Global warming and the frequency of intense Atlantic hurricanes: model results
Could global warming increase wind shear over the Atlantic, potentially leading to a decrease in the frequency of Atlantic hurricanes? There is a growing consensus among hurricane scientists that this is indeed quite possible. Two recent studies, by Zhao et al. (2009), "Simulations of Global Hurricane Climatology, Interannual Variability, and Response to Global Warming Using a 50-km Resolution GCM", and by Knutson et al. (2008), "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", found that global warming might increase wind shear over the Atlantic by the end of the century, resulting in a decrease in the number of Atlantic hurricanes. For example, the second study took 18 relatively coarse (>60 km grid size) models used to formulate the 2007 IPCC climate report, and "downscaled" them using a higher-resolution (18 km grid size) model called ZETAC that was able to successfully simulate the frequencies of hurricanes over the past 50 years. When the 18 km ZETAC model was driven using the climate conditions we expect in 2100, as output by the 18 IPCC models, the authors found that a reduction of Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century resulted. An important reason that their model predicted a decrease in the frequency of Atlantic hurricanes was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.
Figure 1. Top: predicted change by 2100 in wind shear (in meters per second per degree C of warming--multiply by two to get mph) as predicted by summing the predictions of 18 climate models. Bottom: The number of models that predict the effect shown in the top image. The dots show the locations where tropical storms formed between 1981-2005. The box indicates a region of frequent hurricane formation where wind shear is not predicted to change much. Image credit: Geophysical Research Letters, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", by Vecchi and Soden, 2007.
Since the Knutson et al. study using the 18 km resolution ZETAC model was not detailed enough to look at what might happen to major Category 3 and stronger hurricanes, a new study using a higher resolution model was needed. This was done by a team of modelers led by Dr. Morris Bender of NOAA's GFDL laboratory, who published their results in Science in February. The authors used the GFDL hurricane model--the model that has been our best-performing operation hurricane track forecasting model over the past five years--to perform their study. The GFDL hurricane model runs at a resolution of 9 km, which is detailed enough to make accurate simulations of major hurricanes. The researchers did a double downscaling study, where they first took the forecast atmospheric and oceanic conditions at generated by the coarse (>60 km grid) IPCC models, used these data to initialize the finer resolution 18 km ZETAC model, then used the output from the ZETAC model to initialize the high-resolution GFDL hurricane model. The final results of this "double downscaling" study suggest that although the total number of hurricanes is expected to decrease by the end of the century, we should expect an increase of 81% in the number of Category 4 and 5 storms in the Atlantic. This trend should not be clearly detectable until about 60 years from now, given a scenario in which CO2 doubles by 2100. The authors say that their model predicts that there should already have been a 20% increase in the number of Category 4 and 5 storms since the 1940s, given the approximate 0.5°C warming of the tropical Atlantic during that period. This trend is too small to be detectable, given the high natural variability and the difficulty we've had accurately measuring the exact strength of intense hurricanes before the 1980s.The region of the Atlantic expected to see the greatest increase in Category 4 and 5 storms by the year 2100 is over the Bahama Islands (Figure 2), since wind shear is not expected to increase in this region, and sea surface temperatures and atmospheric instability are expected to increase there.
The net effect of a decrease in total number of hurricanes but an increase in the strongest hurricanes should cause an increase in U.S. hurricane damages of about 30% by the end of the century, the authors compute, assuming that hurricane damages behave as they did during the past century. Over the past century, Category 4 and 5 hurricanes made up only 6% of all U.S. landfalls, but accounted for 48% of all U.S. damage (if normalized to account for increases in U.S. population and wealth, Pielke et al., 2008.)
Figure 2. Expected change in Atlantic Category 4 and 5 hurricane per decade expected by the year 2100, accord to the Science paper by Bender et al. (2010).
Commentary
These results seem reasonable, since the models in question have been successfully been able to simulate the behavior of hurricanes over the past 50 years. However, the uncertainties are high and lot more research needs to be done before we can be confident of the results. Not all of the IPCC models predict an increase in wind shear over the tropical Atlantic by 2100, so the increase in Category 4 and 5 hurricanes could be much greater. Also, the GFDL model was observed to under-predict the strength of intense hurricanes in the current climate, so it may not be creating enough Category 4 and 5 hurricanes in the future climate of 2100. On the other hand, IPCC models such as the UKMO-HadCM3 predict a very large increase in wind shear, leading to a drastic reduction in all hurricanes in the Atlantic by 2100, including Category 4 and 5 storms. So Category 4 and 5 hurricane frequency could easily be much greater or much less than the 81% increase by 2100 found by Bender et al.
The estimates of a 30% increase in hurricane damages by 2100 may be considerably too low, since this estimate assumes that sea level rise will continue at the same pace as was observed in the 20th century. Sea level rise has accelerated since the 1990s, and it is likely that this century we will see much more than than the 7 inches of global sea level rise that was observed last century. Higher sea level rise rates will sharply increase the damages due to storm surge, which account for a large amount of the damage from intense Category 4 and 5 hurricanes.
Keep in mind that while a 30% in hurricane damage by the end of the century is significant, this will not be the main reason hurricane damages will increase this century. Hurricane damages are currently doubling every ten years, according to Pielke et al., 2008. This is primarily due to the increasing population along the coast and increased wealth of the population. The authors theorize that the Great Miami Hurricane of 1926, a Category 4 monster that made a direct hit on Miami Beach, would have caused about $150 billion in damage had it hit in 2005. By 2015, the authors expect the same hurricane would do $300 billion in damage. This number would increase to $600 billion by 2025 (though I think it is likely that the recent recession may delay this damage total a few years into the future.) It is essential that we limit coastal development in vulnerable coastal areas, particularly along barrier islands, to reduce some of the astronomical price tags hurricanes are going to be causing. Adoption and enforcement of strict building standards is also a must.
