THE CLIMATE CHANGE FACTOR IN HURRICANE HARVEY

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27 august 2017


Did Climate Change Intensify Hurricane Harvey?


“The human contribution can be up to 30 percent or so up to the total rainfall coming out of the storm.”


Robinson Meyer, The Atlantic, August 27, 2017

Every so often, the worst-case scenario comes to pass.

As of Sunday afternoon, the remnants of Hurricane Harvey seem likely to exceed the worst forecasts that preceded the storm. The entire Houston metropolitan region is flooding: Interstates are under feet of water, local authorities have asked boat owners to join rescue efforts, and most of the streams and rivers near the city are in flood stage.

Some models suggest that the storm will linger over the area until Wednesday night, dumping 50 inches of water in total on Houston and the surrounding area.

“Local rainfall amounts of 50 inches would exceed any previous Texas rainfall record. The breadth and intensity of this rainfall are beyond anything experienced before,” said a statement from the National Weather Service. “Catastrophic flooding is now underway and expected to continue for several days. ” (In years of weather reporting, I have never seen a statement this blunt and ominous.)

This means that thousands of people—and perhaps tens of thousands of people—are facing a terrifying and all-too-real struggle to survive right now. In an age when the climate is changing rapidly, a natural question to ask is: What role did human-caused global warming play in strengthening this storm?

Climate scientists, who specialize in thinking about the Earth system as a whole, are often reticent to link any one weather event to global climate change. But they say that aspects of the case of Hurricane Harvey—and the recent history of tropical cyclones worldwide—suggest global warming is making a bad situation worse.

It may not be obvious why global warming has anything to do with hurricane strength. Climate change is caused by the release of greenhouse gases like carbon dioxide and methane into the atmosphere. These gases prevent some of the sun’s rays from bouncing back into space, trapping heat in the planetary system and raising air temperatures all over the world.

This warmer air causes evaporation to happen faster, which can lead to more moisture in the atmosphere. But that phenomenon alone does not explain climate change’s effects on Harvey.

Storms like Harvey are helped by one of the consequences of climate change: As the air warms, some of that heat is absorbed by the ocean, which in turn raises the temperature of the sea’s upper layers.

Harvey benefitted from unusually toasty waters in the Gulf of Mexico. As the storm roared toward Houston last week, sea-surface waters near Texas rose to between 2.7 and 7.2 degrees Fahrenheit above average. These waters were some of the hottest spots of ocean surface in the world. The tropical storm, feeding off this unusual warmth, was able to progress from a tropical depression to a category-four hurricane in roughly 48 hours.

“This is the main fuel for the storm,” says Kevin Trenberth, a senior scientist at the U.S. National Center for Atmospheric Research. “Although these storms occur naturally, the storm is apt to be more intense, maybe a bit bigger, longer-lasting, and with much heavier rainfalls [because of that ocean heat].”

This also suggests an explanation for one of Harvey’s strangest and scariest behaviors. The storm intensified up until the moment of landfall, achieving category-four strength hours before it slammed into the Texas coast. This is not only rare for tropical cyclones in the western Gulf of Mexico: It may be unique. In the past 30 years of records, no storms west of Florida have intensified in the last 12 hours before landfall.

Why do storms normally weaken—and why didn’t Harvey? As mentioned above, hurricanes feed and grow on warm ocean surface waters. But as they grow, their strong winds often pick up seawater, churning the oceans and moving the warmest waters deep below the surface. The same winds also bring newer, colder water closer to the atmosphere, which usually serves to drain energy and weaken the storm.

That didn’t happen with Harvey. The hurricane churned up water 100 or even 200 meters below the surface, said Trenberth, but this water was still warm—meaning that the storm could keep growing and strengthening. “Harvey was not in a good position to intensify the way it did, because it was so close to land. It’s amazing it was able to do that,” he told me.

All of this said, a storm like Harvey could have happened even if there was no climate change. Planning experts have long fretted over the possibility of a major hurricane striking Houston. Harvey is also a powerful hurricane forming in one of the most hurricane-friendly regions of the world at the peak of hurricane season. Storms similar to it would form in any climate.

But Trenberth says that the extra heat could make the storm more costly and more powerful, overpowering and eventually breaking local drainage systems.

“The human contribution can be up to 30 percent or so up to the total rainfall coming out of the storm,” he said. “It may have been a strong storm, and it may have caused a lot of problems anyway—but [human-caused climate change] amplifies the damage considerably.”

