A cryptocurrency marketed as a green alternative to bitcoin is actually consuming 18 times more energy than its makers originally claimed. Even management at the Chia Network admits the figures are “not wildly off”.

Bitcoin requires miners to do vast amounts of otherwise useless calculations to maintain the network, a system known as proof of work. Analysis suggests that bitcoin currently consumes around 157 terrawatt hours per year – about as much as the entire country of Poland. But Chia instead uses a proof-of-space-and-time approach that ditches these calculations and relies on empty hard disk space. The more space a miner devotes to the task and the longer they leave it, the higher their probability of receiving new coins.

Using Chia involves two main steps: plotting and farming. Plotting is a memory- and processor-intensive task that creates the data to be stored, while farming involves simply storing that data and occasionally proving to the network that it still exists. Speedy solid-state drives (SSDs) are often used for plotting, while cheaper and slower hard disks are used for farming.

Soraya Djerrab at the Higher School of Computer Science and Digital Technologies in Algeria and her colleagues investigated Chia and found that plotting wears out significant numbers of SSDs, and that the embodied carbon of those devices would drive up the network’s carbon emissions. The researchers also used hardware fitted with accurate wattmeters and ran various Chia tasks to see how much power devices consumed at all stages of the process, and worked that into their calculations.

They found that Chia’s annual carbon footprint likely lies between 0.584 and 1.402 million tonnes per year. Even the average of those bounds would mean that Chia’s emissions were about 18 times the claimed 50,000 tonnes – and two orders of magnitude larger than mainstream blockchains such as Ethereum.

“Mainly it’s from embodied emissions,” says Djerrab. “To use Chia, people have to buy hardware. When you buy them, energy is used to create them. Chia didn’t include this when they calculated the energy used.”

The researchers estimate that going through the process to create a plot just 160 times would destroy a brand-new SSD, and that attempts by Chia to lower the burden on hard disks with other ways to create plots require more RAM and GPUs, which bring their own carbon emissions. As co-author Clémentine Gritti at the Institut National des Sciences Appliquées de Lyon puts it: “it’s still better than bitcoin, but it doesn’t save the planet”.

Gene Hoffman, the CEO of Chia Network, told New Scientist that the cryptocurrency makes extensive use of hard disks from data centres that would otherwise be thrown away at the end of their life, at least for the farming stage, and that the figures in the paper are an overestimate.

“I don’t think [they’re] wildly off,” says Hoffman. “I think [they’re] off a bit. We’re being charged with the full carbon footprint of building drives that ran for four years in an enterprise data centre and then, otherwise, would have landfilled,” he says. “It [Chia farming] really created a market for something that was otherwise a cast-off. “

In any case, Hoffman claims that wide changes to the Chia network – due for launch in two months and called Proof of Space 2.0 – will dramatically cut emissions. “We feel like we’re doing a pretty good job and we think that Proof of Space 2.0 is going to be even better.”