The senior British economic thinker on climate, Sir Nicholas Stern, has estimated that if we don't reverse climate change, the costs of dealing with the resulting catastrophe would be as much as twenty percent of the world's Gross Domestic Product. He's saying that if we do nothing about climate change, then we will have to spend a full fifth of our planet's economic energy on dealing with the floods, hurricanes, droughts, food shortages, and epidemics that will result.