Last week, I posted a commentary on NASA scientist James Hansen’s study and op-ed, which attribute recent extreme weather to global climate change. In the op-ed, Hansen stated:

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.

What Hansen is claiming, however, is not that U.S. temperatures are causing drought but that global warming is causing drought. So Pat and Chip attempt to determine the influence of global temperatures on U.S. temperatures. They find that about 33% of U.S. temperature trends is explained by global temperature variations, although there is little relationship from year to year.

Figure explanation: The observed annual U.S. temperatures from 1895 through 2011 (open circles) and that part of them which is explained by global temperatures (black circles).

Pat and Chip then compare the black part of the chart above (the portion of U.S. temperatures influenced by global temperatures) with the PDSI. They find no relationship between global temperature variations and U.S. drought conditions (graph below, left) but a significant relationship between PDSI and non-global warming factors (graph below, right).

Pat and Chip conclude: “In other words, the situation is as it always has been. And the 2012 drought conditions, and every other drought that has come before, is the result of natural processes, not human greenhouse gases emissions.”