Tesla's boss Elon Musk suddenly accepted bitcoin to buy his electric vehicles, putting the cryptocurrency's energy use in the headlights.
Some Tesla investors, along with environmentalists, are increasingly critical of the way bitcoin is "mined", using large amounts of electricity generated from fossil fuels.
Musk said on Wednesday that he supported that concern, particularly "the use of coal, which has the worst emissions of any fuel."
So how dirty is virtual currency?
Unlike mainstream traditional currencies, bitcoin is virtual and is not made of paper or plastic, or even metal. Bitcoin is virtual but power hungry because it is built using high-powered computers worldwide.
At current rates, such bitcoin "mining" consumes as much energy annually as the Netherlands did in 2019, data from the University of Cambridge and the International Energy Agency shows.
Some bitcoin proponents noted that the current financial system also uses large amounts of energy with its millions of employees and computers in air-conditioned offices.
The largest cryptocurrency in the world, which was once a fringe asset class, has become increasingly mainstream as it becomes more prominent in the U.S. Accepted by companies and financial firms.
More and more demand, and higher prices, leads to more miners competing to solve puzzles in the fastest time to win a coin using increasingly powerful computers that require more energy.
Bitcoin is created when high-powered computers compete against other machines to solve complex mathematical puzzles, an energy-intensive process that often relies on fossil fuels, especially coal, among the dirtiest of them all. .
A 2019 study in the scientific journal Joule has found that production of bitcoin is estimated to generate between 22 and 22.9 million metric tons of carbon dioxide emissions annually, or levels produced by Jordan and Sri Lanka.
Efforts to reduce the environmental harm of mining in the cryptocurrency industry are increasing and the entry of large corporations into the crypto market could boost incentives to produce "green bitcoins" using renewable energy.
Some sustainability experts say that companies can buy carbon credits to offset the impact.
And blockchain analysis firms say that it is theoretically possible to track the source of bitcoin, raising the possibility that a premium may be charged for green bitcoin. Climate change policies by governments around the world can also help.
Projects from Canada to Siberia are striving for ways to move bitcoin mining away from fossil fuels, such as using hydropower, or at least to reduce its carbon footprint, and the currency is being pushed by mainstream investors. Make it more attractive to.
Some are attempting to reuse the heat generated by mining to meet agriculture, heating and other needs, while others are using flare gas-generated electricity for crypto mining - from oil extraction to sub-. The product is usually burned.
Some industry players and academics have warned that the dominance of Chinese miners and the lack of motivation to swap cheap fossil fuels for more expensive renewable energy means there are some quick solutions to the emissions problem of bitcoin.
Chinese miners produce around 70%, data from the Center for Alternative Finance of the University of Cambridge shows. They use renewable energy - mostly hydropower - during the rainy summer months, but fossil fuels - mainly coal - for the rest of the year.