Did climate change cause Hurricane Harvey?

By Roddy Scheer
Climate change can contribute to a storm's severity.
Climate change can contribute to a storm's severity.
Dear EarthTalk: Was Hurricane Harvey caused by global warming? 
– Tom Dell, Bern, North Carolina 
    The short answer is no. No single hurricane or weather event can be directly linked to the general phenomena known as climate change. “Climate change does not cause things, because climate change is not a causal agent,” writes David Roberts on Vox.com. “Climate change is a descriptive term — it describes the fact that the climate is changing.”
    That said, global warming likely contributed to the severity of Harvey and has created an overall climate more hospitable to the formation of extreme weather events of every stripe. "For hurricanes, we would ask the question as to what are the possible hurricane developments in the world we live in and compare that to the possible hurricane developments in a world without climate change," said Friederike Otto, lead researcher at the Oxford Martin TNC Climate Partnership-University of Oxford, in an interview with BBC News. 
    One definite fingerprint of global warming on Harvey is the intensity and amount of rainfall. Climatologists cite the Clausius-Clapeyron equation (a hotter atmosphere holds more moisture: for every extra degree Celsius in warming, the atmosphere can hold 7 percent more water) as one link between global warming and stronger storms. Houstonians have witnessed a 167 percent increase in the frequency of the most intense downpours since the 1950s. 
    Adam Sobel, atmospheric scientist at Columbia University's Initiative on Extreme Weather and Climate, estimates that as much as 10 percent of Harvey’s rainfall could be blamed on global warming. Kevin Trenberth, senior scientist at the U.S. National Center for Atmospheric Research, pegs the number at closer to 30 percent. “It may have been a strong storm, and it may have caused a lot of problems anyway—but [human-caused climate change] amplifies the damage considerably,” Trenberth reports in The Atlantic
    We’re also heating up our seas. "The waters of the Gulf of Mexico are about 1.5 degrees Celsius warmer above what they were from 1980-2010," reports Sir Brian Hoskins, director of the Grantham Institute for Climate Change at Imperial College London. "That is very significant because it means the potential for a stronger storm is there.” 
    Meanwhile, even the fact that Harvey hung around so long and dumped rain on and around southeast Texas for nearly four days suggests a climate connection: A recent report from climate scientist Michael Mann suggests that near-stationary summer weather patterns are more common in a warmer world. 
    But others think we are focusing too much on the climate underpinnings of Harvey. Ilan Kelman, researcher at University College London’s Institute for Risk and Disaster Reduction, tells the BBC that the real human contribution to the catastrophe in Houston is more about the type of development we allow than about the emissions we are pumping skyward. 
    "The hurricane is just a storm; it is not the disaster," says Kelman. "The disaster is the fact that Houston population has increased by 40 percent since 1990 [and] that many people were too poor to afford insurance or evacuate.” He adds: "Climate change did not make people build along a vulnerable coastline so the disaster itself is our choice and is not linked to climate change."


     U.S. National Center for Atmospheric Research

     Initiative on Extreme Weather and Climate

    “Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events,” by Michael E. Mann, et al, Scientific Reports, March 2017.


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