A surge in the value of crypto-currencies provokes alarm

It is hard to predict when bubbles will pop, in particular when they are nested within each other. It helps to keep this image in mind when considering one of the biggest surges in asset values of recent years: the market value of all the world’s crypto-currencies has trebled since the beginning of the year, and is now worth more than $60bn.

Bitcoin is the best known of these currencies, especially after hackers this month instructed victims to pay ransoms in the anonymous digital cash in order to get their computer files decrypted. Not that many bitcoins exist: there are about 16.3m of them, with only 1,800 new ones minted every day. But growing demand has pushed bitcoin’s price to a record recent high of about $1,830, up from $450 a year ago.