A new U.S. Geological Survey analysis finds that, as climate scientists have been predicting for decades, the Southwestern U.S. is drying in part because of rising levels of carbon dioxide:
The decrease of floods in the southwestern region is consistent with other research findings that this region has been getting drier and experienced less precipitation as a likely result of climate change.
The study, “Has the magnitude of floods across the USA changed with global CO2 levels?” appearing in Hydrological Sciences Journal, however, relies on dubious and “absurd” assumptions, according to a number of climate scientists I spoke with. Amazingly, the lead author seems to lack an understanding of core issues germane to his analysis, as we’ll see.
The finding about SW drying that I’ve focused on isn’t the main spin the USGS and media have given the study. The USGS focused on what they claim is the lack of a “significant relationship between carbon dioxide (CO2) in the atmosphere and the size of floods over the last 100 years” in the three other regions they rather arbitrarily divide the country into — northeast, southeast, and northwest.
Interestingly the NE “stretches from the middle of the Dakotas and Nebraska all the way east to the New York and New England area,” and it “shows a tendency towards increases in flooding over this period.” But in the USGS analysis, the tendency isn’t statistically significant. I’ll address in a later post why that isn’t particularly surprising given how the USGS does its analysis.
But it’s worth noting that Kevin Trenberth, senior scientist at the National Center for Atmospheric Research, and a leading expert on the impact of global warming on extreme weather and precipitation, thinks it is “absurd” that USGS was looking for a relationship between global CO2 levels and flooding. Other climate scientists I spoke to expressed similar reservations about this. One called it, “simply wrong.” Why?
CO2 levels have been rising pretty steadily for many decades, but flooding is primarily linked to warming (through the greater water vapor in the atmosphere and things like early snowmelt).
Many studies and all global climate models have made clear that global temperatures don’t rise in lockstep with global mean carbon dioxide concentrations (GMCO2) — thanks to aerosols, volcanoes, and the El Niño Southern Oscillation (ENSO) and so on. And, of course, the temperatures relevant to U.S. flooding don’t rise in lockstep with global temperatures thanks to ENSO and the like. So finding correlations between flooding and GMCO2 is not terribly dispositive.
Yet even with that big flaw and many others, the study still found one trend that is similar to what the IPCC models have predicted:
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President Obama yesterday 
