Bitcoin has become less green since China cracked down on mining the cryptocurrency, new research suggests.

Mining refers to the process of creating new bitcoins using banks of powerful computers.

The share of renewable energy powering mining fell from 41.6% in 2020 to 25.1% last August as miners stopped using Chinese hydro and moved to the US, where gas supplies much of their power.

Researchers estimate Bitcoin now causes carbon emissions comparable to Greece.

"We see the network becoming less green than ever before," Alex de Vries, one of the authors of the peer-reviewed study published in the journal Joule, told the BBC.

Industry groups had been more optimistic about the use of renewables.

Previously, the Bitcoin Mining Council estimated that the "global mining industry's sustainable electricity mix had grown to approximately 58.5%".

Bitcoin miners will generally attempt to move to places with plentiful cheap electricity in order to maximise their profits.

The specialised computers that verify crypto transactions consume large amounts of electricity.

In the past, bitcoin miners in China would move in the wet season from provinces with inexpensive electricity generated by fossil fuels, to those with an abundance of hydropower.

But after bans in key provinces, mining was more or less eliminated in China by June 2021.

Forced out of China, mining moved to countries such as the US and Kazakhstan.

Researchers say this could be why Bitcoin has become less environmentally-friendly.

The move to the US increased the proportion of fossil fuels used, particularly natural gas.

"The US grid only has a small part of its entire system sourced from renewable energy sources," Mr De Vries said. He added that many of the states popular with crypto-miners, such as Texas, Kentucky and Georgia, performed worse than the national average in terms of the amount of power generated from renewables.

Kentucky, for example, "grants tax breaks to attract Bitcoin miners and thus saves coal companies", the researchers noted.

And Kazakhstan, the researchers say, has coal-fired power stations burning "hard coal", which are more polluting than their Chinese counterparts.

The net result of these shifts, the research estimated, was that the carbon intensity of Bitcoin increased by about 17%.

Bitcoin, the research suggests, produces more than 65.4 megatonnes of carbon dioxide per year.

By comparison, Greece produced 56.6 megatonnes of carbon dioxide emissions in 2019.

"The carbon footprint per single bitcoin transaction is going to be something like 669 kilograms of carbon dioxide," said Mr De Vries.

He said it was comparable to the per-passenger carbon footprint of a flight from Amsterdam to New York.

An economy-class return flight from London to New York emits an estimated 670kg of CO2 per passenger, according to the calculator from the UN's civil aviation body, the International Civil Aviation Organization.

Mr De Vries and his co-authors say their research highlights the need for the crypto industry to accelerate efforts to decarbonise.

The Crypto Climate Accord is one such initiative.

Its members pledge to achieve net-zero emissions from electricity consumption related to their crypto operations by 2030.

But the report suggests that is a long way off.

"A rapid solution to Bitcoin's carbon footprint is not within sight," the authors conclude.

Mr De Vries says it shows the need for international co-operation.

"China decided to ban cryptocurrency mining, but we see that that actually had an adverse effect", he said.