Michael Bloomberg called 'bullsh*t' on this energy technology.

Michael Bloomberg, an outspoken environmentalist and former New York City mayor, had some harsh words for carbon capture and storage, the unproven technology that proponents say will turn fossil fuels into “clean” energy sources.

“Carbon capture is total bullshit” and “a figment of the imagination,” Bloomberg said on Monday, addressing a crowd at the Bloomberg New Energy Finance summit in New York.

Carbon capture involves taking the emissions from coal and natural gas-burning power plants and industrial facilities, then burying the carbon deep underground or repurposing it for fertilizers and chemicals. The idea is that by trapping emissions before they enter the atmosphere, we can limit their contribution to human-caused climate change.

Climate experts say it will be next to impossible to eliminate the world’s emissions without carbon capture systems. The International Energy Agency has called the technology “essential,” given that countries are likely to keep burning coal, oil, and natural gas for decades to come.

But to Bloomberg and other critics, that’s precisely the problem. By investing billions of dollars into carbon capture, countries can effectively delay the inevitable — the end of fossil fuels — and postpone investments in genuinely cleaner energy, such as wind and solar power.

So far, only a handful of carbon capture projects even exist around the world, and many of them have faced steep cost overruns and delays. The Kemper Project in Mississippi — billed as America’s “flagship” carbon capture project — is more than $4 billion over budget and still not operational.

Yet President Donald Trump and many coal industry leaders talk about carbon capture as if it’s already solved the nation’s energy challenges. If we have “clean coal,” why invest in alternatives?

Bloomberg has also used aggressive language to express disdain for the coal industry.

“I don’t have much sympathy for industries whose products leave behind a trail of diseased and dead bodies,” he wrote in his new book, Climate of Hope, which he co-authored with former Sierra Club executive director Carl Pope.

“But for everyone’s sake, we should aim to put them out of business,” Bloomberg said.

The billionaire media mogul has donated some $80 million to the Sierra Club to help the environmental group shut down coal-fired power plants as part of its Beyond Coal campaign.

More than 250 U.S. coal plants have shut down or committed to retire since the campaign began in 2011. Many of those closures came as natural gas prices plummeted, prompting utilities to ditch coal, and as federal clean air and water rules made it too costly to upgrade aging coal plants.

Of the nation’s more than 500 coal plants, only 273 now remain open, and Bloomberg’s philanthropy arm and the Sierra Club are working to shutter those, too.

The former mayor also recently announced a new coal-related donation. Bloomberg told the Associated Press that he plans to donate $3 million to organizations that help unemployed coal miners and their communities find new economic opportunities.

Bloomberg Philanthropies highlighted the struggles of miners in a new film, From the Ashes, to be featured at the Tribeca Film Festival in New York this week.

Coal miners “have paid a terrible price,” he told the AP.