Angel Marshman wades through floodwaters from Tropical Depression Imelda after trying to start his flooded car on Sept. 18 in Galveston, Tex. (David J. Phillip/AP)

Lumbering Tropical Storm Imelda dumped up to 43.39 inches of rain in Southeast Texas, between Houston and Port Arthur, on Sept. 18 and 19. At least $1 billion in damage was probably incurred, along with at least five deaths from the epic deluge, which scientists estimate had a return period of once in 1,200 years.

But the flooding wasn’t a freak occurrence in this region, having followed other heavy rains in 2016, 2017 and 2018. The biggest and most damaging event was 2017′s Hurricane Harvey, which set a national rainfall record for the heaviest rain in a tropical system, at 60.58 inches.

A new study examines Tropical Storm Imelda which, like Harvey, lingered in one general area for days on end, and any ties between the heavy rainfall totals and long-term, human-caused climate change.

A scientific consortium known as World Weather Attribution, which conducts rapid analyses of whether and how climate change played a role in extreme weather events, analyzed Imelda in a similar way to a previous analysis of Hurricane Harvey.

The Harvey study found that global warming increased the intensity of rainfall from that devastating storm by about 15 percent, while the probability of its occurrence went up by a factor of three, because of increased concentrations of greenhouse gases in the atmosphere and corresponding changes to ocean temperatures and the amount of water vapor available for storms to tap into as energy.

The new study, which uses similar methods and has not yet been peer-reviewed, comes to similar conclusions, finding that although this event involved a weaker storm with slightly lower rainfall totals, it, too, was supercharged by a warming climate.

Since 1900, the chances of receiving such an amount of rain has more than doubled, the study found, while the amount of rainfall in such an event has increased by about 18 percent. This is largely because warmer ocean waters and increasing average air temperatures provide additional moisture that storms can tap into for energy and wring out in the form of precipitation.


Satellite loop showing heavy rains along the Texas Gulf Coast from tropical depression Imelda. (NOAA)

One robust conclusion of climate science studies in recent years is that we’re already seeing an uptick in extreme precipitation events in many parts of the United States and worldwide as a result of climate change, though the signal is not uniform everywhere.

Scientists are working toward the goal of being able to detect the fingerprints of human-caused climate change on extreme weather and climate events in near-real-time. Their methods for such studies are well-established.

According to study co-author Friederike Otto of the University of Oxford, researchers first combed through observed data on how much rain fell and when it fell. They then used climate models “to simulate what is possible rainfall in the Gulf region in the world today and compare it with possible rainfall amounts in the same world but with the greenhouse gases from burning fossil fuels removed from the models,” she said via email.

At the same time, as the odds of extreme rainfall events has increased along the Texas Gulf Coast, the region has grown, with about 6.6 million residents affected by T.S. Imelda. After decades of free-for-all development, including in flood zones, lawmakers have been working to encourage smarter growth practices that expose fewer people to flooding in future storms.

Ominously, the study notes that the occurrence of three similar flooding events in the past four years needs to be examined in case it’s “more than an unfortunate coincidence.”

“We expect more rainfall from tropical storms and hurricanes,” said study co-author Geert Jan van Oldenborgh at the Royal Netherlands Meteorological Institute. “Note that extreme precipitation on the Gulf Coast can also come from other types of meteorological events, the 2016 Louisiana floods were due to a cut-off low,” Oldenborgh said. “We analyze extreme precipitation from all these sources, not only tropical storms and cyclones.”

“We will have to work out the chances of three extreme precipitation events in four years on the Gulf Coast to check whether this is just bad luck or signifies [a] stronger increase in the most extreme events than we thought,” he said via email.

The intersection between extreme rainfall events as well as societal vulnerability to them is playing out increasingly along the Texas Gulf Coast. “This shows again that climate change is happening here and now, and it is affecting people in all parts of the world. With such repeated events it will be those with the least money that will take the longest to recover,” Otto said.