The digital mining of cryptocurrencies required more energy per US dollar generated than the mining of physical metals between January 2016 and June 2018, according to a new study published in the British journal Nature Sustainability.

Digital currency and physical metals aren’t “functional substitutes,” said author Max Krause to BuzzFeed News. The primary aim of the study is to create awareness: “Just because something is digitally processed does not mean it does not consume a considerable amount of energy.”

Cryptocurrency is a form of electronic cash that’s managed by a decentralized network of computers, rather than a government or bank. Over the past two years, cryptocurrency networks have experienced a kind of frenzy, with wildly fluctuating prices that triggered a boom in giant “mines” full of computer processors, alongside bitcoin scammers and multimillion-dollar Ponzi schemes. In December 2017, the value of bitcoin, the most popular cryptocurrency, peaked at $19,666 (it has since fallen to about $6,410) — and according to the latest research, the computing power required to fuel the crypto craze consumed a massive amount of electricity, about as much as consumed by Ireland or Hong Kong per year.

Krause and coauthor Thabet Tolaymat found that it takes more energy to produce $1 worth of bitcoin or the cryptocurrency Monero than $1 worth of copper or gold. Also, mining $1 of bitcoin and Monero consumed more energy (17 and 14 megajoules, or MJ, respectively), compared to Ethereum and Litecoin (both 7 MJ on average). Meanwhile, the conventional mining of $1 worth various physical metals required less energy — rare earth metals (9 MJ), precious metals (7 MJ), gold (5 MJ), and copper (4 MJ). The only exception is aluminum, which required 122 MJ.

Krause and Tolaymat also estimated that, during the 2.5-year period, the four cryptocurrency networks (bitcoin, Ethereum, Litecoin, and Monero) generated between 3 million and 15 million tons of carbon emissions (1 metric ton is equivalent to 1.1 US tons).

The carbon footprint of any cryptocurrency depends hugely on where the coins are generated. Digital mining in China, where a significant percentage of mining occurs, created four times more CO2 than Canada, where 60% of electricity is generated by hydropower.

“At least for that two-and-a-half-year period, mining a dollars’ worth of bitcoin took about three times as much energy as mining a dollars’ worth of gold. I was shocked to see that the numbers were so high,” Krause told BuzzFeed News.

Here’s why some cryptocurrencies are so energy intensive: The blockchain technology it’s built on is a race for digital cash. Crypto “miners” compete to finish the complicated equations required to verify the network’s transactions. Whoever completes the calculation first is rewarded with coins. Bitcoin, the most popular cryptocurrency, awards 12.5 bitcoins to the winning miner which, currently, is worth about $80,000. Anyone can join the verification network and “mine” for digital money. To increase their chances, opportunists beef up their computing power with more energy-intensive mining rigs.

However, Krause noted the study focuses solely on each item’s mining and creation process, not the products’ impact over time. According to Krause, cryptocurrency mining facilities with thousands of computers require expansive cooling systems, and those systems, which use electricity, are not factored in his report’s energy consumption costs, because they are unknown. “In the future, once we get those cooling costs, we’ll have a more complete picture of energy consumption estimates for cryptocurrencies,” he said.

As for what happens after the asset is mined, “While mining bitcoin is energy intensive, for the rest of its lifecycle as a digital asset, [bitcoin] will require much less energy than any metal material, which has to be shipped for transport and physically reshaped,” Krause said.

Both researchers are employed by the US Environmental Protection Agency (Krause as a post-doctoral fellow and Tolaymet as an environmental engineer), but, according to Krause, they conducted the study independently, on personal time, without funding.