Hurricane Florence - A Climate Change Triple Threat

Here's the "Director's Cut" of my recent op-ed in The Guardian (includes nearly three extra paragraphs toward the end)

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Hurricane Florence - A Climate Change Triple Threat

Just a year ago we were dealing with an historically devastating Atlantic hurricane season, marked by the strongest Hurricane—Irma--ever observed in the open Atlantic, the near total devastation of Puerto Rico by a similarly powerful cat 5 monster Maria, and Hurricane Harvey—the worst flooding event in U.S. history. At the time, I commented here and elsewhere about the role climate change had played in amplifying the destructive characteristics of these storms.

Not to be outdone, the 2018 Atlantic hurricane season, initially predicted to be quiet--quelled by an incipient El Nino event and cool early summer ocean waters--has suddenly erupted. If the current disturbance in the western Gulf of Mexico known as “95L” earns the status of tropical storm,  the 2018 season will be tied for the most tropical storms (five) ever simultaneously present in the Atlantic basin.

What happened to cause all of this? An early autumn ocean “heat wave” has brought sea surface temperatures in the western Atlantic to bathtub-level warmth. Just as  summer heat waves on land are greatly increased in frequency and intensity by even modest overall warming, so too are these ocean heat waves becoming more frequent and more extreme as the oceans continue warm. All else being equal, warmer oceans mean more energy to intensify tropical storms and hurricanes.

But when it comes to coastal threat, it hardly matters how many tropical storms there are over the course of the season. A single landfalling hurricane can wreak havoc and destruction.  Think Katrina in 2005, Irene in 2011, Sandy in 2012, either Harvey or Maria in 2017 and now Florence in 2018.

In this sense, the sometimes fractious debate about whether we’ll see more or fewer storms in a warmer world is somewhat misplaced. What matters is that there is a consensus we’ll see stronger and worse flood-producing storms—and in fact we’re seeing them already. That brings us to Hurricane Florence—a climatologically-amplified triple threat.

First, there is the threat of wind damage. Florence strengthened into a monster category 4 hurricane with 140 mile per hour winds over those very warm western Atlantic waters. Past studies indicate a roughly 7% increase in the peak wind speed of a cat 4 storm for each 1C warming of ocean surface temperatures. So the roughly 1.5C warmer-than-normal waters in the subtropical Atlantic where the storm intensified (and keep in mind that “normal” as modernly defined by NOAA as the average during the 1981-2010 period is itself already about 1C warmer than pre-industrial times prior to advent of human-caused greenhouse warming) corresponds to a roughly 11% increase in peak winds. But the. destructive potential of a storm goes as the cube of the wind speed. So that 11% increase in wind speed corresponds to a 33% increase in destructive potential.  That’s not a subtle effect.

Fortunately those winds decreased substantially as the storm approached landfall, but as a strong, very slowly-moving landfalling hurricane pounding structures with near 100 mile per hour winds for hours on end, Florence is doing considerable damage as it skirts the long Carolina coastline, taking down trees and powerlines and rendering large areas without electricity.

That brings us to the second, even greater threat: storm surge. Though the storm weakened as it approached the coast, the storm surge was built up over of a period of several days, including the several days during which it existed as a cat 4 or strong cat 3 storm. That means the catastrophic ~10 foot storm surge from Cape Hatteras to Myrtle Beach was baked in well in advance of the landfall of the storm.

Don’t forget to add to that the 1 foot of sea level rise that has occurred along the southeastern U.S. coast, mostly due to climate change (there’s a small contribution owing to the geological subsidence of the coast).

The wonderful little coastal North Carolina town of New Bern particularly hard hit, with the downtown area flooded by the 10 foot storm surge and 200 people requiring rescue. It is eerily ironic  and seemingly prescient that I gave a lecture in New Bern last year, during last year’s storm season, warning about the coastal threat from climate change, in a church that has now been flooded by Florence.

Last but not least, we have the threat of inland flooding. Warmer oceans mean more moisture in the atmosphere. It’s one of the simplest relationships in all of meteorology; for each 1C of warming, there is about 7% more moisture in the air. That means those 1.5C above normal ocean temperatures have given the storm about 10% more moisture. All other things being equal, that implies about 10% more rainfall.

But that’s not the whole story. What made Harvey a record flooding event last year and makes Florence such a flooding threat now, is the slow-moving nature of the storm. The slower the storm moves, the more rainfall that accumulates in any one location and the more flooding you get. Such was the case with other devastating past storms like Harvey or 2011’s Hurricane Irene that caused historic flooding in my own state of Pennsylvania. There is evidence that the forward motion of these storms is slowing., with climate change to blame. My own research suggests that we’re seeing more stalled summer weather systems due to the amplified warming of the Arctic, which slows down the summer jet stream and favors the stalling of weather systems.

We will see post-hoc analyses based on so-called “attribution” studies which compare the results of climate models with and without human-caused greenhouse gas increases to assess the impact that climate change may have had in impacting a particular weather event (indeed, a pre-event attribution study has even been publicized for Florence).  If anything, these sorts of studies are likely to underestimate the impact of climate change on extreme weather events, as I noted earlier this summer in The Guardian, because the models don’t do a good job capturing the physical processes we’ve identified that are leading to stalled summer weather patterns.

 If anything, the scientific projections—of sea level rise, and the increase in extreme weather events including intense hurricanes—have been too conservative. Yes there is uncertainty to be found here. But it is not our friend.

Some headlines have reported that Florence is a warning of what is to come. But in reality, it is a warning of what has already arrived. Far worse is to come if we don’t get serious, in a hurry, about acting on climate. We must transition away from fossil fuels toward renewable energy even more rapidly, and we must elect politicians who will support such efforts. In the U.S., there’s an opportunity to do so in less than two months now in the upcoming mid-term elections, where we must elect politicians who support enlightened policies on energy and climate, and vote out of office those who don’t.

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Michael E. Mann is Distinguished Professor of Atmospheric Science and Director of the Earth System Science Center at Penn State University. He is co-author of The Madhouse Effect: How Climate Change Denial is Threatening our Planet, Ruining our Politics and Driving us Crazy.