Climate change doesn’t create every hot spell from scratch. But it does change the odds. Think about it. We don’t get that many days that are very hot or very cold. Most days are closer to average. Plot them on a graph and they look a bit like a bell, with the very hot and very cold days at either end. Global temperature has increased by around 1.4C since pre-industrial times. That may not sound much. But now look at the second curve. The hot end has shifted quite a bit. That means temperatures that used to be rare are now closer to normal - more days cross the line into extreme heat. So heatwaves become more likely. And when they happen, they are more likely to be hotter and last longer, because they are happening in a world that is already warmer. - Justin Rowlatt, BBC Climate Editor