Climate change made Hurricane Harvey’s rainfall worse, study finds

Updated 2:52 PM EST, Wed December 13, 2017
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(CNN) —  

Climate change is partly to blame for the record rainfall that fell over Texas and Louisiana in the days after Hurricane Harvey’s landfall on August 25, according to new scientific analysis.

Human-caused climate change made the rainfall from Harvey, which dumped more than 19 trillion gallons of water and brought devastating floods to the Houston area, roughly three times more likely to occur and 15% more intense, according to World Weather Attribution (WWA), an international coalition of scientists led by nonprofit scientific research group Climate Central, in a report published Wednesday.

The question of whether climate change played a role in the disaster came up almost immediately, as it so frequently does after extreme weather events.

It was assumed that climate change contributed to Harvey’s rainfall, as intense rainfall events are one of the most often cited consequences of a warming climate. The observational data and computer models used in this study supported that assumption.

As air temperature warms, it can hold more moisture (about 7% more per degree Celsius of warming), which means there is more water vapor in the air that can be squeezed out as rainfall in a world warmed by climate change.

Other factors, such as changes in upper level winds and sea level rise can also contribute to increased flood events.

The WWA study examined extreme precipitation events in the US Gulf Coast region since 1880 and found “a clear positive trend between 12% and 22%.” Using computer models loaded with this data and comparing it to similar models that do not show the climate warming, the authors attempted to quantify the role that the warming climate had on the 2017 event.

Harvey, which rapidly intensified to a Category 4 storm just before landfall near Corpus Christi, Texas, stalled after it moved inland and remained nearly stationary for four days. This allowed rainfall totals to add up in and around Houston, reaching levels never before seen in a landfalling tropical system.

Cedar Bayou, about 30 miles east of Houston, recorded 51.89 inches of rainfall by the morning of August 31. That eclipsed the 48-inch rainfall record from Tropical Storm Amelia in 1978 and set a new record for the continental US.

The biblical flooding across Harris and surrounding counties required more than 120,000 people to be rescued or evacuated, and about 80 people died.

Though the economic impact from Harvey is still being calculated, “it is likely a $100 billion disaster,” according to Adam Smith, lead researcher at the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information.

“Harvey is the closest modern disaster comparison we have in relation to Katrina in terms of damage costs,” Smith told CNN.

The WWA study looked only at the role climate change played in the amount of rainfall Harvey produced, ignoring other variables that made Harvey so exceptional, such as the rapid intensification before landfall and the unusual lack of movement it had in the following days.

“It’s clear that Harvey’s exceptional rainfall was due to the stalling and looping nature of the track of the storm – which this study does not address in the slightest,” Ryan Maue, a meteorologist with weather.us who was not affiliated with the study, told CNN.

But that doesn’t mean Maue is refuting its conclusion.

“There’s little doubt that global warming has and will continue to enhance rain rates and intensity, but it’s not clear that this paper has teased out all of the possible mechanisms that led to Harvey’s extreme flooding impacts,” Maue said.