More generally, it’s still unclear what effect climate change is having on hurricane formation across the greater Atlantic Ocean. A draft version of a major U.S. government review of climate science due out later this year says there is “medium confidence” that human activities “have contributed to the observed upward trend in North Atlantic hurricane activity since the 1970s.”

Houston has been ground zero for super-damaging storms lately. It has seen four 100-year flooding events since the spring of 2015, according to the meteorologist Eric Holthaus. The city also sees more than 156 percent more heavy downpours than it did in the 1950s. Meanwhile, only one-sixth of its residents have federal flood insurance, though that program has struggled to adjust to the increased flooding risk associated with climate change.

Yet even compared to recent storms, Harvey is unprecedented—just the kind of weird weather that scientists expect to see more of as the planet warms. Harvey has already dumped more water on Harris County than Tropical Storm Allison, the area’s previous worst-ever flooding disaster in 2001, though it has only lasted half the time of that earlier storm.

And it will keep raining. As of Sunday afternoon, Buffalo Bayou, a major river near downtown Houston, is one foot above flood stage. It is projected to rise as much as another 12 feet today alone.

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It's a fact: climate change made Hurricane Harvey more deadly


Michael E Mann, The Guardian, August 28, 2017

We can’t say that Hurricane Harvey was caused by climate change. But it was certainly worsened by it

What can we say about the role of climate change in the unprecedented disaster that is unfolding in Houston with Hurricane Harvey? There are certain climate change-related factors that we can, with great confidence, say worsened the flooding.

Sea level rise attributable to climate change – some of which is due to coastal subsidence caused by human disturbance such as oil drilling – is more than half a foot (15cm) over the past few decades (see here for a decent discussion). That means the storm surge was half a foot higher than it would have been just decades ago, meaning far more flooding and destruction.

In addition to that, sea surface temperatures in the region have risen about 0.5C (close to 1F) over the past few decades from roughly 30C (86F) to 30.5C (87F), which contributed to the very warm sea surface temperatures (30.5-31C, or 87-88F).

There is a simple thermodynamic relationship known as the Clausius-Clapeyron equation that tells us there is a roughly 3% increase in average atmospheric moisture content for each 0.5C of warming. Sea surface temperatures in the area where Harvey intensified were 0.5-1C warmer than current-day average temperatures, which translates to 1-1.5C warmer than “average” temperatures a few decades ago. That means 3-5% more moisture in the atmosphere.

That large amount of moisture creates the potential for much greater rainfalls and greater flooding. The combination of coastal flooding and heavy rainfall is responsible for the devastating flooding that Houston is experiencing.

Not only are the surface waters of the Gulf of Mexico unusually warm right now, but there is a deep layer of warm water that Harvey was able to feed upon when it intensified at near record pace as it neared the coast. Human-caused warming is penetrating down into the ocean. It’s creating deeper layers of warm water in the Gulf and elsewhere.

Harvey was almost certainly more intense than it would have been in the absence of human-caused warming, which means stronger winds, more wind damage and a larger storm surge. (As an example of how this works, we have shown that climate change has led to a dramatic increase in storm surge risk in New York City, making devastating events like Hurricane Sandy more likely.)

Finally, the more tenuous but potentially relevant climate factors: part of what has made Harvey such a devastating storm is the way it has stalled near the coast. It continues to pummel Houston and surrounding regions with a seemingly endless deluge, which will likely top out at nearly 4ft (1.22m) of rainfall over a days-long period before it is done.

The stalling is due to very weak prevailing winds, which are failing to steer the storm off to sea, allowing it to spin around and wobble back and forth. This pattern, in turn, is associated with a greatly expanded subtropical high pressure system over much of the US at the moment, with the jet stream pushed well to the north. This pattern of subtropical expansion is predicted in model simulations of human-caused climate change.

More tenuous, but possibly relevant still, is the fact that very persistent, nearly “stationary” summer weather patterns of this sort, where weather anomalies (both high-pressure dry hot regions and low-pressure stormy/rainy regions) stay locked in place for many days at a time, appears to be favoured by human-caused climate change. We recently published a paper in the academic journal Nature on this phenomenon.

In conclusion, while we cannot say climate change “caused” Hurricane Harvey (that is an ill-posed question), we can say is that it exacerbated several characteristics of the storm in a way that greatly increased the risk of damage and loss of life. Climate change worsened the impact of Hurricane Harvey.

Michael E Mann is distinguished professor of atmospheric science at Pennsylvania State University, director of the Penn State Earth System Science Center and author of three books, including The Hockey Stick and The Climate Wars, Dire Predictions, and The Madhouse Effect.


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