Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wet-bulb temperature TW, is surprisingly similar across diverse climates today. TW never exceeds 31?

The discovery in Colombia of a giant species of fossil snake is news in itself. But a wider, more controversial inference to be drawn is that tropical climate in the past was not buffered from global warming.