Imprisoned two years ago for scaling the tallest skyscraper in the U.K., free climber George King-Thompson has scaled another London skyscraper in a stunt aiming to urge action on climate change.

The 21-year-old climbed the 36-story Stratosphere Tower, a residential block in east London's Stratford neighborhood, unaided Tuesday morning and reached the top in less than half an hour.

The former personal trainer said he picked the skyscraper because he was shocked by the flash floods that recently hit the area. Pudding Mill Lane, a subway station in Stratford, was one of eight stations that closed due to flooding on July 25, when almost a month's worth of rain fell on central London.

"I wanted to raise awareness to the seriousness of climate change at the moment, because only a week before that, there was a heat wave throughout London," King-Thompson said. "I hope to raise awareness by climbing this building to urge political leaders to take action immediately."

The young climber said he spent a week preparing for the feat, studying every aspect and surface of the 147-meter (482-foot) building and trying to climb a few floors overnight in secret.

But there was one thing he wasn’t prepared for.

"First eight floors, the windows were sticky. They had grip. But eight floors and above, they didn’t. So, it’s very slippery, which I underestimated," he said.

King-Thompson admits climbing buildings is dangerous but said he planned the climb so he would not be a danger to the public or cause disruptions.

Police dispatched a helicopter to search the area Tuesday after they received a call that a man "was spotted at height" in Stratford. They said officers attended but did not locate the climber.

King-Thompson was arrested in 2019 after scaling the Shard in London – the U.K.’s tallest skyscraper at 310 meters (1,017 feet) tall – after the owners of the building pressed charges against him for trespassing. He was sentenced to six months in prison in October 2019 and served three.