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A clarification of information in an earlier version of this article has been appended to the end of the article.

Ben Santer has clung to sheer granite walls. He’s hoisted himself onto narrow ledges. He’s inched his way to survival out of a deep, dark and deadly crevasse.

Decades of stressful high-stakes mountaineering have prepared the Lawrence Livermore National Laboratory scientist for his latest perilous challenge: refuting the Trump Administration’s denial of climate change.

Santer’s conclusion — that human activities are warming Earth’s surface — have triggered hate mail, searing criticism from climate change contrarians, even a dead rat on his doorstep.

There could be a personal cost. As a lab employee, his livelihood depends on continued federal funding.

But he has refused to stay silent when administration officials and GOP leaders assert that human-fueled activities aren’t warming the planet, calling it nonsense, “a hoax,” and a “contrived phony mess.”

“They’re wrong. Demonstrably wrong,” said Santer, 62, from his closet-sized office inside the Lab’s high-security facility. Soft-spoken and meticulous, he picks his words as carefully as he chooses his routes over rocks and what he calls “trickle down ignorance.”

“It was necessary to set the record straight,” he said.

Environmental Protection Agency Administrator Scott Pruitt, Texas Sen. Ted Cruz and others have cited analyses that there’s been no global warming trend over the past 18 to 20 years.

So Santer and his colleagues went back to the lab — and, in a tedious and time-consuming year-long effort, updated their previous research. They pored over climate model simulations. They retrieved updated satellite data of atmospheric temperature changes from sites all over the world. They ran calculations, looked for trends and double-checked their math.

“He is not political,” Brenda Ekwurzel, director of Climate Science at the Union of Concerned Scientists, a research and advocacy group, said of Santer. “If you were to go to a medical doctor and ask for a diagnosis, wouldn’t you want them to talk about what he or she found? Not tiptoeing around, but giving you the facts and honest science?”

His doubled-down and fact-checked scientific conclusion: The planet is indeed warming — and unless we do something, will continue to warm.

“These were testable claims,” Santer said. “It seemed necessary to do the science to determine whether the claims had merits or not. That’s what scientists do.”

Now he’s moving on to his next big project: Are humans changing the seasonal cycles? Are both summer and winter warming? If so, the implications are profound.

Problem-solving is a skill acquired over a lifetime for Santer, who lives in San Ramon.

As a boy, his family moved to Europe when his father, a manager in a food services company, was transferred to Düsseldorf, Germany. Speaking no German, Santer was sent off to British Army School.

“As the only kid whose parent wasn’t in the military and the only kid who couldn’t play soccer, I had to learn. Otherwise,” he laughed, “life didn’t look good.”

While in college in England, he yearned to explore the British countryside but didn’t have a car — so he joined the rock climbing club.

Mountaineering has taken him to rock walls over the world, from the snowy Alps and sea cliffs of Wales to Eichorn’s Pinnacle in Yosemite. He was a member of the last team to climb Mount St. Helens before it erupted.

“It’s vertical chess,” he said. “You’re always thinking one or two moves ahead. If I do this, where will I be? If I fall, will this rope hold me?”

Inside, there’s a voice: ” ‘Get this calculation right, Ben.’ ”

One mistake nearly cost him his life. As a young man climbing on the Milieu Glacier in Chamonix, France, he was distracted; his thoughts were with his father, who was battling cancer. A bad jump sent him falling backwards into a deep crevasse.

Tumbling 120 feet, he stopped only when his body lodged into a V-shaped narrowing of the ice. It was cold, and very dark. But, remarkably, he was unhurt.

“You think: ‘I’m not going to get out of here,’ ” he recalled. “I couldn’t move. I feared I would freeze to death.”

“There was just a thin slit of blue sky,” he said. “I recognized if I could reach that, I would be in the world of the living.”

The only solution: climb his own rope. He tied a series of sliding loops, called Prusik knots, which hold by friction when weighted. He secured one knot to his waist harness, then another for a foot loop. He despaired at first; climbing ropes stretch, and there was little progress. But slowly he inched his way to safety.

The lesson? “Don’t be afraid,” he laughs. “But the consequence of your errors are serious.”

“I feel like all of this stuff was training for the experiences I’ve encountered in life, in science,” he said. “You can’t afford to just crap out and give up.”

Boldly, he led a group of the 1995 Intergovernmental Panel on Climate Change and wrote this world-changing consensus opinion two decades before Donald Trump even decided to run for president: “The balance of evidence suggests a discernible human influence on global climate.”

“The 12 words that changed my life,” Santer said.

Since then, he has been awarded the MacArthur “genius” grant; gained membership into the exclusive U.S. National Academy of Sciences and received numerous prizes — among them, an award by Republican former President George H. W. Bush. He even made a guest appearance on Late Night with Seth Meyers.

“Ben Santer was there at the beginning, leading the earliest science,” said Ekwurzel, of the Union of Concerned Scientists.

It’s also earned him critics.

Late one night, he heard a knock on his front door in San Ramon. He opened it to discover a dead rat on his stoop — and a fleeing driver in a yellow Hummer, shouting curses.

Now, Santer’s home phone number is unlisted. So is his home address.

The Heartland Institute, a conservative think tank in Illinois, asserts that Santer’s position runs counter to that of other scientists. Most scientists do not believe human greenhouse gas emissions are a proven threat to the environment or to human well-being, it says.

“Santer and other alarmists have gotten the science wrong for decades,” said Jim Lakely of the The Heartland Institute.

“Ask any honest scientist: Being wrong, and admitting error, is part of the job. The trouble with Santer and others on the alarmist side of this debate about the causes and consequences of climate change is they insist they are infallible,” he said. “They also use their positions to silence any scientists who dare question them. Those are not the actions of a confident scientist, but a polemicist and a bully.”

Will the government continue to support his research, when funding expires towards the end of this year? He shrugs. Unlike rock climbing, computers and data analysis, that’s something far beyond his control.

But there are hints that his efforts may be paying off.

No one is repeating the claim that satellite data shows a leveling off of warming. Instead, the Trump administration is taking a different tack: Even if climate change is occurring, Pruitt said this month in Las Vegas, a warmer atmosphere might not be so awful for humans.

“It’s as if there’s some recalibration of language,” said Santer. “Our findings diminished the ‘wiggle room.’

“We’ll see,” he said. “But you can’t be fearful, or you would do nothing.”

MEET BEN SANTER

Ben Santer is speaking at Livermore’s Bankhead Theater on Thursday, March 29. To order tickets, go to http://raedoroughspeakerseries.org/speaker5/.

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Correction: March 21, 2018

An earlier version of this article did not make clear the details of Ben Santer’s employment.

While he is an employee of Lawrence Livermore National Laboratory, he is not technically a government employee. Instead, his salary comes from one of the many government contracts that sustain the lab.