New York — Meteorologists explain that Hurricane Harvey stalled off the Texas coast because two high-pressure atmospheric masses — huge bookends made out of air — have squeezed it in place, and there haven’t been any high-level currents to help steer it away.
Harvey is yet another of several recent weather disasters marked by such shocking staying power, punishing whole regions for days or weeks on end — and longer. Others include a huge heat wave over Russia and flooding in Pakistan in 2010, the Texas drought of 2011, the California drought that began around the same time and continued into this year, and the flooding last year in Texas’s neighbour to the east, Louisiana.
Sluggishness in storms is a big deal, particularly if they’re increasing in frequency. "It turns a garden-variety disaster into a catastrophe," says Paul Douglas, a broadcast meteorologist and weather entrepreneur. As Harvey stays put, it functions as a fire hose that sucks warm water from the Gulf of Mexico and the atmosphere, and dumps it inland. As of this writing, meteorologists predict Harvey will move north-northeast on Wednesday, and up into the Mississippi basin on Friday.
Just to be clear, climate change itself is no longer up for debate — both in the scientific community and among national governments (with a few notable exceptions). Understanding it is relatively simple. Global average temperatures are rising. There’s at least 4% more water vapour in the air than 70 years ago. Ice sheets are melting. Seas are swelling. These are the direct effects of warming, and humans are causing it.
"That’s the part of the science everyone agrees about," says Adam Sobel, professor of applied physics and maths and of earth and environmental sciences, at Columbia University.
What experts don’t agree on, however, is whether — or how much — man-made climate change is responsible for this meteorological stickiness that’s kept Harvey over South Texas. The questions scientists are now asking about stalling storms aren’t along the lines of "did climate change cause the hurricane?" Climate change didn’t cause the hurricane, but today’s warmer water and more humid air provided it with rocket fuel, making it more intense, and humanity did conjure those conditions.
Instead, the 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 summer 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 on 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 favoured by human-caused 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 co-author of the paper and head of earth systems analysis at the Potsdam Institute for Climate Impact Research, explains there may really be several things going on. In general, the jet stream, the high-flying river of air that flows west to east, has slowed and gone all wavy in recent summers, with pronounced north-south 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 analysed in their paper — California drought, Russia’s 2010 heat wave and Pakistan’s related flood — the meandering north-south river of the jet-stream stabilises for periods of time in some places, creating an insurmountable wave-like band.
The researchers looked for some kind of misbehaviour in atmospheric circulation after realising that heat-related effects alone couldn’t explain the extreme nature of some disasters.
Not everybody’s sold on either the general jet-stream wobbliness from a warming Arctic, or the stabilising atmospheric waves described by Mann, Rahmstorf, and colleagues. "It’s still controversial," Sobel says. Even Mann, the lead author, introduced the pattern as "tenuous", and Rahmstorf says that more real-world case studies and more years of observations are needed.
With Houston still under siege and the 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.