A new paper in today’s Nature by Justin Sheffield and colleagues suggests we’ve been using simplistic calculations that have overestimated the increase in drought globally over the last 60 years in response to greenhouse warming. Scientists typically use the Palmer Drought Severity Index, a bit a statistical black box that bases its drought estimate on a formula that combines precip and temperature data with normals for the geographical spot in question. Using PDSI, people have calculated increasing global drought as a result of global warming.

Sheffield and colleagues argue PDSI is too simplistic to capture what’s really going on:

More realistic calculations, based on the underlying physical principles8 that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years.

From an accompanying Nature “news and views” piece by Sonia Seneviratne:

The findings imply that there is no necessary correlation between temperature changes and long-term drought variations, which should warn us against using any simplifications regarding their relationship.