In the wake of Harvey, many researchers pointed out that a warmer atmosphere holds more water vapor and that, as a result, a warmer planet should see more extreme rains. But Emanuel’s study goes beyond this general statement to support the idea that the specific risk of such an extreme rain event is already rising because of how humans have changed the planet.
Via climate modeling, Emanuel generated 3,700 computerized storms for each of three separate models that situated the storms in the climates of the years from 1980 to 2016. All of the storms were in the vicinity of Houston or other Texas areas. He examined how often, in his models, there would be about 20 inches of rain in one of these events.
Harvey produced closer to 33 inches over Houston. But in the tests under the 1980 to 2016 conditions, getting 20 inches of rain was rare in the extreme.
“By the standards of the average climate during 1981-2000, Harvey’s rainfall in Houston was ‘biblical’ in the sense that it likely occurred around once since the Old Testament was written,” wrote Emanuel, adding that in the much larger area of Texas, such rains did occur once every 100 years.
Then Emanuel performed a similar analysis, this time in the projected climates of the years 2080 to 2100, assuming the climate changes in some of the more severe ways scientists suggest it could.
The odds, accordingly, shifted toward a much greater likelihood of such events by 2100. Harvey’s rains in Houston became a once-in-100-years event (rather than a once-in-2,000-years event), and for Texas as a whole, the odds increased from once in 100 years to once every 5½.
This also meant, Emanuel calculated, that Harvey was probably more likely in 2017 than in the era from 1981 to 2000. In 2017, Harvey would be a once-in-325-years event. For Texas as a whole, in 2017 it would be a once-in-16-years event.
“It was a very unusual event,” Emanuel said. “Less unlikely than it might have been 30 years ago, but even now very unusual.”
Emanuel conceded that precisely why the simulations changed the odds with greater global warming wasn’t clear — whether it had something to do with more water vapor or other storm characteristics. “This is left to future work,” he wrote.
Several researchers praised the study, including Noah Diffenbaugh, a climate expert at Stanford University who has focused on the science of attributing extreme events to climate change.
“Harvey was a complex event with lots of contributing ingredients. This study breaks new ground by isolating the role that global warming played in upping the odds that a storm like Harvey produces very heavy rainfall,” he said.
“The 20-fold future increase in the probability of Harvey-level rainfall points toward a markedly increasing vulnerability of Gulf Coast communities — one that they are not well prepared to adjust to,” added Greg Holland, a hurricane scientist at the National Center for Atmospheric Research.
Shane Hubbard, a researcher at the Space Science and Engineering Center of the University of Wisconsin–Madison who has also studied the odds of Harvey’s rains, did question some aspects of the presentation.
He suggested that Harvey was such an extreme event — producing as much rainfall as three prior Texas flood disasters combined — that Emanuel’s approach of looking at rainfall at a single point “does not accurately represent what happened during Harvey.”
Still, looking toward the future, Hubbard found the research helpful.
“This work suggests that with landfalling hurricanes, the amounts of precipitation will dramatically increase, meaning the risk to populations along streams and rivers will also dramatically increase. Hurricanes are not a single hazard, but multiple hazards,” he added.
The new study will probably be followed by many others on the link between the devastating 2017 hurricane season and climate change.
“I think humans have changed the odds quite a bit,” Emanuel said.