Human activity is making hurricanes worse, according to a pair of studies published on Wednesday in the journal Nature.
Climate change increased rainfall from 5 percent to 10 percent in hurricanes Katrina (2005), Irma (2017) and Maria (2017), wrote two researchers at Lawrence Berkeley National Laboratory. The other study, by scientists from Princeton University and the University of Iowa, finds that Houston's urban footprint increased the odds of extreme flooding seen during Hurricane Harvey (2017) by about 21 times.
Hurricanes are expected to grow only more intense and drop more precipitation in a warmer, wetter climate, even if they may also occur less frequently in the future. The conventional focus on the damage hurricanes cause cities has obscured, until this new paper about Harvey's effects on Houston, that urban asphalt and buildings cause atmospheric drag and friction that can change storms for the worse.
Hurricanes are difficult to study. The swirling weather systems occur just a few dozen times annually, and the satellites capable of tracking them have been aloft less than 40 years. Air temperature, in contrast, is far simpler to examine. Nearly continuous measurements from all over the world date back more than a century.
Berkeley researchers Christina Patricola and Michael Wehner identified for the first time long-projected jumps in hurricane rainfall attributable to global warming. Loath to make blanket statements, the pair stipulate that "additional cases are needed before making a general conclusion" for all such storms. Under the bleakest warming scenario, hurricane rainfall could eventually rise by up to 30 percent.
The study also brought researchers closer to solving a meteorological mystery. Physicists have known since the middle of the 19th century that as air warms, there's a predictable increase in the amount of moisture it can hold-about 7 percent per degree Celsius. However, abnormally rainy storms such as Hurricane Harvey dump far more water than expected.
The reason, Patricola and Wehner write, may be that the structure of storms is changing along with the increase in atmospheric moisture. Hurricanes appear to be dumping more water than they should, because they are concentrating rainfall more tightly in the center, draining precipitation from the edges.
The pair did not find an increase in storm intensity in the data to go along with the increase in storm rainfall. (Storm intensity refers to high wind speed and low air pressure at sea level.)
Previous studies have suggested that this expected increase in cyclone intensity may be masked by aerosol pollution, a byproduct of industrial activity. Unlike greenhouse gases, aerosols have a temporary cooling effect on the atmosphere. That effect may be canceling out the longer-term warming effect of carbon dioxide and other gases, dampening the expected intensification of storms, according to Tom Knutson, a research meteorologist at the National Oceanic and Atmospheric Administration.
The Houston study, meanwhile, takes a comprehensive view of how the city itself contributed to Hurricane Harvey's peculiar staying power. The storm parked itself above the U.S.'s fourth most populous city in August 2017 and delivered about 1.3 meters of rainfall.
Deploying the same major weather model used in the Berkeley study, researchers basically reran Harvey over southeast Texas in two otherwise-identical worlds: one with Houston and one with cropland covering the same area. The goal, according to Gabrielle Villarini, associate professor of civil and environmental engineering at the University of Iowa, was to determine whether the city itself affected Harvey's rainfall.
The answer was a resounding yes. The city's hard surfaces and buildings added friction to the storm as it whirled overhead, and the heat "associated with urbanization and urban land-use change may lead to the destabilization of the atmosphere, enhancing rainfall over the eastern side of Houston," the authors wrote.
The study is important because it asks a question that's received too little scholarly attention, said Kerry Emanuel, a professor of atmospheric science at MIT: namely, how hard surfaces and tall buildings create friction and slow storms-thereby increasing the opportunity for rain to fall on population centers. "This paper represents a real advance in our understanding of hurricane impacts on urban areas," he said. The findings have "important implications for urban planning."