In recent times, cryptocurrencies, of which the leading example is Bitcoin, have been used worldwide. One might suppose that being a record of currency transactions that exists only digitally, on computers, this might be an amazingly sustainable way to track money. Gone are all those specialty, holographic papers to prevent counterfeiting. Gone are all those giant printing presses. Nothing but some little computer calculations. Wrong!
It turns out that there is an immense amount of computation going on to support Bitcoin transactions. The entire record of all the transactions that have taken place since the last time the database was started is saved on a giant database with specialty hashtags and enormous collections of data. And this giant database is distributed to millions of computers that are participating. Then some arcane calculations have to be performed on the databases and the first person to finish that and prove it is right, gets a reward. Which means there are millions of computers trying to be first. All of this competitive effort uses up gigantic amounts of energy. Estimates range upwards from 20%, of all the energy used by the internet, is used for cryptocurrencies. It seems doubtful that all the energy used up to make that former holographic paper and its associated printing presses and the bureaucrats organizing it would be as much as this quantity of energy. And new, faster computers are constantly needed to process the databases. So there ends up being an enormous amount of tangible electronic waste (in a world where garbage rules and little is reused).
Here is a preliminary paper on the subject from Chemical and Engineering News in February 2019. BitcoinWaste