'Free Solo' climber Alex Honnold often sees melted glaciers on his climbs. It's changed the course of his career.

• 'Free Solo' climber Alex Honnold witnessed the effects of climate change throughout his career.
• He's seen glaciers and ice features melt away in Patagonia and Alaska.

In 2023, world-famous climbers Alex Honnold (of "Free Solo" fame) and Tommy Caldwell tackled the Devils Thumb, a hazardous mountain in southeastern Alaska.

Before their ascent, Honnold and Caldwell hiked to the mountain's base, tripping over piles of boulders. The path didn't always look so treacherous.

"What used to be relatively benign snowfields have melted down into shattered glaciers and exposed rock," Honnold, 39, says in "The Devil's Climb," a new documentary about the journey.

Honnold told Business Insider that the mountain changed significantly over the past decade, which made planning the climb more challenging.

"A lot of the photos were from 2014 or 2016, and in just the span of seven or eight years, the mountain had melted out like a ton," he said. In particular, the northwest face had a different terrain. "It was a different mountain, which kind of changed our climbing experience."

Throughout his climbing career, Honnold has witnessed similarly stark environmental changes. It inspired him to start the Honnold Foundation in 2012, a non-profit that builds renewable energy resources around the world.

"It makes it easy to talk about climate change to people because you see it with your own eyes," he said.

He's seen 'dramatic' landscape changes on his climbs

Honnold and Caldwell previously planned an ice climb in Patagonia, Argentina. But once they got to the area, they learned the ice feature had fallen down the year before.

"It was just gone and would probably never re-form again," Honnold said, noting that it takes hundreds of years for snow to compress back into an ice wall.

In that same area, he's seen a glacial lake extend "double the distance" due to receding glaciers. "Year over year, you see the glaciers move so much that it changes the way in which you can hike into the mountains," he said. "It's really dramatic."

His foundation focuses on communities

In 2023, the Honnold Foundation helped launch the first community-led microgrid project in Puerto Rico. Comprised of rooftop solar panels, it was intended to be a solution to the island's electricity shortage after Hurricane Maria in 2017.

Honnold said the foundation's projects focus on helping communities because "there's no real point in solving environmental issues if you're not also improving human living conditions."

The organization has also installed solar panels in US cities like Detroit and Memphis to help families reduce their electricity bills.

He mentions climate change at corporate events

In a recent interview on the "Rich Roll" podcast, adventure journalist Adam Skolnick shared an anecdote about how Honnold raises money for his foundation by hosting climbing and outdoor events with top business leaders where he mentions the impacts of climate change.

Skolnick said that Honnold will "scold them for their environmental issues and implore them to do good and do better," before taking them on excursions.

While Honnold said said while he rarely leads events this way, climate change conversations come up a lot in his career "because as a climber, you see it so acutely."

"I fundraise for the foundation really in any way that I can," Honnold said. "I'm happy to take money from anybody if it means supporting community solar around the world."

"The Devil's Climb" premieres October 17 at 9pm EST on National Geographic and streams the next day on Disney+ and Hulu.