Cryptocurrencies are heralded as the technology of the future. But Bitcoin, and others like it, are putting that same future at risk, with their devastating effect on the environment. According to research by Digiconomist.net, Bitcoin operations create as much carbon dioxide as the entire City of London and consume almost 100 TWh per year, and as the value of digital currency grows the problem is only getting worse. So, what is the real-world impact of digital currency, and what, if anything, can be done about it?