If you spend any time arguing about climate on the internet, sooner or later someone will show you a graph of global temperatures in the early 20th century. Between 1900 and the mid-1940s the world warmed by around 0.4C, a rapid rise at a time when human emissions of CO2 were only around 15% of today’s levels. The implication (sometimes stated, sometimes just implied) is that if the world could warm that quickly back then, maybe the modern warming isn’t fully human-driven after all.