Global warming and the frequency of intense Atlantic hurricanes: model results

By: Dr. Jeff Masters , 1:37 PM GMT on April 05, 2010

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 hurricanes per decade expected by the year 2100, according 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

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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1355. Floodman
 Okay kids...I'm out! Play nice and save some trolls for me!
1354. KEEPEROFTHEGATE (Mod)
 Quoting CycloneOz:I can imagine a scenario this year where I fly into Raleigh, NC...rent a car, and drive straight to the outer banks and then its a race to the north and Virginia Beach before the hurricane spanks me.remember friend a hurricane can strip the land of everything including all life so that nothing remains but the dirt and water
1352. hydrus
 Quoting Floodman:You have to shave that fly-away hair off them though...you can't amke their little heads to stay in the settings otherwise If the customer demands the fly away hair look ya just stringem like poils.
Member Since: December 31, 1969 Posts: Comments:
1351. RitaEvac
 Really need this squall line to develop SW so I can get some rain. Severe storm over Huntsville, line trying to build SW, needs to get going, need at least 1/2 inch of rain. Getting dry and pollen is bad
1350. hydrus
Member Since: December 31, 1969 Posts: Comments:
1349. Floodman
 Quoting hydrus:We save the heads and make jewelery with them. It can be a lucrative business for considerable financial gain.You have to shave that fly-away hair off them though...you can't amke their little heads to stay in the settings otherwise
1348. RitaEvac
 Quoting Floodman:RitaEvac, you're not a troll, you're a curmudgeon. There is a clear and definite separation between trolls and curmudgeons: curmudgeons say things because they don't care what others think, trolls say things because they don't thinkI find you to be an intelligent person whose posts are fairly well thought out, and therein lies the difference...LOLSweet! thats what makes me unique
1347. Floodman
 Quoting RitaEvac:Why you pinching my head of FloodmanRitaEvac, you're not a troll, you're a curmudgeon. There is a clear and definite separation between trolls and curmudgeons: curmudgeons say things because they don't care what others think, trolls say things because they don't thinkI find you to be an intelligent person whose posts are fairly well thought out, and therein lies the difference...LOL
1346. hydrus
 Quoting Floodman:The warming of the El Nino is a momentary variance...we're going to see neutral conditions soon enough and likely a mild La Nina before the end of storm season. Hang on to your hats, kids, this is going to be a very, ummm, interesting storm season...I guess probably 17-8-6? I expected to see some high numbers for this season. I am amazed at how high.
Member Since: December 31, 1969 Posts: Comments:
1345. NRAamy
 yes, I read psot #1322...what does that have to do with Homer?you sayin' you gonna pinch off my head?!;)
1344. FloridaTigers
 I love the lack of trolls on the blog at this time.
1342. Floodman
 The warming of the El Nino is a momentary variance...we're going to see neutral conditions soon enough and likely a mild La Nina before the end of storm season. Hang on to your hats, kids, this is going to be a very, ummm, interesting storm season...I guess probably 17-8-6?
1341. hydrus
 Quoting Floodman:hydrus, I have laways had the utmost of respect and admiration for you, and now I find that you enjoy my favorite sport: pinching the tiny little pointed heads off trolls...I say tally ho! We save the heads and make jewelery with them. It can be a lucrative business for considerable financial gain.
Member Since: December 31, 1969 Posts: Comments:
1340. RitaEvac
 Why you pinching my head of Floodman
1339. winter123
 Still looks like some snow for the great lakes including western PA and west-central NY on friday morning to saturday night. Doubt it will accumulate, but still unexpected as it hit 84 here twice last week.
Member Since: December 31, 1969 Posts: Comments:
1338. Floodman
 Amy, you throw Prilosec at trolls? That's no way to treat an effective ant-acid!
1337. Floodman
 Quoting hydrus:Told ya you had da class Floodman.Thank you; I was raised to be courteous to a fault...LOL
1336. hydrus
 Quoting NRAamy:They do it in Star Trek, so its fine by me to call women "sir"Homer Simpson sleeps at his desk at work...so, it's fine by me!;) Did you read post # 1322.?
Member Since: December 31, 1969 Posts: Comments:
1335. Floodman
 Quoting hydrus:We will handle trolls accordingly, as a team. Trolls are idiots in a brain less void. It is fun to pinch their heads off. hehehydrus, I have laways had the utmost of respect and admiration for you, and now I find that you enjoy my favorite sport: pinching the tiny little pointed heads off trolls...I say tally ho!
1334. hydrus
 Quoting Floodman:I am old; my memory is not to be trusted (unless it has to do with history, then I am spot on)...I apologize wholeheartedly for my mistake **bows low, removes hat with a flourish** Told ya you had da class Floodman.
