Just a year ago we were dealing with a historically devastating Atlantic hurricane season. It was marked by the strongest hurricane — Irma — ever observed in the open Atlantic; the near-total devastation of Puerto Rico by a similarly powerful Category 5 monster, Maria; and Hurricane Harvey — the worst flooding event in US history.
At the time, I commented about the role climate change had played in amplifying the destructive characteristics of these storms.
Not to be outdone, this year’s 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 disturbance in the western Gulf of Mexico known as “95L” earns the status of a tropical storm, this season would be the second time in recorded history that we have seen five tropical storms simultaneously present in the Atlantic basin — the last time was in 1971.
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.
However, 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 last year and Florence this year.
In this sense, the sometimes fractious debate about whether we can expect to see more or fewer storms in a warmer world is somewhat misplaced. What matters is that there is a consensus that we will see stronger and worse flood-producing storms — and we are 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 225.3kph winds over those very warm western Atlantic waters.
Past studies indicate about a 7 percent increase in the peak wind speed of a Category 4 storm for each 1°C warming of ocean surface temperatures.
So the about 1.5°C warmer-than-normal waters in the subtropical Atlantic where the storm intensified — and keep in mind that “normal” as modernly defined by the US National Oceanic and Atmospheric Administration as the average during the 1981 to 2010 period is itself already about 1°C warmer than pre-industrial times prior to advent of human-caused greenhouse warming — corresponds to about an 11 percent increase in peak winds.
However, the destructive potential of a storm is the cube of the wind speed. So that 11 percent increase in wind speed corresponds to a 33 percent increase in destructive potential, which is not a subtle effect.
Fortunately those winds decreased substantially as the storm approached landfall, but even as a strong, very slowly moving hurricane pounding structures with near 160.9kph winds for hours on end, Florence did considerable damage as it skirted the long Carolina coastline, taking down trees and power lines, and rendering large areas without electricity.
That brings us to the second, even greater threat: storm surge.
Although the storm weakened as it approached the coast, the storm surge was built up over a period of several days, including the time during which it existed as a Category 4 or strong Category 3 storm.
That means the catastrophic, about 3m storm surge from Cape Hatteras to Myrtle Beach was baked in well in advance of the landfall of the storm.
Do not forget to add to that the 0.3m of sea level rise that has occurred along the southeastern US coast, mostly due to climate change — there is a small contribution owing to the geological subsidence of the coast.
The wonderful little coastal North Carolina town of New Bern has been particularly hard hit, with the downtown area flooded by the 3m storm surge and 200 people requiring rescue.
It is 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 is one of the simplest relationships in all of meteorology: For each 1°C of warming, there is about 7 percent more moisture in the air. That means those 1.5°C above-normal ocean temperatures have given the storm about 10 percent more moisture. All other things being equal, that implies about 10 percent more rainfall.
Yet that is not the whole story. What made Harvey a record flooding event last year, and makes Florence such a flooding threat, 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 storms such as Harvey or 2011’s Hurricane Irene that caused historic flooding in the state of Pennsylvania.
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 everyone does not get serious, in a hurry, about acting on climate change.
The world must transition away from fossil fuels toward renewable energy even more rapidly, and people must elect politicians who will support such efforts.
In the US, there is an opportunity to do so in less than two months with the upcoming mid-term elections, where Americans must elect politicians who support enlightened policies on energy and climate, and vote those who do not out of office.
Michael Mann is distinguished professor of atmospheric science and director of the Earth System Science Center at Pennsylvania State University.
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