Climate change has been the main driver of mass extinctions over the last 500 million years. This column argues that current evidence provides a stark warning. Human activity is producing greenhouse gases, and as a consequence global temperatures and ocean heat content are rising. Such trends raise the risk of tipping points. Economic analysis offers a number of ideas, but a key problem is that distributions of climate variables can shift, invalidating stationarity-based analyses, and making action to avoid possible future shifts especially urgent.