(Bloomberg) -- A U.S. House subcommittee hearing on “Cleaning Up Cryptocurrency” and the energy usage of blockchains kicked off Thursday with one representative calling the energy consumption of some Bitcoin mines “deeply concerning.”

The hearing comes as the outsized power consumption of cryptocurrency mining is increasingly questioned by politicians and climate advocates. Mining companies run thousands of computers to solve the calculations that underpin certain cryptocurrencies, especially Bitcoin and Ethereum.

Rep. Diana DeGette, a Colorado Democrat, said in her opening statement Thursday that blockchain technology has promising applications but mining’s energy efficiency and carbon emissions need to be at forefront of any discussion about crypto. Lawmakers specifically raised concerns about the repurposing of old coal plants in New York and Pennsylvania to support mining operations.

“We cannot bring retired fossil fuel plants back online or delay the retirement of some of our oldest and least efficient plants in support of energy intensive crypto mining,” said Rep. Frank Pallone, the Democrat who heads the Energy and Commerce Committee.

The hearing was held by the House Committee on Energy & Commerce subcommittee and witnesses included a utility executive and BitFury Chief Executive Officer Brian Brooks. 

A report cited by the subcommittee estimates that together Ethereum and Bitcoin mining operations emitted more than 78 million tons of carbon dioxide into the atmosphere last year --an amount equivalent to the annual tailpipe emissions of more than 15.5 million cars.

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