Why Forecasts Of Increased Drought May Be Completely Wrong
A Princeton scientist suggests that the index most commonly used to predict droughts may not be reliable, raising questions about whether the world might get wetter as it gets warmer. Some scientists agree with him.
Princeton scientist Justin Sheffield suggests that, contrary to popular belief, we may not be heading towards a drier world as global temperatures rise. His claim is based on a reexamination of the Palmer Drought Severity Index (PDSI), a measurement used by climate scientists for over 40 years. In an influential 2007 report, PDSI-based studies were cited to conclude that droughts would increase over time.
What’s the Big Idea?
Sheffield says the problem with the PDSI is that it focuses on the difference between precipitation and evaporation using temperature as the guide, when temperature isn’t the only factor that influences evaporation. By applying another equation that involves wind speed and humidity, he deduced that there hasn’t been much of a change in global drought occurrences over the last 60 years. Some scientists think Sheffield may be right: “There has been a growing acknowledgement that the PDSI should not be trusted when doing climate change studies.” A US expert says that if Sheffield is correct and “warmer temperatures are accompanied by more rainfall and lower evaporation rates, then a warmer wetter world would [mean] a more benign climate.”