Hurricane Harvey, What impact of climate change ?

In full political news on global climate change, the terrible Hurricane Harvey questioned some scientists about the link between warming due to human activities and intense weather events increasing in recent years.

Source : National Weather Service

Michael E. Mann, 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, develops a long commentary in theguardian.com (¹) about it, freely translated by myself.

What can we say about the role of climate change in the unprecedented disaster unfolding in Houston with Hurricane Harvey ? Il existe certains facteurs liés au changement climatique que nous pouvons, with a 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 – est d’un demi-pied (15 cm wide) au cours des dernières décennies. That means the storm surge was half a foot higher than it would have been just decades ago, ce qui signifie beaucoup plus d’inondations et de destruction.

En plus de cela, sea surface temperatures in the region have risen about 0.5°C over the past few decades from roughly 30°C at 30.5°C, which contributed to the very warm sea surface temperatures (30,5-31°C).

There is a simple thermodynamic relationship known as the Clausius-Clapeyron equation that tells us there is a roughly 3% de la teneur moyenne en humidité atmosphérique pour chaque 0,5°C de réchauffement. Sea surface temperatures in the area where Harvey intensified were 0.5-1°C warmer than current-day average temperatures, which translates into "average" temperatures of 1° to 5° warmer than a few decades earlier. Cela signifie 3 down to 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, ce qui signifie des vents plus forts, more wind damage and a larger storm surge. (À titre d’exemple de la façon dont cela fonctionne, nous avons montré que le changement climatique a entraîné une augmentation spectaculaire du risque de marées de tempête à New York, 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 1.20 m. 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, presque « stationnaires » de ce type, où les anomalies météorologiques (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. Nous avons récemment publié a paper dans la revue académique Scientific Reports sur ce phénomène.

In conclusion, alors que nous ne pouvons pas affirmer que le changement climatique a « causé » le cyclone 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.

Screenshot Weather4D 2.0 by Huig @doubleonine

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(¹) Source : The Guardian
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