Bitcoin fell more than 10 percent after the tweet, while Tesla’s share price fell.
Tesla’s announcement in March that it would accept cryptocurrencies came with objections from some environmentalists and investors.
The electric car manufacturer disclosed in February that it had bought $1.5 billion from the world’s largest digital currency.
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But on Thursday it again went back to previous comments.
“We are concerned about the rapid increase in the amount of fossil fuels used in mining and bitcoin trading, especially coal, which emits the most fuel,” Musk wrote.
“Cryptocurrencies are a good idea but it can’t be a big loss for the environment.
He also said the electric car manufacturer does not sell Bitcoin and intended to trade them as soon as the mining industry switches to more sustainable energy.
Market analysts said the move is Tesla’s attempt to concerns about climate change and sustainability.
“Environmental, social and corporate governance (ESG) issues are now the main incentives for many investors. As a clean energy-focused company, Tesla may want to do better in the ESG environment sector,” he said.
Last month, Tesla announced a profit of $438 million in the first three months of the year, up from $16 million a year earlier driven by bitcoin sales and environmental credit.
Tesla’s profits are driven by Bitcoin and credit green lines
Musk has long been one of the world’s most supportive high-profile cryptocurrencies, often tweeting about Bitcoin and other digital currency releases.
In recent months, his tweets have helped turn Dogecoin, originally a social media prankster, into the world’s fourth-largest cryptocurrency.