Hurricanes And Climate Change — An Explainer

After wrecking large swaths of the northwestern reaches of the Bahamas, leaving at least 30 dead and a nation in crisis, Hurricane Dorian is currently making its way up the east coast of the United States. Emergency alerts have gone out for the entire South Carolina coastline; Charleston has seen flooding, high winds, and downed power lines; North Carolinians are being told to shelter in place; and experts are forecasting storm surges and possible flash flooding in coastal North Carolina and Virginia through Friday evening, with predicted rainfall of “6 to 12 inches in the coastal Carolinas and 3 to 8 inches in far southeast Virginia.”

Though the destruction wrought by hurricanes isn’t new by any stretch of the imagination, over the past decade, we’ve collectively witnessed storms like 2012’s Hurricane Sandy, which damaged or destroyed 650,000 homes, left 7.5 million people without power, and led to over 285 deaths, or 2017’s Hurricanes Harvey and Maria, the latter of which plunged Puerto Rico into darkness, and is estimated to have caused at least 2,975 deaths. Irma and Jose also did a fair amount of damage in 2017. Last year, Florence and Michael brought punishing rains to the Atlantic coast as part of a 22-hurricane season that “began earlier, continued later, and some took unexpected paths before hitting land.” Five of the 35 recorded category 5 hurricanes in 169 years of record-keeping have occurred in the past four years.

So what gives? Is this just Mother Nature flexing her muscles? Part of shifting and completely normal trends? Bad luck? Or is an influx of powerful hurricanes linked to climate change? We break it down.

What happened in the Bahamas?

First: let’s go over what’s going on with Dorian, the most recent hurricane to form in the Atlantic Ocean.

As of the morning of Friday, September 6, Dorian seems to be widening and weakening along the coast of North Carolina. But before Dorian made its way up the east coast of the U.S., it first made landfall in the Abaco Islands — an outcropping of smaller islands in the Bahamas populated by fishermen and Haitian immigrants — as a category five storm early on Monday, September 2. It then stalled out over Grand Bahama, the popular tourist island, for an entire day, pummeling the island with “sustained winds as high as 180 miles per hour.”

Of the damage to the Abacos, Lia Head-Rigby, who runs the hurricane relief group HeadKnowles, told the Associated Press, “It’s total devastation. It’s decimated. Apocalyptic.” The death toll in the Bahamas is at least 30, but expected to rise in the coming days and weeks as rescue efforts get underway.

The storm moved toward Florida on Tuesday, September 3, and, though it’s now hovering between a category two and three storm, it’s making its way up the east coast of the U.S. with a substantial forecast cone, which effectively means: while they can guess, experts can’t say exactly where it’ll go next.

Was climate change a contributing factor to the intensity of Hurricane Dorian?

Maybe, according to Tom Knutson, a research meteorologist at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory. Knutson researches the intersection between tropical cyclones (hurricanes) and climate change, so he would certainly know.

And while Knutson is careful not to make assumptions about what caused something as complicated as a hurricane — which depends on a multitude of factors, from sea surface temperatures to the amount of moisture in the air, to variables in wind shear — he’s quick to point out promising research from colleague Jim Kossin about what the human-caused warming of the planet could be doing to the speed of storms. There’s preliminary evidence that, as the atmosphere warms, it slows down atmospheric circulation, which, in turn, causes storms such as hurricanes to slow down. These stall-outs can cause extensive damage if they happen when the storms make landfall — and that is precisely what happened in the Bahamas. Dorian stayed put over the Bahamas for almost two days.

“Potentially it’s another way in which climate change could lead to greater impacts on people,” Knutson says, “because if storms start slowing down, they can hang around longer, rain longer. As is the case with Dorian, [hurricanes like that] just hang around longer with high winds, and cause more damage that way.”

So wait, does climate change have anything to do with hurricanes?

This is a loaded question with a complicated answer. The simple version is, “yes.” Many experts believe climate change is creating certain conditions that could be conducive to larger, more destructive storms. Still, the evidence to support climate change’s impact on hurricane patterns isn’t as clear as, say, the evidence showing that global warming is largely anthropogenic.

Here’s what we mean.

