Slowing Tropical Cyclones Bring More Mayhem

Tropical cyclones are slowing down.  Hurricanes have lost their hurry.  Paradoxically, this is bad news:  they have more time to work their mischief.

Tropical cyclones are moving more slowly.  As temperatures rise, the pace at which a hurricane storms across a landscape has slowed perceptibly in the last 70 years.  But the slowdown means each hurricane has more time to do more damage and deliver more flooding.

“Tropical cyclones over land have slowed down 20% in the Atlantic, 30% in the northwestern Pacific and 19% in the Australian region,” said James Kossin, of the US National Oceanic and Atmospheric Administration’s national centers for environmental information.

“These trends are almost certainly increasing local rainfall totals and freshwater flooding, which is associated with a very high mortality risk.”

He reports in the journal Nature that thanks to atmospheric warming as a consequence of the profligate combustion of fossil fuels in the last century, the summer tropical circulation has slowed and, along with it, hurricane and typhoon speeds.  Overall, since 1940, cyclone movements have slowed by 10%; over some land areas, they have slowed much more.

But as the temperature goes up, the capacity of the atmosphere to hold moisture increases – by at least 7% with each degree Centigrade.  That means a tropical cyclone – a whirling system of terrifying winds bearing huge quantities of water – has both more water, and more time to drop it over land.

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