The authors of the GFDL hurricane model study have put together a nice web page with links to the paper and some detailed non-technical explanations of the paper.
References
Bender et al., 2010, "Modeled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes", Science, 22 January 2010: Vol. 327. no. 5964, pp. 454 - 458 DOI: 10.1126/science.1180568.
Vecchi, G.A., B.J. Soden, A.T. Wittenberg, I.M. Held, A. Leetmaa, and M.J. Harrison, 2006, "Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing", Nature, 441(7089), 73-76.
Vecchi, G.A., and B.J. Soden, 2007, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", Geophysical Research Letters, 34, L08702, doi:10.1029/2006GL028905, 2007.
Jeff Masters
Reader Comments
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oh my god you guys enough with the pens & SSN number.
oz you seem like a very smart guy, would you really try to be so close in a cat5 storm? don't you have family that is going to be upset and scared if you do this? i'm sorry it's really none of my business but i just had to comment.
Strange, i wonder if the csu and the el nino advisory looked at this.
on our local weather they said the forecast for this year is for more hurricanes to impact Florida on the EAST coast. so i guess that lets us off the hook here in the GOM?
The only problem with that is once the storm passes over FL and enters the GOM, Now you got a problem.
I'm amazed that anyone would be forecasting any increased liklihood for a specific area this early...but then again, there are a lot of TV mets that get their forecasts from Dr. Bunson Honeydew
It's coming down on average. It won't hold on much longer with the huge SOI positive burst we have going on right now. Besides, with the falloff that is inevitable, a procrastinating death of this El Nino is a bad thing for the U.S. regarding the hurricane season.
I was at a talk with the late Dick Fletcher from Channel 10 here in Tampa. Some guy said if he was in a surge, he would just swim to safety.
Dick Fletcher told him that's kind of like trying to swim in a running garbage disposal with all the debris in it.
The dude blanched and changed his tune...
Dont you have a Family?
I hope your Heavily insured Oz.
I can guarantee if your near a Cat 5 Surge Zone,,with a Aluminum 80Cu Ft Scuba Tank with BC and a Regulator. Your going to drown.
Period.
And for sure 2000 folks wouldnt be near you..cuz the Local Emg Mgt isnt going to let your ignorance,draw a crowd.
Seems reasonable.
And with what Drakoen posted last night about the subsurface with those blues expanding,this El Nino is toasted.
I thought it was common knowledge that all storms head to Florida?
Most retirees do....
Klotzbach and Gray have the GOM at roughly the same probability as the east coast:
PROBABILITIES FOR AT LEAST ONE MAJOR (CATEGORY 3-4-5) HURRICANE LANDFALL ON EACH OF THE FOLLOWING COASTAL AREAS:
1) Entire U.S. coastline - 69% (average for last century is 52%)
2) U.S. East Coast Including Peninsula Florida - 45% (average for last century is 31%)
3) Gulf Coast from the Florida Panhandle westward to Brownsville - 44% (average for last century is 30%)
I myself put much less weight in landfall forecasts than seasonal forecasts, and seasonal forecasts have little to no demonstrated skill.
Better read all of those Disclaimers on that Life Insurance Declarations Page. It probably would not pay out.
Yup, cold water associated with a cold PDO signature is squeezing down on the warm anomalies from both the north and south in the eastern Pacific. It's worth noting that the PDO index only peaked at +0.83 during this El Nino, which is quite low considering how strong this Nino was. That says a lot about how cold the PDO signature is outside of the equatorial region. That's why this El Nino became so central-pacific based, "El Nino Modoki" as you guys like to call it.
I have one question: what keeps the water bourne building debris from doing you serious bodily harm, my one-man demolition derby friend?
Crazy. Swim? I don't think so.
Plus, I have had the occasion to swim a mile in a small lake. Could do a couple of miles.
Not 10 miles, in rough seas. There are coastal places that wouldn't have dry land for 10 miles, though, unless you want to wait 12 hours or more...in the right surge scenario.
and more specifically to JFV's bathroom....
;)
They do indeed, with the exception of the 1998 El Nino signature much farther to the east in the Pacific. What's scary is then looking at 2005 and comparing the Atlantic and the equatorial Pacific. The similarities are impressive. The only real difference in 2005 was the warm anomalies in the NE Pacific, due to 2005 being embedded within a warm PDO period, but the overall profile in the Pacific and Atlantic is very similar with a dying central Pacific-based El Nino and warm water in the tropical Atlantic with cold water to the north. 1998 exhibits these similarities in the Atlantic as well.
1998:
2005:
Current:
lol
by the way you could always strap yerself to a flagpole and rise up and down with the surge that would look cool on the cam i pay money to see that and a cat 5 too
U the first person to get me to crack a smile on this season's potentialities...
So you do not really intend to document CAT 5 conditions? Generally these conditions are limited to a very confined area around the immediate eyewall. From what your saying, you only intend on being in less than CAT 5 conditions.
I had some family friends who were in Country Walk and featured in Inside Edition after Andrew. They had neighbor's whose houses were being destroyed, and they were flocking to other people's houses to seek refuge. There were 8 people in her car in a small garage for four hours, as her house around her was torn apart.
Who said your going to get shut down?
I thought you wanted to Draw 2000folks at the Coast Ozzy.
I hope the Live Video Feed goes right to the Climax OZ.,as Ill be recording it as well hopefully.
But I stress Hurricane Prep,not "come flock to me in the Cat 5 Surge" on the wunderground.
So we have diametrically opposing views on advice.
Good Luck with your continuing Saga.
Hurricane Preparation 2010
IRLoopCATL
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