John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Places like Atlanta could see a 100 percent increase in the number of days with damaging winds, hail, and tornadoes.
Scientists who probe the atmosphere is search of tomorrow's weather have already picked up signs of major heat waves looming for American cities, particularly eastern 'burgs like Boston, Baltimore, and Philadelphia.
Now NASA is giving us a heads-up that residents of yet more cities might soon face increased risks of getting frizzled by lightning or charley-horsed by walnut-sized hailstones. The space agency has released new weather models based on a 2007 study by Purdue University's Robert Trapp, who examined what might happen if the concentrations of atmospheric greenhouse gas were to continue increasing until the end of the century. Trapp's evidence suggested that by the late 21st century the United States (and no doubt other places) will have many more days with ideal conditions for severe thunderstorms – you know, the ones with towering convective clouds and associated "high-impact weather such as destructive surface winds, hail and tornadoes."
These models of the projected summer climate from 2072 to 2099 illustrate how Trapp and other climatologists are reaching their worrisome conclusion. They're basically comparing how two atmospheric phenomena that determine how strong a storm gets might change in a globally warmer world. The first is called "convective available potential energy" or CAPE, which is a measure of how much energy a storm is packing based on things like air temperature and moistness. With emissions causing hotter days and more water evaporation, the net CAPE over the U.S. is likely to increase. The swell in potential energy looks like it'll be particularly intense in the southeast region:
The researchers also looked at vertical wind shear, the profile of the speed and direction of winds in the atmospheric column. Strong shear can "pull and twist weak storms into strong, windy ones,” as one NOAA meteorologist puts it, potentially setting the stage for the arrival of vicious tornadoes. The good news is that climate change is likely to make the Arctic nice and toasty, which in turn will reduce the wind shear over much of the U.S. The wussiest winds will probably blow over the Rocky Mountain area:
The bad news is that the sheer amount of energy in the atmosphere will likely overpower the altered winds, making our thunderstorms more frequent, window-shattering affairs. This bounce in CAPE could prompt even more storms than we currently expect: Trapp's research indicated that many of the contemporary climate models put out "significant" underestimates of future severe-weather events, and that cities from Atlanta to New York could see a 100 percent increase in the number of days with mega-nasty storms:
The ratcheted-up storm threat looks like it will develop most ripely over the east-southeastern U.S., and "especially in Missouri and coastal North and South Carolina," says NASA. Commence preemptively nailing plywood over windows... now? (Here's a fuller discussion of the models.)
Top photo of a thunderstorm by John Kerstholt on Wikipedia