Summary: The heavy rains hitting Texas last May were record-breaking extreme weather evens. But were they natural (records routinely break as time passes) or the result — partially or mostly — the result of anthropogenic effects? Here an eminent climatologist gives a preliminary answer, based on solid evidence (not modeling). There is no visible trend to rainfall in Texas, but its distribution has changed — more concentrated in large storms.

“With the lack of a positive trend in monthly springtime precipitation, there is no direct observational evidence that the record-setting May 2015 statewide rainfall total in Texas had an anthropogenic component.”

The mainstream news media have become more cautious in their coverage of extreme weather, unlike their previous uncritical reporting of everything as climate change. For example, see this by USA Today: “Wild weather shifts in Texas spark concern about “new normal’.” And “Climate Change May Have Souped Up Record-Breaking Texas Deluge” by Elizabeth Harball and Scott Detrow at Scientific American on 27 May 2015 — “Deadly downpours flooded Texas and Oklahoma and may have been exacerbated by global warming.” The link to climate change is strongly implied, but not stated as definite. The text reports climate scientists’ uncertainty about attribution of events to climate change.

Activists ignore the science, preferring simple narratives. Bill Nye, the children’s science guy, says on CNN: “The floods in Texas, the strengthening storms… these things are a result of human activity making things worse.” As usual, the most over-the-top story comes from fantasy writer – climate activist Robert Marston Fanney (bio here) at his blog RobertScribbler: “The Merciless Rains of Climate Change Hammer Houston, Southeast Texas.”

Eventually scientists will produce papers with more definitive information. Here’s an excerpt from an early analysis (click on the link to read it in full)…

By John W. Nielsen-Gammon (see his bio)

Texas state climatologist and professor of atmospheric sciences at Texas A&M

From NOAA’s Annual Climate Diagnostics and Prediction Workshop, Oct 2015

(1) Introduction

Texas received its all-time wettest month of rainfall in May 2015, with an average of 9.05″ (230 mm) across the state… The purpose of this talk is to put the extreme rainfall events in Texas in 2015 in a historical perspective and to consider the possible role of contributing factors, including anthropogenic climate change …

Figure 1: Monthly precipitation totals in Texas, with 2015 totals in red.

(2) Monthly rainfall totals

Figure 1 shows the historical distribution of rainfall in Texas for those four months, with the four months of 2015 highlighted in red. May 2015 was an extreme outlier. The gap between May 2015 and the second largest total (6.66”, or 170 mm) is as large as the gap between the second largest total and the 88th largest total. The May 2015 total was easily sufficient to break the record for wettest 31 consecutive days as well. Longer-duration records were also broken, such as the wettest first six months of the year.

…When the mud settled, Texas had experienced its wettest year on record, breaking the previous record by nearly an inch. Both May and October effectively ended droughts in Texas. …

(3) Attribution and the faucet

The direct thermodynamic effect of climate change is to increase the water vapor carrying capacity of the atmosphere. All else being equal, a saturated atmosphere that is warmer will produce more precipitation.

…A good analogy is a water faucet. The direct thermodynamic effect is comparable to the size of the pipe, which controls how much water can be delivered to the faucet. The remaining dynamic and thermodynamic effects are comparable to the handle of the faucet, which may be closed, slightly open, or fully open. The net resulting precipitation depends on both the size of the pipe and the position of the handle. However, when the handle is wide open, the precipitation intensity is controlled only by the size of the pipe.

Over the past 121 years, there is essentially no trend in springtime precipitation in Texas. If anthropogenic climate change has had an effect, it has been offset by natural variability. It is thus difficult to argue that climate change played a direct role in the record-setting May rainfall.

An upward trend does exist in intense one-day and two-day rainfall events in the south-central United States (e.g., Janssen et al. 2014). This means either that the precipitation handle is wide open more frequently, or that on days in which the precipitation handle is wide open, the atmosphere is delivering more precipitation.

Since overall precipitation has not increased, we presume that the pipe has become wider rather than the handle position becoming more favorable. In other words, climate change is increasing the amount of precipitation on those days in which ideal intense precipitation conditions are present.

As for a possible interaction effect between natural variability and climate change, Wang et al. (2015) have found that global warming may have enhanced the atmospheric response to El Niño in Texas, which even without climate change favors enhanced springtime precipitation under developing El Niño conditions.

(4) The pipe: Heavy rainfall events during May 2015

During May 2015, near-ideal intense precipitation conditions were present in various locations across Texas. On sixteen different days, some locations in Texas received at least 6″ (152 mm) of rainfall. These events occurred within every climate division of the state, and included major flooding events north of Fort Worth, along the Blanco River in Wimberly and San Marcos, and in parts of Houston. Individual events such of these appear to have been made more likely due to climate change.

(5) Summary

With the lack of a positive trend in monthly springtime precipitation, there is no direct observational evidence that the record-setting May 2015 statewide rainfall total in Texas had an anthropogenic component. One study has found a possible enhancement of the springtime Texas rainfall response to El Niño. Much more apparent is the likely contribution of anthropogenic climate change to individual intense rainfall events within the month of May. This contribution is analogous to the effect of a wider pipe on water delivered by a faucet.

Editor’s note: for greater clarity to non-scientist readers, I added the three graphs other than “figure one” to this text.

————————- End paper ————————-

More about all our weather is climate change

One of climate activists’ major tactics is to blame every extreme weather event on CO2 emissions. Eventually one or two large events will occur, panicking Americans into acceptance of major public policy changes.

There are other ways to play the game, such as predictions that cover the all bases, as in this by Andrea Thompson at Climate Central, 5 June 2015.

“The seemingly endless and often torrential rains that deluged Texas and Oklahoma in May are in some ways a harbinger of what the South Central states can expect to see as the world warms. But the region also could be in store for just the opposite – more long bouts of hot, dry days that could cause the Southern Plains to be even more susceptible to drought than they already are.”

For More Information

About modern weather in Texas

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For examples of activists exaggerating what scientists know about anthropogenic effects on climate change see these posts: key facts about the often misreported California drought, “Climate Change Is Strangling Our Oceans”, Assigning blame for the flooding of Pacific atolls, 90% of the biggest Yosemite glacier has melted – Did we do it?, and Rising seas alert! Watch how science becomes a sensational news story.

To better understand today’s extreme weather

To learn more about the state of climate change see The Rightful Place of Science: Disasters and Climate Change by Roger Pielke Jr. (Prof of Environmental Studies at U of CO-Boulder, and Director of their Center for Science and Technology Policy Research).