Global Warming and Hurricanes

1. Summary Statement

Two frequently asked questions on global warming and hurricanes are the following:

• Have humans already caused a detectable increase in Atlantic hurricane activity or global tropical cyclone activity?
• What changes in hurricane activity are expected for the late 21st century, given the pronounced global warming scenarios from current IPCC models?

In this review, we address these questions in the context of published research findings. We will first present the main conclusions and then follow with some background discussion of the research that leads to these conclusions. The main conclusions are:

• It is premature to conclude that human activities–and particularly greenhouse gas emissions that cause global warming–have already had a detectable impact on Atlantic hurricane or global tropical cyclone activity. That said, human activities may have already caused changes that are not yet detectable due to the small magnitude of the changes or observational limitations, or are not yet confidently modeled (e.g., aerosol effects on regional climate).
• Anthropogenic warming by the end of the 21st century will likely cause tropical cyclones globally to be more intense on average (by 2 to 11% according to model projections for an IPCC A1B scenario). This change would imply an even larger percentage increase in the destructive potential per storm, assuming no reduction in storm size.
• There are better than even odds that anthropogenic warming over the next century will lead to an increase in the occurrence of very intense tropical cyclone in some basins–an increase that would be substantially larger in percentage terms than the 2-11% increase in the average storm intensity. This increase in intense storm occurrence is projected despite a likely decrease (or little change) in the global numbers of all tropical cyclones.
• Anthropogenic warming by the end of the 21st century will likely cause tropical cyclones to have substantially higher rainfall rates than present-day ones, with a model-projected increase of about 10-15% for rainfall rates averaged within about 100 km of the storm center. 

 Klimaforsker Rasmus Benestad: On tropical cyclone frequency and the warm pool area
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As Hurricane Harvey approaches Texas, an old argument has again resurfaced about whether climate change is to blame.
In the past, scientists have had a rather unsatisfying answer for both environmentalists and skeptics, essentially saying that while climate change increases the frequency and severity of extreme weather, no individual event could be attributed to it.
But that may be changing.
Climate scientists now say that they can use models and historical data to evaluate with increased precision how global warming has affected the odds of a given individual weather event.

So far, scientists have not found an extreme weather event that could not have occurred without climate change, but they have found several instances where it multiplied the odds of one that occurred.
As extreme weather events increase in severity and frequency in the coming years, scientists say the odds are increasing that there eventually will be a storm that they can unequivocally say would not have occurred without climate change.

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Michael E. Mann on his Facebook page:
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 is due to coastal subsidence due to human disturbance e.g. oil drilling) is more than half a foot over the past few decades (see http://www.insurancejournal.com/…/sou…/2017/05/31/452704.htm for a decent discussion).

That means that the storm surge was a half 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 (30.5-31 C or 87-88F). There is a simple thermodynamic relationship known as the "Clausius-Clapeyron equation (see e.g. https://en.wikipedia.org/…/Clausius%E2%80%93Clapeyron_relat…) that tells us there is a roughly 3% increase in average atmospheric moisture content for each 0.5C (~1F) 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 the 'average' temperatures a few decades ago. That means 3-5% more moisture in the atmosphere.

That large amount of moisture meant 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 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 warming not just the surface but creating deeper layers of warm water in the Gulf and elsewhere.

So 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 Superstorm #Sandy more likely (http://www.pnas.org/content/112/41/12610.full).

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 right near the coast, continuing to pummel Houston and surrounding regions with a seemingly endless deluge which will likely top out at nearly 4 feet of rainfall over a several 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 like a top with no direction. This pattern, in turn, is associated with a greatly expanded subtropical high pressure system over much of the U.S. right now, 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 favored by human-caused climate change. We recently published on this phenomenon:

https://www.nature.com/articles/srep45242


In conclusion, while we cannot say climate change "caused" hurricane Harvey (that is an ill-posed question), we can say that it exacerbate 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.


Christopher W. Landsea fra NOAA forklarer hvordan global oppvarming fører til færre men kraftigere ekstremvær og kommer med en god analyse av skadeomfanget til disse ekstremværene de siste 100 år:

"Any manmade alterations to the air's moisture, thunderstorm activity, vertical shear, and originating disturbances may be as or even more important that changes to the ocean temperatures themselves. All climate models predict that for every degree of warming at the ocean that the air temperature aloft will warm around twice as much7. This is important because if global warming only affected the earth's surface, then there would be much more energy available for hurricanes to tap into. But, instead, warming the upper atmosphere more than the surface along with some additional moisture near the ocean means that the energy available for hurricanes to access increases by just a slight amount. Moreover, the vertical wind shear is also supposed to increase, making it more difficult (not easier) for hurricanes to form and intensify"
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"Hurricane Irma was born far out in the Atlantic Ocean, where it is usually not hot enough for a hurricane of this strength to occur. We also see that Irma built up in strength very quickly, says Bjørn Hallvard Samset, climate researcher at the Center for International Climate Research (Oslo CICERO), adding:

"90 percent of the energy from global warming goes right into the ocean, and the ocean saves this energy. We have put on a solid kettle that drives and warms up our oceans. The hot sea temperatures also cause the hurricanes to increase as they approach land, contrary to what is normal.

Kevin Trenberth at the National Center for Atmospheric ResearchHe is a leading expert on precipitation change, and believes the strong storms are due to global warming.

"With man-made global warming, heat is getting more and it has to go somewhere. Most people go to warm up the seas. One way the climate system can relieve pressure is through hurricanes that provide a kind of push for the heat out of the ocean, and into the atmosphere, he writes to VG in an email.

He explains that this happens through increased evaporation of moisture out of the ocean, which cools down the ocean and gives a more humid atmosphere and more heavy rainfall.

- At the same time, the heat is released into the atmosphere. This is what drives the storm, he says and adds:

"It can lead to more storms, more intense storms, bigger storms and storms that last longer. We expect a large storm to cool down the ocean as much as four or more minor storms.

https://www.theatlantic.com/science/archive/2017/08/did-climate-change-intensify-hurricane-harvey/538158/

http://www.vg.no/nyheter/utenriks/vaer-og-uvaer/klimaforsker-om-monsterorkanene-vi-har-slaatt-paa-en-gedigen-vannkoker/a/24135613/

Forskerne har forutsett at ekstremværene blir mer intense;

https://goo.gl/xkNbse

https://goo.gl/4hVcbG