A study by the University of Cambridge found that Bitcoin has struggled to be greener in the year to January as it made a modest gain in its use of sustainable energy.
The processing of Bitcoin transactions and the "mining" of new tokens is performed by powerful computers connected to a global network.
The process relies heavily on polluting fossil fuels such as coal, which consumes a lot of electricity.
It has drawn criticism from policymakers, investors and environmentalists concerned about its impact on global warming.
There are projects looking for ways to switch bitcoin mining to cleaner energy such as repurposing heat byproducts from oil extraction for crypto mining.
Still, fossil fuels account for about 62% of Bitcoin's energy mix in January 2022, up from 65% a year ago, according to Cambridge's Bitcoin Electricity Consumption Index (CBECI) study.
Bitcoin's greenhouse gas emissions are expected to reach 48.4 million tonnes of carbon dioxide equivalent this year, about 14% less than its estimated 2021 emissions, CBECI said.