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Floodwaters cover Austin’s North Lamar Boulevard near Shoal Creek on May 25

By Bill Dawson
Texas Climate News

May’s deadly and disastrous flooding in Texas and Oklahoma was aggravated by manmade global warming, researchers at Utah State University and in Taiwan have concluded in a new study.

Their computer analysis is the second one released this month that attributes the severity of a catastrophic weather event partly to human causation.

A separate research project by different scientists concluded that heat-trapping pollution is making California’s devastating, years-long drought up to 25 percent worse than it would have been otherwise.

The Utah State and Taiwan researchers who analyzed the factors underlying the Texas and Oklahoma floods did not calculate such a percentage for manmade global warming’s impact on the storms that caused them.

But John Nielsen-Gammon, the Texas state climatologist and an atmospheric sciences professor at Texas A&M University, used the same data to calculate the magnitude of the effect they had identified.

“The size of the estimated impact is about 0.5 inch,” he told CBS News. “In the context of 9 inches of rain in Texas and 14 inches of rain in Oklahoma, it’s only an extra 5 percent or so, but that does change the odds of such an extreme event quite a bit. I estimate it doubles the chances of getting something as extreme as what happened.”

This statement by Nielsen-Gammon echoed more general remarks that he and other Texas scientists had made at the time of the May flooding – essentially, noting that the event was consistent with climate scientists’ projections of an increased risk of such weather extremes in Texas and elsewhere.

That was not a cautionary message that some leading political figures such as Sen. Ted Cruz, who is seeking the Republican presidential nomination, wanted to hear, however.

Cruz, who has been consistently dismissive of scientists’ warnings about manmade climate change, was asked in May about the possible role of global warming in the flooding. “At a time of tragedy, I think it’s wrong to try to politicize a natural disaster,” he replied.

Climate scientists in Texas fired back at Cruz, however. For instance, Andrew Dessler, a colleague of Nielsen-Gammon’s on the A&M atmospheric sciences faculty, said climate change is “absolutely a reasonable thing to talk about” at a time of a weather disaster.

The Utah State and Taiwan researchers who produced the new study on the 2015 Texas-Oklahoma flooding did not address that issue directly, but they did assert in their paper that their analysis shows global warming should be factored into seasonal weather predictions as a precautionary measure.

Several interconnecting weather processes this year, along with their calculations, “point toward the exacerbating effect of increasing [greenhouse gases] on the springtime precipitation over Texas and Oklahoma during a developing El Niño – this being so currently (i.e. 2015) and in the future,” they wrote.

“Furthermore, the diagnostic analyses detailed here, in which increased extreme events and a warmer climate were shown to be dynamically linked, is key in the provision of seasonal predictions as a guide to future occurrences and intensities of extreme weather events.”

In their paper, to be published by the scientific journal Geophysical Research Letters of the American Geophysical Union, the researchers reported that their analysis had determined there was “a significant increase” in above-normal precipitation caused by the current El Niño weather pattern “when increases in anthropogenic [human-caused] greenhouse gases were taken into account.”

They noted that “El Niño tends to increase late-spring precipitation in the southern Great Plains and this effect has intensified since 1980,” adding that there was a “detectable effect of anthropogenic global warming in the physical processes that caused the persistent precipitation in May of 2015.”

Those processes, the scientists wrote, included warming in the tropical Pacific that strengthened El Niño’s influence on North America, modification of circulation that “deepened” a region of low pressure west of Texas, and enhanced southerly winds in the Great Plains “increasing moisture supply from the Gulf of Mexico.”

In a comment to CBS, the study’s lead author, S.-Y. Simon Wang of Utah State’s Utah Climate Center, noted the study’s identification of a confluence of several factors in the severity of the 2015 Texas-Oklahoma flooding: “Everything has to work the way it did to make an extreme event. But without [manmade] warming, the floods in May wouldn’t be so extreme.”

Nielsen-Gammon, elaborating on his own comment to CBS, told Texas Climate News by email:

An earlier study had shown that May precipitation in Texas and Oklahoma is particularly enhanced when the El Niño is developing.  I don’t know whether the magnitude of the climate change impact is similarly enhanced, but it would make sense. Also, the [Utah State and Taiwan] study does not address any possible enhancement of baseline precipitation due to climate change independent of whether an El Niño is present; that enhancement, if it is an enhancement, is probably smaller than the climate-El Niño interaction. We also don’t know whether the frequency or intensity of El Niño itself is being affected by climate; this study only addresses what would happen given an El Niño of a particular strength.

The new study on the impact of human-caused climate change on the California drought, also appearing in Geophysical Research Letters, was carried out by researchers at Columbia University’s Earth Institute and the University of Idaho who analyzed data on precipitation, temperature, humidity, wind and other factors from 1901-2014.

Besides attributing up to about a quarter of the current drought’s severity to human causes, the scientists calculated that the odds of such dry spells in California have roughly doubled in the past 100 years along with rising temperatures. They also cautioned that future droughts there will be more severe and persistent as global warming intensifies.

“A lot of people think that the amount of rain that falls out the sky is the only thing that matters,” lead author A. Park Williams of Columbia said in a university-released statement. “But warming changes the baseline amount of water that’s available to us, because it sends water back into the sky.”

At least three studies likewise attributed part of the intensity of the especially severe 2011 portion of Texas’ recent, multi-year drought to manmade climate change, including one for which Nielsen-Gammon was a co-author. He told TCN at the time that he and his colleagues had detected a “significant global warming contribution” to the Texas drought, although smaller than another widely publicized study had calculated.


John Nielsen-Gammon is a member of TCN’s Advisory Board.

Image credit: Emmanuelle Bourgue / Cropped photo used under a Creative Commons license.