Member Since: December 31, 1969 Posts: Comments:
1333. NRAamy
 They do it in Star Trek, so its fine by me to call women "sir"Homer Simpson sleeps at his desk at work...so, it's fine by me!;)
1332. RufusBaker
 Above average year this year! there may be something forming below cuba now!!! the two tone talk lives on!!!!!
1331. hydrus
 Quoting Bordonaro:Looks like TDude g=has missed out on the worst of the Severe T Storms, so far. Hydrus, here's a look at your Severe Weather Threat: We may luck out if the storms weaken a bit, but I never trust a squall line moving through at night. It is rare, but I have seen them strengthen in the overnight hours just like they do with the daytime heating. Then we are not ready and get our buts handed to us. I,ll be up.
Member Since: December 31, 1969 Posts: Comments:
1330. Floodman
 Quoting twhcracker:i am a female, which is really hard for me to understand your confusion since we have actually talked on the phone during the haiti portlite thing :)I am old; my memory is not to be trusted (unless it has to do with history, then I am spot on)...I apologize wholeheartedly for my mistake **bows low, removes hat with a flourish**
1329. FloridaTigers
 Quoting hydrus: Its 2010, I thought it can be a sign of respect to call someone sir even if it is a female. Am I wrong?They do it in Star Trek, so its fine by me to call women "sir"
1328. hydrus
 Quoting twhcracker:i am a female, which is really hard for me to understand your confusion since we have actually talked on the phone during the haiti portlite thing :) Its 2010, I thought it can be a sign of respect to call someone sir even if it is a female. Am I wrong?
Member Since: December 31, 1969 Posts: Comments:
1327. CybrTeddy
 Quoting Stormchaser2007:Nino still hanging on. Even warmed a bit over the past few days. Now near 1CCurious.. CSU thinks its going to dissipate. I think its going to hang on a bit then start to drop by next week or the week after words.
1326. Bordonaro
 Looks like TDude has missed out on the worst of the Severe T Storms, so far. Hydrus, here's a look at your Severe Weather Threat:
1325. twhcracker
 Quoting Floodman:I lived in New Orleans, sir...before, during and after that little storm we had there a few years ago...what was it called...oh yeah, Katrina!i am a female, which is really hard for me to understand your confusion since we have actually talked on the phone during the haiti portlite thing :)
1324. indianrivguy
 Quoting CycloneUK:That eyjafjallajokull webcam really is great. I see people http://eldgos.mila.is/eyjafjallajokull-fra-fimmvorduhalsi/Yea, it a lot more wound up now as opposed to 3pm est. I tried to find an aerial that would give me perspective on the distance away those folks standing on the little peak with no luck. They opened up the view earlier when the rift got active to the left bit now in darkness I guess they figure only the splashy stuff is of interest. You can see the clouds moving from left to right indicating activity out of our view.
1323. Stormchaser2007
 Nino still hanging on. Even warmed a bit over the past few days. Now near 1C
1322. hydrus
 Quoting NRAamy:I feel sorta left out of the fray because I have not made a prediction as to how many storms we will have.I have a prediction...as to how many trolls we will have....more than my Prilosec can handle...;) We will handle trolls accordingly, as a team. Trolls are idiots in a brain less void. It is fun to pinch their heads off. hehe
Member Since: December 31, 1969 Posts: Comments:
1321. KEEPEROFTHEGATE (Mod)
 WOCN11 CWTO 071907Special weather statementIssued by Environment Canada Ontario region.Wednesday 7 April 2010.Special weather statement issued for..City of TorontoLondon - MiddlesexOxford - BrantCity of HamiltonHalton - PeelYork - DurhamHuron - PerthWaterloo - WellingtonDufferin - InnisfilBarrie - Orillia - MidlandPeterborough - Kawartha LakesBancroft - Bon Echo ParkCity of OttawaGatineauPrescott and RussellSmiths Falls - Lanark - Sharbot LakeParry Sound - MuskokaHaliburtonRenfrew - Pembroke - Barry's BayAlgonquinBurk's Falls - Bayfield Inlet... Rain heavy at times tonight and Thursday morning. Possiblefreezing rain between Northern Georgian Bay and the Québec Border...A low pressure system over Illinois this afternoon will move towards Southern Ontario tonight. Rain ahead of this system will becomeHeavy tonight and continue Thursday morning.Present indications are that total rainfall amounts will reach 30 to 40 mm with local 50 possible as rain could be accompanied by thunderstorms.In addition fog is likely to develop overnight in many areas asWell.Further north over Algonquin Burk's Falls - Bayfield InletShort period ofFreezing rain is possible towards Thursday morning as strong northeasterly winds bring cold air.Listen for further statements. Additional informationmay also be found by consulting the latest public forecast.The next public forecast will be issued by 5.30 AM.END/OSPC
1320. WaterWitch11
 Quoting twhcracker:thats like that Ms. Evans teacher story. no one knew she had a brain because you couldn't see it.god you people from la are driving me crazy. I WANT TO GO TO LOUISIANA! I'M TRYING FOR MAY-WILL SEEmoral of mrs. evans story:just because you can't see it or explain it doesn't mean that it doesn't exist!
