Michael Lucy
Sluggish superstorms like Hurricane Harvey – which loitered over eastern Texas for four days in 2017, dumping up to a metre of rain and causing unprecedented floods – may be commonplace in the future, if a newly identified 70-year slowdown in tropical cyclone speeds continues.
In a climate change-inspired study of tropical cyclones around the world, published in the journal Nature, James Kossin from the US National Oceanic and Atmospheric Administration (NOAA) found an average 10% drop since 1949 in “translation speed”, or how fast the storm moves from place to place.
All other things being equal, according to the paper, a slower storm has more time to dump rain on a given location and is more likely to cause flooding.{%recommended 7015%}
“[Translation speed] is not something a lot of people have looked at,” says Kevin Walsh, a cyclone expert at the University of Melbourne in Australia. “This will spark a lot of effort to find out what’s going on and explain it physically.”
Earlier research had shown that tropical cyclones are gradually becoming more intense and moving away from the equator toward the poles, in changes that largely agree with what climate models predict will happen as the world warms.
This study began with the idea that rising temperatures might also have affected how fast tropical cyclones move around. Tropical cyclones form around a region of low air pressure over a warm ocean, but once formed they are pushed around by bigger currents of air.
“Global warming causes changes in the global wind patterns,” says Kossin. “Since tropical cyclones are mostly just carried along in these winds, those changes should show up in their translation speed.”
Large-scale atmospheric circulation in the tropics during the summer cyclone season appears to be slowing down, he says, so finding evidence that cyclones are also moving slower wasn’t entirely surprising.
“But I was surprised at how large and significant a signal it was,” he adds.
Sifting through decades of records from around the world, Kossin found tropical cyclones slowing down everywhere they occur except the northern Indian Ocean. Particularly large drops occurred along the western edge of the North Pacific (30%), the west of the North Atlantic (20%) and around north-eastern Australia (19%).
While the evidence for the drop is clear, the physical mechanism behind it – and any connection to global warming – is still up in the air.
“This will hopefully galvanize new research that explores the cause of the slowdown,” says Kossin. “But until then, we can only speculate and say that the findings are consistent with what might be expected in a warming world.”
At this stage, the results are unlikely to change anyone’s cyclone forecasts.
“Global warming presents a very real threat of flooding potential from tropical cyclones,” says Thomas Loridan, a co-founder of the Australian company Reask, based in Australia, which studies climate-related risk for the insurance industry. In Australia, he says, slower-moving cyclones are most likely to affect the Queensland coast: “Increased flooding risk can be expected all the way down to Brisbane.”
However, precisely because Kossin’s research is consistent with theoretical expectations for a warming world, it won’t yet affect projections for the future.
“The physical arguments [for cyclones slowing down] are very strong and well documented,” says Loridan. “The evidence from the data doesn’t necessarily provide us with a lot of additional confidence.”
Kossin stands by the work, arguing that it adds plenty to our confidence in the theory of human-induced global warming and the broader climatic changes it is driving.
“It’s not enough to just expect something to be true,” he says. “Someone needs to go in and do the analysis.”