Human-caused global warming has caused the earth to warm by approximately 0.8 degrees celsius (1.4 degrees Fahrenheit) and the sea surface to warm by approximately 0.55 degrees celsius (1 degree Fahrenheit). We can say this definitively. (It’s also worth noting that, though these numbers may seem low, scientists at NASA stress that a one to two degree drop in temperature was enough to plunge the earth into the Little Ice Age, and a five-degree drop “was enough to bury a large part of North America under a towering mass of ice 20,000 years ago.”)

And right now, scientists strongly believe that the warming of the oceans has created conditions — particularly, warm water and warm air needed to fuel a hurricane’s creation — that have been conducive to developing longer-lasting, more destructive cyclones.

Knutson cautions, however, that there are several factors which prevent researchers from saying that climate change is the cause behind these super-storms. One? Aerosols. Humans have released enough aerosols into the air to have a significant impact on atmospheric conditions. Most aerosols reflect sunlight back toward space, and their cooling effects complicate the science enough for Knutson to hedge about the connections between today’s current hurricane conditions and climate change. There’s just not enough evidence to show that human-caused global warming is intense enough to give hurricanes the temperatures throughout the troposphere that they need to flourish.

“We think that human emissions leads to climate warming,” he says. “And that also causes hurricanes to be at least slightly more intense. But there’s some issues around here about where we are on that point because of aerosols. If you look for long-term trends of intensity, you don’t see a clear signal connecting the two.”

Are we looking at a future with increasingly worse hurricane seasons?

That said, Knutson says one thing we can count on is the effect human-caused climate change will have on hurricanes and their impact in the future.

Since the late 1800s, the planet has warmed a little less than 1-degree celsius (with two-thirds of that warming occurring after 1975). At two degrees celsius warming, the consequences of climate change (when it comes to hurricanes specifically) will be markedly different. Unlike the tangled web of hypotheticals and unknown variables that prevents scientists from saying that climate change is causing the recent spate of super-storms, Knutson says that models show what will very likely happen in the future if the planet continues to warm.

We project that two degrees of warming will likely lead to about a five percent increase of maximum wind speeds of hurricanes — globally and in the Atlantic,” Knutson says. Recent research shows that the earth is very likely to warm more than two degrees celsius by the end of the 21st century. One study shows a median outcome of a 3.2 degree celsius increase by the end of the century.

Again, this may not sound like much, but according to Knutson, “Hurricane damage tends to go up non-linearly with wind speed. Maybe a third power of the wind speed. A five percent increase of intensity implies something like a 15 percent increase in damage for a storm.” While it’s not creating the entire problem, it’s certainly exacerbating it.

The same goes for rainfall. Knutson says research shows that “for every one degree celsius warming in sea surface temperature, you get a seven percent increase in the rainfall rate.”

In other words: global warming will make hurricanes more destructive: more rain, higher wind speeds, slower movement, which means potentially — as we saw with Harvey, which stalled out over Houston and dumped on the city, leading to record-breaking floods — more time to do damage in populated areas. (Oh, also population density along the coasts certainly affects hurricane casualties, injuries, and damage.)

Why should we care?

Knutson says, “Hurricanes can be very damaging events. They can do tremendous damage at the coast. So anything that affects their currents and their intensity should be of interest to people who are in hurricane-prone regions.”

Think of the damage done by Harvey in 2017. Record rainfall, approximately 30,000 people displaced, at least 68 people dead as a direct result of the storm. And Maria, only a month later, did unspeakable damage to Puerto Rico. Two years on, and people are still recovering from the storm, still dealing with power outages, using tarps instead of roofs. One year ago, Florence shattered “at least” 28 flood records when it stalled over the Carolinas and dumped rain on the coast.

But people in coastal regions aren’t the only ones who should care, according to Knutson, because these outcomes are going to hurt future generations no matter where they are. Pragmatically speaking, hurricanes are costly. The Congressional Budget Office reported that hurricane damage costs Americans about $54 billion annually. That number is expected to rise significantly as climate change worsens and so do the natural disasters related to the conditions created by climate change.

But it’s more than just a matter of pragmatism or what’s right in front of us. Knutson continues, “Many of your readers won’t be around when we get to two degrees of global warming. We are passing these problems along to our children and grandchildren. I think they are going to care since they are going to experience what is going to happen. As for the people in today’s world, maybe the effect on them personally won’t be as great as it will on their grandchildren. I’ll let them ponder whether that’s important or not.”