A new study by a scientist with the National Oceanic and Atmospheric Administration has found that over the past seven decades, tropical cyclones have slowed down near coastlines around the world.
The findings published in the journal Nature describe a clear link between global warming and the behavior of these severe storms — with potentially devastating consequences for the people that live near them.
According to overwhelming scientific consensus, the climbing average global temperature over the past century has been fueled largely by human activities that have released heat-trapping greenhouse gases like carbon dioxide into the atmosphere. Among its negative effects, that rising heat has been linked to the melting of glaciers, the creeping rise of oceans, the bleaching of corals, the spread of human diseases and the worsening of drought.
Scientists have also been trying to understand the link between global warming and the behavior of extreme weather events, such as the hurricanes that pound Atlantic coastlines during the summer and fall.
Certainly there’s one clear physical link, said Christina Patricola, an atmospheric scientist at Lawrence Berkeley National Laboratory who was not involved in the study. For every 1 degree Celsius rise in temperature, the atmosphere is able to hold 7% more moisture. That means that when it finally does rain, there’s a lot more water coming down — which could raise the risk of flooding.
The relationship of global warming to other aspects of such storms, such as frequency and intensity, has been more difficult to pin down, said study author James Kossin, a climate scientist with the National Oceanic and Atmospheric Administration in Madison, Wis. That’s partly because it’s hard to compare older data to the information that’s been gathered in recent decades with more sophisticated instruments.
And of course, in the pre-satellite era (before the 1960s), a lot of storms that lived and died over the open ocean would have gone unnoticed, which could potentially make them seem less frequent in the past than they really were.
But one of the hallmarks of Hurricane Harvey, which ravaged the Texas coastline in 2017, was how it slowly it moved across the Houston region, dumping roughly 50 inches of rain in some areas.
“A lot of the initial research has been focused largely on numbers of tropical cyclones and then of course on intensity because that’s the way that we feel the impacts,” Patricola said. But when events like Hurricane Harvey hit, she added, “it’s a reminder that we need to consider the other characteristics of tropical cyclones.”
Kossin wanted to see if, on average, the traveling speed of such tropical cyclones was slowing down. To find out, he used data from the U.S. National Hurricane Center and the Joint Typhoon Warning Center to study the position of such storms’ centers every six hours. From those, he was able to calculate the moving speed of these coastal storms between 1949 and 2016.
Kossin found that, on average globally, the speed at which these tropical cyclones moved had fallen by about 10% with just 0.5 degrees Celsius of warming. In certain regions, that slowdown was even more extreme: about 30% over land affected by tropical cyclones from the western North Pacific and 20% over land affected by tropical cyclones from the North Atlantic.
“I was surprised at how strong a signal it was and how large a signal it was,” Kossin said. “Ten percent over 70 years is really quite a lot.”
Still, the finding fits with what is known about global warming and the dynamics of the atmosphere, he said. Tropical cyclones are largely carried along by atmospheric circulation — and that has been slowing down, as the temperature gradient between the poles and the tropics begins to shrink (because the poles are warming faster than the tropics are).
A storm that lingers longer over inhabited coastlines is a more dangerous storm, Kossin said, because it allows more rain to fall in a local area, raising the risk of flooding and storm surge. It may even increase the damage from hurricane winds, simply by battering the same structures over a longer period.
“These tropical cyclones arrive carrying many, many hazards with them — none of them good — and the longer it’s in your area, the worse it’s going to be,” Kossin said. “You don’t want them to move slowly.”
Kossin was quick to point out that his results only show what had happened over the 68-year period, not what may happen in the future.
Patricola, who called the findings “quite convincing,” said study opened new questions that need answering. For example: How has tropical cyclone rainfall changed in the recent historical record, and how did the moving speed of these storms affect those regional rainfall totals?
Scientists will also need to look more closely at the moving speeds of stalled tropical cyclones — the subset of these storms that are moving extremely slowly, and are much rarer, she said.