 Quoting hydrus: Thank you for taking the time to post it T-Dude. The brunette is hot.:)agreed ;)
1318. Stormchaser2007
 One can not help but notice how quickly the Gulf has warmed over the past week. Should be near normal or slightly above normal by earl-mid May.
1317. NRAamy
 I feel sorta left out of the fray because I have not made a prediction as to how many storms we will have.I have a prediction...as to how many trolls we will have....more than my Prilosec can handle...;)
1316. hydrus
 Quoting tornadodude:here is that commercial: linkthanks to Greyelf for finding it for me! Thank you for taking the time to post it T-Dude. The brunette is hot.:)
Member Since: December 31, 1969 Posts: Comments:
1315. hydrus
 Quoting Floodman:Drak, you saying that makes it sound like church bells on a foggy morning; ominous... Speaking of ominous, the computer models are looking ominous for Haiti. Also NOAA,s Puerto Rico area forecast discussion mentions the possibility of flash floods in the region.
Member Since: December 31, 1969 Posts: Comments:
 Quoting hydrus: Great. We are going to get whacked golf ball size hail whizzin along at 80 mph. That,s how I would have all my days.....not.here is that commercial: linkthanks to Greyelf for finding it for me!
1313. hydrus
 Quoting Floodman:I lived in New Orleans, sir...before, during and after that little storm we had there a few years ago...what was it called...oh yeah, Katrina! I feel sorta left out of the fray because I have not made a prediction as to how many storms we will have. This year I am going to make May-15 the day I post my estimate for the number of named storms and hurricanes that form and where they might go. Last year I forecasted 13 named storms. Not too good. I saw a couple of systems that could have been named and never were. I saw some systems that were named and looked awfully ragged. LOL
Member Since: December 31, 1969 Posts: Comments:
1312. Floodman
 Quoting Drakoen:Perhaps a total like 1995 with 19 named storms 11 hurricanes and 5 majors is not out of the question.Drak, you saying that makes it sound like church bells on a foggy morning; ominous...
1311. Floodman
 Quoting Patrap:As a rule I usually dont comment nor run with any of the Forecast numbers,as they cant tell us Where,nor when,.But I feel that CSU and da Boyz held back their Gut feelings a tad, and went with the "adjusted down" Public release.So..I focus on Prep and Education...the NWS has a Lot of new Products warning and watch times, and will be noting them as we close on May 15thYou got that right...I love the questions like "How many hurricanes in south Florida this year?"
1310. Floodman
 Quoting NRAamy:1292. Floodman 1:13 PM PDT on April 07, 2010 Quoting hydrus:Kind of a nasty way to get the bad news...lolt'Bob covered in roaches coming from Jerry Garcia, I'm not sure how to take that....;)Hiya, toots! How's t'ings?
1309. Drakoen
 2010:2005:
1308. Drakoen
 Quoting BahaHurican: Agree with all this. I wouldn't be surprised to see 19 named storms this year (though I'm not "forecasting" any numbers)and I don't think I'm the only one thinking this way....Perhaps a total like 1995 with 19 named storms 11 hurricanes and 5 majors is not out of the question.
1307. BDAwx
 the ocean temps in BDA jumped 3.4F between the 5th and 6th :O(65.3 - 68.7)
1306. BahaHurican
 Quoting Patrap:As a rule I usually dont comment nor run with any of the Forecast numbers,as they cant tell us Where,nor when,.But I feel that CSU and da Boyz held back their Gut feelings a tad, and went with the "adjusted down" Public release.So..I focus on Prep and Education...the NWS has a Lot of new Products warning and watch times, and will be noting them as we close on May 15th Agree with all this. I wouldn't be surprised to see 19 named storms this year (though I'm not "forecasting" any numbers)and I don't think I'm the only one thinking this way....
1305. NRAamy
 1292. Floodman 1:13 PM PDT on April 07, 2010 Quoting hydrus:Kind of a nasty way to get the bad news...lolt'Bob covered in roaches coming from Jerry Garcia, I'm not sure how to take that....;)

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