage if we want to have any say in how trillions of dollars in financial investments, policies and decisions are made.”

On Monday Mr. Trump also signed a letter to the United Nations, which administers the pact, notifying the world body of the withdrawal. The withdrawal will become official one year after the submission of the letter.

U.S. efforts to reduce greenhouse gas emissions were already stalling in 2024, and Mr. Trump’s entry into office makes it increasingly unlikely the United States will live up to its ambitious pledges to cut them even further. Emissions dropped just a fraction last year, 0.2 percent, compared with the year earlier, according to estimates published this month by the Rhodium Group, a research firm.

Despite continued rapid growth in solar and wind power that was spurred by the previous administration’s signature climate legislation, the Inflation Reduction Act, emissions levels stayed relatively flat last year because demand for electricity surged nationwide, which led to a spike in the amount of natural gas burned by power plants.

The fact that emissions didn’t decline much means the United States is even further off-track from hitting Mr. Biden’s goal, announced last month under the auspices of the Paris Agreement, of slashing greenhouse gases 61 percent below 2005 levels by 2030. Scientists say all major economies would have to cut their emissions deeply this decade to keep global warming at relatively low levels.

In a scenario where Mr. Trump rolled back most of Mr. Biden’s climate policies, U.S. emissions might fall only 24 to 40 percent below 2005 levels by 2030, the Rhodium Group found.

“President Trump is choosing to begin his term pandering to the fossil fuel industry and its allies,” the Union of Concerned Scientists said in a statement. “His disgraceful and destructive decision is an ominous harbinger of what people in the United States should expect from him and his anti-science cabinet.”

Since 2005, United States emissions have fallen roughly 20 percent, a significant drop at a time when the economy has also expanded. But to meet its climate goals, U.S. emissions would need to decline nearly 10 times as fast each year as they’ve fallen over the past decade.

The United States is also a major exporter of emissions. Because of policies promoted by both Republicans and Democrats, the United States is now producing more crude oil and natural gas than any nation in history. Mr. Trump has vowed to further ramp up production and exports.

While the United States may not be party to the Paris Agreement, it will still be part of the U.N. Framework Convention on Climate Change, which hosts annual climate negotiations known as COPs. This year’s COP will be held in Brazil in November and nations will be announcing new pledges for emissions reductions.

One recent study by Climate Action Tracker, a research group, found that, if every country followed through on the pledges they have formally submitted so far, global average temperatures would be on track to rise roughly 2.6 degrees Celsius, or 4.7 degrees Fahrenheit, above preindustrial levels by the end of the century, well above the 1.5 degrees Celsius the Paris Agreement originally set as a goal.

“Trump’s irresponsibility is no surprise,” said Christiana Figueres, a Costa Rican diplomat and an architect of the Paris Agreement in 2015. “In time, Trump will not be around but history will point to him and his fossil fuel friends with no pardon.”

Chimpanzees live only in African rainforests and woodlands. Orangutans live only in the jungles of Indonesia. But humans live pretty much everywhere. Our species has spread across frozen tundras, settled on mountaintops and called other extreme environments home.

Scientists have historically seen this adaptability as one of the hallmarks of modern humans and a sign of how much our brains had evolved. But a new study hints that maybe we aren’t so special.

A million years ago, researchers have found, an extinct species of human relatives known as Homo erectus thrived in a harsh desert landscape once considered off limits before Homo sapiens came along.

“It’s a significant shift in the narrative of adaptability, expanding it beyond Homo sapiens to include their earlier relatives,” said Julio Mercader, an archaeologist at the University of Calgary and an author of the study, which was published Thursday in the journal Communications Earth and Environment.

Fossils of our early forerunners collected over many decades seemed to confirm the special adaptability of our species. Our ancestors, known as hominins, split off from other apes in Africa about six million years ago and lived for millions of years in open woodlands. They did not seem to live in extreme environments.

Dr. Mercader and his colleagues closely examined environments in East Africa, which has yielded some of the richest troves of hominin fossils. They picked a site in northern Tanzania called Engaji Nanyor where paleoanthropologists had previously found fossils of Homo erectus.

Homo erectus is believed to have evolved about 2 million years ago in Africa. They were the first to reach the stature of modern humans, and they had long slender legs to run on. Their brains were also larger than those of earlier hominins, though only about two-thirds the size of our own.

At some point, Homo erectus expanded out of Africa, getting as far as Indonesia, where they became extinct about 100,000 years ago. In Africa, many researchers suspect, they gave rise to our own species in the past several hundred thousand years before disappearing there as well.

Dr. Durkin and his colleagues set out to determine exactly what kind of environment Homo erectus lived in a million years ago at Engaji Nanyor. They looked at fossil pollen grains, analyzed the chemistry of the rocks and searched for other clues to the landscape.

“These studies are an immense amount of work,” said Elke Zeller, a climate scientist at the University of Arizona who was not involved in the project.

For hundreds of thousands of years, the researchers determined, Engaji Nanyor had been a comfortable open woodland. But around a million years ago, the climate dried up and the trees vanished. The landscape turned to a Mojave-like desert shrub land — an extremely arid place that seemed inhospitable for early hominins.

“The data led us to a pivotal question: How did Homo erectus manage to survive and even thrive under such challenging conditions?” Dr. Mercader said.

Instead of fleeing, the hominins figured out how survive in their changing home. “Their greatest asset was their adaptability,” Dr. Mercader said.

They changed the way they searched for animal carcasses to scavenge, for example. The hominins found the ponds and streams that sprang into existence after storms. They didn’t just drink at these fleeting watering holes. They hunted the animals that also showed up there, butchering their carcasses by the thousands.

The hominins also adapted by upgrading their tools. They took more care when chipping flakes from stones to give them a sharper edge. Rather than just pick up rocks wherever they were, they preferred material from particular places. And once they made a tool, they carried it with them.

“They may have had strategies where they basically say, ‘This is a good tool. I should bring it with me and be ready if we find food,’” said Paul Durkin, a geologist at the University of Manitoba who also worked on the study.

Dr. Durkin and his colleagues found that Engaji Nanyor was at the southern edge of a vast belt of desert shrub lands that stretched out of Africa, across much of the Middle East and into Asia. It’s possible that the adaptability that Homo erectus displayed at Engaji Nanyor helped them expand to other continents.

Dr. Zeller and her colleagues have taken a different approach to studying hominins: creating large-scale climate models to figure out what conditions were like during our evolution. Their models, like the new study, suggest that Homo erectus may have thrived in environments that were once thought too harsh for species other than our own.

Studies like the ones Dr. Zeller and the Engaji Nanyor team are conducting “are all starting to tell the same story,” she said. “We definitely have to look further back in time to understand our adaptability.”