But two aspects of this system for measuring surface temperatures have long been worrying a growing array of statisticians, meteorologists and expert science bloggers. One is that the supposedly worldwide network of stations from which GHCN draws its data is flawed. Up to 80 per cent or more of the Earth’s surface is not reliably covered at all. Furthermore, around 1990, the number of stations more than halved, from 12,000 to less than 6,000 – and most of those remaining are concentrated in urban areas or places where studies have shown that, thanks to the “urban heat island effect”, readings can be up to 2 degrees higher than in those rural areas where thousands of stations were lost.