Sun Sentinel Palm Beach Edition
Scientists find Harvey’s idling a sticky issue
Study says climate change may help storms stay locked in place. Not all agree.
Meteorologists explain that Hurricane Harvey stalled off the Texas coast because two high-pressure atmospheric masses — huge bookends made out of air — squeezed it in place, and there weren’t any high-level currents to help steer it away.
Harvey is another of several recent weather disasters marked by such staying power, punishing whole regions for days or weeks and longer. Others include a massive 2010 heat wave over Russia and flooding in Pakistan; the Texas drought of 2011 and the California drought that began around the same time and continued through this year; and the flooding last year in Louisiana.
Sluggishness in storms is a big deal, particularly if they’re increasing in frequency. “It turns a gardenvariety disaster into a catastrophe,” said Paul Douglas, a broadcast meteorologist and weather entrepreneur. As Harvey stays put, it functions as a firehose that sucks warm water from the Gulf of Mexico and the atmosphere, dumping it inland.
Experts don’t agree on whether — or how much — human-caused climate change is responsible for the meteorological stickiness that kept Harvey over South Texas.
Scientists aren’t asking questions such as, “Did climate change cause the hurricane?” Climate change didn’t cause the hurricane, though today’s warmer water and more humid air provided it with rocket fuel, making it more intense.
Instead, experts are asking how huge, “dynamic” atmospheric systems may be changing, at least indirectly, because of humanity’s prodigious greenhouse gas pollution. So, a better question might be, “Does humanity have anything to do with summertime Northern Hemisphere storms that are prevented from moving off because of changes in the jet stream?” The answer is there’s not enough evidence in yet.
Michael Mann, distinguished professor of atmospheric science at Penn State University, published an essay Monday in which, among items better understood about hurricanes and warming, he included:
“More tenuous, but possibly relevant still, is the fact that very persistent, nearly ‘stationary’ summer weather patterns of this sort, where weather anomalies ... stay locked in place for many days at a time, appears to be favored by humancaused climate change.”
In March, Mann and several colleagues published a study in the journal Scientific Reports that demonstrates a relationship between extreme events, such as the 2011 Texas drought and 2010 Pakistan flooding, and a rare stationary phase that upper atmospheric currents sometimes go through in the mid-latitudes.
Stefan Rahmstorf, a coauthor of that paper and head of Earth Systems Analysis at the Potsdam Institute for Climate Impact Research, explained that there may be several things going on. In general, the jet stream, the high-flying river of air that flows west-toeast, has slowed and gone all wavy in recent summers, with pronounced northsouth meanders. That’s one thing that may have helped hold Harvey in place. Researchers have sparred since 2012 over whether Arctic warming, which is occurring at twice the global average, is driving this atmospheric wobble, consequently creating more opportunities for persistent weather farther south.
In a number of extreme cases analyzed by their paper, the north-south meander of the jet-stream stabilizes for periods in some places, creating an insurmountable wavelike band. The researchers looked for some kind of misbehavior in atmospheric circulation after realizing that heat-related effects alone couldn’t explain the extreme nature of some disasters.
Not everybody’s sold on either the general jetstream wobbliness from a warming Arctic, or the stabilizing atmospheric waves described by Mann, Rahmstorf and colleagues.
“It’s still controversial,” said Adam Sobel, a professor at Columbia University who specializes in extreme weather.
Even Mann, the lead author, introduced the pattern as “tenuous,” and Rahmstorf said that more realworld case studies and more years of observations are needed.
With Houston still recovering, with floodwaters rising Wednesday east of there and with hurricane season still churning, the thought of more case studies is the last thing anybody wants to consider.
And yet, just as it took decades to prove climate change, time and more studies will likely show whether humans have gone beyond global warming and in fact changed which way the wind blows.