(Bloomberg) -- Elon Musk stepped up his criticism of Bitcoin by saying he believes in digital currencies but that environmentally unsustainable mining of cryptocurrency outweighs its benefits.The Tesla Inc. chief executive officer said in a tweet Thursday he worries about “massive use” of coal and other carbon-intensive energy to generate electricity needed to mine digital currency. He also reiterated his call for a carbon use tax in a separate tweet.Earlier, Musk posted a chart on Twitter from the University of Cambridge showing Bitcoin’s electricity consumption has skyrocketed this year. It’s the second day he’s criticized crypto mining and follows a decision to suspend Tesla car purchases using Bitcoin.In February, Tesla said it had purchased $1.5 billion in Bitcoin and planned to accept it as a payment, triggering a rally in its own stock as well as the currency. But the about-face on Wednesday sent the value of it and other cryptocurrencies sliding.Bitcoin remained not far from the lowest levels of the day after Musk’s latest tweet. The largest cryptocurrency was down about 9% to $49,700 as of 5:30 p.m. In New York, though off the lows of around $46,000 reach earlier.Musk has been an early corporate champion of adopting digital currencies and has helped spur the popularity of Dogecoin, a cryptocurrency started as a joke in 2013.Tesla shares rose 0.3% in postmarket trading after closing down 3.1% to $571.69. The stock is down about 19% this year.(Corrects to state in that Elon Musk helped popularize Dogecoin in sixth paragraph.)For more articles like this, please visit us at bloomberg.comSubscribe now to stay ahead with the most trusted business news source.©2021 Bloomberg L.P.