Flawed USGS Study Still Links Southwestern Drying to Increasing Carbon Dioxide Pollution and Climate Change

Flawed USGS Study Still Links Southwestern Drying to Increasing Carbon Dioxide Pollution and Climate Change


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:


One region, the southwest, showed a statistically significant negative relationship between GMCO2 and flood magnitudes….

The similarity is strong in terms of the trend towards drying conditions in the Rocky Mountains and arid southwest.

In ClimateWire (subs. req’d), the lead author, Robert Hirsch, is quoted rather bizarrely on this statistically significant negative correlation in the SW:

But even that shred of correlation could be dismissed. “It has more to do with a general decrease in precipitation that has been observed by many people,” said Hirsch.

That is a head-exploding quote.  There have been several major studies published in which climate scientists have projected a decrease in precipitation in the U.S. Southwest thanks to rising CO2 levels and global warming — one dating back to 1990.  I have a comment piece in Nature on this very subject coming out Wednesday.  You can see some of the literature here: “USGS on Dust-Bowlification: Drier conditions projected to accelerate dust storms in the U.S. Southwest.”

The serious hydrological changes and impacts known to have occurred in both historic and prehistoric times over North America reflect large-scale changes in the climate system that can develop in a matter of years and, in the case of the more severe past megadroughts, persist for decades. Such hydrological changes fit the definition of abrupt change because they occur faster than the time scales needed for human and natural systems to adapt, leading to substantial disruptions in those systems. In the Southwest, for example, the models project a permanent drying by the mid-21st century that reaches the level of aridity seen in historical droughts, and a quarter of the projections may reach this level of aridity much earlier.

Again, the USGS release itself statesThe 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.”

Apparently Hirsch doesn’t even read his own agency’s press releases!  The USGS paper doesn’t cite a single one of the studies above on SW drying.  The entire analysis is weak, and Hirsch’s quote suggests a severe lack of understanding of core issues germane to his analysis.

The ClimateWire piece headline was “Flood sizes not rising along with CO2 levels, ‘at odds’ with IPCC — study.”  As I’ll discuss in a later post, that headline may reflect the study, but the study itself is just too flawed to warrant such a strong conclusion.  UPI did better with its headline, “USGS finds complex link to floods and CO2.”

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