Climate change is making hurricanes slower, and that’s terrifying

Three new studies make clear we've entered a dangerous era of ever-worsening coastal flooding.

Epic flooding inundates Houston after Hurricane Harvey, August 28, 2017. CREDIT: Joe Raedle/Getty Images
Epic flooding inundates Houston after Hurricane Harvey, August 28, 2017. CREDIT: Joe Raedle/Getty Images

We have entered a dangerous new era of coastal flooding, where slow-moving, city-inundating hurricanes like Harvey are the new normal, new research finds.

NOAA reported Wednesday that, thanks to the rise in sea levels and extreme weather, the flooding from high tides has doubled in just 30 years. Also, “more than a quarter of the coastal locations tied or broke their individual records for high tide flood days.” 

The number of days with destructive high tide flooding (bar chart) has doubled in  the past three decades as sea levels have risen (blue line). CREDIT: NOAA
The number of days with destructive high tide flooding (bar chart) has doubled in the past three decades as sea levels have risen (blue line). CREDIT: NOAA

At the same time that seas are rising thanks to global warming, NOAA also reports that hurricanes are moving more slowly — and the slower a hurricane moves, the  more time it has to deluge a city.

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For instance, as climatologist Michael Mann explained during Hurricane Harvey, “the kind of stalled weather pattern that is drenching Houston is precisely the sort of pattern we expect because of climate change.” Climate science predicted a weaker jet stream, and Harvey stalled because the jet stream wasn’t strong enough to move it along.

In addition, as the planet warms, the air can carry more water vapor. When you combine the greater moisture in the air with a slower moving storm, you get a disaster.

Indeed, “as little as a 10 percent slowdown could double local rainfall and flooding impacts caused by 1°C of warming,” NOAA explains.

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Study author Dr. Jim Kossin of NOAA’s National Centers for Environmental Information explains that “Tropical cyclones over land have slowed down 20 percent in the Atlantic” from 1949 and 2016 and 30 percent in the western North Pacific. “These trends are almost certainly increasing local rainfall totals and freshwater flooding, which is associated with very high mortality risk.”

Harvey, for example, virtually stalled in place for days while maintaining much of its strength. This led to five-day rainfall totals that blew past the previous all-time record for the continental United States of 48 inches. The National Weather Service recorded an astonishing cumulative total of 51.88 inches of rain in Cedar Bayou, Texas.

Kossin’s study was not aimed at attributing the cause of this hurricane slowdown, but he notes that the observed global slowdown “occurred in a period when the planet warmed by 0.5°C.” He added, “more study is needed to determine how much more slowing will occur with continued warming.”

Significantly, just two weeks ago, scientists at the National Center for Atmospheric Research (NCAR) published “a detailed analysis of how 22 recent hurricanes would be different if they formed under the conditions predicted for the late 21st century.”

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Based on current projections of climate change, they found that future storms  would, on average, have “6 percent stronger average hourly maximum wind speeds than those in the past.” The storms also moved 9 percent slower and “had 24 percent higher average hourly maximum rainfall rates.”

When scientists modeled a future Hurricane Ike, which slammed Texas in  September 2008, they found it would have “13 percent stronger winds, move 17 percent slower, and be 34 percent wetter if it formed in a future, warmer climate.”

The forecast for the coastal United States is flooding, followed by ever more flooding.