Anyone with even a passing interest in technology...or money...cannot fail to have noticed the advent of cryptocurrencies, of which BitCoin is perhaps the most prominent and well known. The concept does to money, and specifically digital currency transactions, what peer-to-peer (P2P) networks did for the distribution of music, video and software sharing. As with the P2P networks, cryptocurrencies rely not on a central service or server but rather the distributed potential of millions of computers all connected to the internet twenty-four hours a day, seven days a week working together.

Digital currency precludes the need for physical notes and coins. Transactions take place through the shuffling of electrons rather than the shuffling of papers and the chinking of coins and as such they can take place near instantaneously without borders, and in importantly without regulations, bankers, or governments. To keep track of the putative billions of transactions, BitCoin (and other digital currencies) use a technology known as a blockchain. This is effectively an unhackable ledger that records each and every transaction from the inception of a specific BitCoin as it chinks its way through the internet buying and selling digital and other goods and services or paying ransomware demands. The distributed BitCoin software runs on the network keeping track of the blockchains and updating as and when a transaction takes place.

Satoshi Nakamoto's 2008 solution to the problem of "double-spending" of digital cash was to "timestamp" transactions by hashing them into an ongoing chain of hash-based proof-of-work, this is the blockchain and mining is the work done in hashing (or encoding) the values using SHA-256 encryption.

Indeed, each block contains an SHA-256 cryptographic hash of the previous block and so requires a lot of computing power to function. There is therefore a real-world cost to the creation of a new BitCoin and to its subsequent propagation. Aside from the "simple" transaction of buying and being paid with this digital currency, some users and networks of users (both legitimate and criminal) are focused on doing the work to verify the transactions across the BitCoin P2P network. This process is known as mining and is rewarded with, you guessed it, BitCoins. The more computing power a person or network dedicates to mining BitCoins, the bigger the BitCoin reward, although the rewards dwindle exponentially as time goes by. It is, for now, big business and as I alluded to above there are both legitimate miners and those who have hijacked other people's computing power to do the work for them.

BitCoin has now entered its second decade and research Dutch economist Alex de Vries has been thinking long and hard about all this mining and the work and thus electrical power that is needed to run the computers that do the mining. Headlines in August screamed that BitCoin mining was, according to de Vries, using 2.55 Gigawatts of electrical power. That is about the same power usage as the country of Ireland. His projections for the growth in BitCoin in the coming years suggests that the power demands on electricity supplies will quickly reach the level of a country the size and population of Australia, at almost 7.678 GW. Ireland currently uses 3.1 GW, Australia 8.2 GW (which presumably includes both of those countries' BitCoin mining, ironically enough). De Vries suggests that the latter number might be reached in 2018 based on "hash rate", the number of transactions that are processed, and the overall computer power demands.

One commentator on the work of de Vries has estimated that the rate of increase of BitCoin power consumption will follow a trend as follows equivalent to the current power demands of country and region: Quarter 4 (Q4) of 2018, Czech Republic, Q2 2019, Sweden, Q4 2019, Mexico, Q2 2020, Germany, Q4 2020, Japan or more than the whole of the Middle East, Q2 2021, 2144, demand between that of Latin America and Europe, Q4 2021, North America, Q2 2022, all of the BRICS nations (Brazil, Russia, India, China and South Africa). By Q4 2022 it will be a little shy of the entire global energy consumption, and by Q2 2023, more than total global power use.

Now, Jonathan Koomey of Berkeley Lab, California, known for his eponymous computing energy efficiency law suggests that de Vries' estimates are probably way off. It is almost impossible to "calculate" even to the nearest order of magnitude how much energy something as diffuse as BitCoin mining activity is using. It is likely that the same clanging false alarm bells that rang when the Internet and the public Web first took off in the 1990s are now being sounded for the BitCoin revolution. There is some evidence that legitimate BitCoin miners are "green aware" and already utilize sustainable energy sources, but that presumably ignores the many unwitting BitCoin miners whose devices are part of a botnet that is carrying out not distributed citizen science projects, but criminal projects for nefarious purposes.

De Vries himself has responded to the criticism from Koomey, conceding that the data that is available is indeed of pretty poor quality overall. He hopes that his paper in the journal Joule [de Vries DOI: 10.1016/j.joule.2018.04.016] will push researchers to build on the concepts and pin down the true and growing costs of mining BitCoin and other cryptocurrencies.

Whether the estimates are closer to de Vries' calculations or Koomey's sober assertions remains to be seen. Either way, computing certainly uses a lot of power and any growth industry whether digital or not will increase demands on power generation. If that can be delivered sustainably with neutral carbon emissions to boot through novel photovoltaic materials and super-capacitance based on advanced materials we might ultimately have green cryptocurrency. It is likely that a sea change will be needed in sustainable energy efficiency and storage and that will only come from advances in materials science that allow us to tap solar power and other sources without additional waste and carbon emissions compared with conventional technologies. It will be interesting to see whether all that new technology is paid for with cryptocurrency.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.