The deadly European heat wave of 2003, the fiery Russian heat wave of 2010 and catastrophic droughts in Texas and Oklahoma last year can each be attributed to climate change. And once the data are gathered in a few weeks’ time, it’s likely that the same will be true for the extremely hot summer the United States is suffering through right now.
My commentary concluded: “Hansen’s sweeping assertion that global warming is the principal cause of the European and Russian heat waves, and the Texas-Oklahoma drought, is not supported by event-specific analysis and is implausible in light of previous research.”
Although Hansen does not explicitly attribute the ongoing U.S. drought to global warming, he does blame global warming for both the 2011 Texas-Oklahoma drought and the current summer heat. And in his study, Hansen states: “With the temperature amplified by global warming and ubiquitous surface heating from elevated greenhouse gas amounts, extreme drought conditions can develop.”
This week on World Climate Report, Pat Michaels and Chip Knappenberger argue that the current U.S. drought “is driven by natural variability not global warming.” Their post (“Hansen Is Wrong“) is concise and layman-friendly. Here I offer an even briefer summary.
A standard measure of drought in the U.S. is the Palmer Drought Severity Index (PDSI), which measures the combined effects of temperature (hotter weather = more soil evaporation) and precipitation (more rainfall = more soil moisture). “The more positive the PDSI values, the wetter conditions are, the more negative the PDSI values, the drier things are.” The PDSI for the past 117 years (1895-2011) shows a small non-significant positive trend (i.e. towards wetter conditions). There is no greenhouse warming signal in this data.