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

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1505. Patrap
 O,..then "nevermind"LOL
Member Since: July 3, 2005 Posts: 437 Comments: 134747
1504. Levi32
 Directly from satellite measurements:April 1st TMI 3-day composite:April 8th (today) TMI 3-day composite:
Member Since: November 24, 2005 Posts: 635 Comments: 26775
1503. hydrus
 Quoting Patrap:Dunno about no candle,but in the entry above,..Jeff says hes on Vacation in Mammoth Cave.But thats cuz we read da entry usually.Jeffradmas is Jeffs9641. I think. Top of da mornin to ya Patrap.Did you make a prediction for the hurricane season. Lemme try and guess....30/28/9!
Member Since: September 27, 2007 Posts: 1 Comments: 25094
1502. Bordonaro
 Quoting twhcracker:he is probably laying flat out on his back with a note pinned to his chest that reads the date and time he will be discovered there with bat guana peppered on his foreheadThey're actually been picking on "Jeff9461", not Jeff Masters, our "blog chief". OK, bring on the "mystery blog" please.
1501. indianrivguy
 Quoting Levi32:March 31st:April 5th:5-day SST Change:That bottom map is telling, thanks Levi.
Member Since: September 23, 2006 Posts: 1 Comments: 2828
1500. twhcracker
 Quoting Patrap:Dunno about no candle,but in the entry above,..Jeff says hes on Vacation in Mammoth Cave.But thats cuz we read da entry usually.he is probably laying flat out on his back with a note pinned to his chest that reads the date and time he will be discovered there with bat guana peppered on his forehead
1499. Bordonaro
 Good day everyone. Alright, where is the "mystery, guest blog guest" hiding at??Looks like a quieter day weather-wise. The Severe threat for today, per the SPC will come at the peak of daytime heating, from NY-FL, and be primarily a high wind threat. Then things should calm down for a few days, in the severe weather department.Here in DFW, TX, the "rain machine" has temporarily been "shut down". It has been about 10 days since the last general widespread rain over N TX. And NWS Ft Worth, TX said our next chance of rain is for next Tu-We and the chances are the majority of the rain will pass north of N TX. So much for our El Nino, it is fizzling quickly!
1498. Levi32
 March 31st:April 5th:5-day SST Change:
Member Since: November 24, 2005 Posts: 635 Comments: 26775
1497. Patrap
 Dunno about no candle,but in the entry above,..Jeff says hes on Vacation in Mammoth Cave.But thats cuz we read da entry usually.
Member Since: July 3, 2005 Posts: 437 Comments: 134747
1496. TampaSpin
 Quoting AussieStorm:Biggest i have seen is 5000.I have seen it slighty over 5000 before too i think.
1495. hydrus
 Quoting twhcracker:where is jeffstradamas when we need him. people are panicking and we need his soothing balm. He is looking into the red candle, meditating, awaiting his next prognosticate vision for todays weather and for this years hurricane season
Member Since: September 27, 2007 Posts: 1 Comments: 25094
1494. Levi32
 Quoting NttyGrtty:Just out of curiosity, are we anywhere close to the longest blog? I've seen 'em with large numbers but is 1500ish high, very high, or highest? Anyone know?Nowhere close lol. We've hit 3500 before.
Member Since: November 24, 2005 Posts: 635 Comments: 26775
1493. Floodman
 Quoting NttyGrtty:Just out of curiosity, are we anywhere close to the longest blog? I've seen 'em with large numbers but is 1500ish high, very high, or highest? Anyone know?I think we've gotten over 3000
1492. Patrap
 Its April 8th..not May 15th Taz.Be a lil patient,we will get there my Man.
Member Since: July 3, 2005 Posts: 437 Comments: 134747
1491. TheOtherWA
 Some minor hail this morning here. Just little bitty stuff, but enough to cover most of the ground.
1490. AussieStorm
 Quoting Tazmanian:is this the day we say Anna?Where?
1489. AussieStorm
 Quoting NttyGrtty:Just out of curiosity, are we anywhere close to the longest blog? I've seen 'em with large numbers but is 1500ish high, very high, or highest? Anyone know?Biggest i have seen is 5000.
1488. AussieStorm
 Good Morning all. i see we are still waiting for the guest blogger to post there blog. Maybe it got sent sea mail.
1487. NttyGrtty
 Just out of curiosity, are we anywhere close to the longest blog? I've seen 'em with large numbers but is 1500ish high, very high, or highest? Anyone know?
Member Since: February 11, 2010 Posts: 1 Comments: 924
1486. Tazmanian
 is this the day we say Anna?
1485. indianrivguy
 1463, thanks BiffWell done and congarats Seen your Cheef
Member Since: September 23, 2006 Posts: 1 Comments: 2828
1484. CycloneUK
 Gulf warmage!
1483. aquak9
 TWH- you betcha! As soon as something is north of Key West, ya oughtta go ahead and stock up.Aw heck, make that west of Miami.
Member Since: August 13, 2005 Posts: 192 Comments: 27857
1482. twhcracker
 Quoting CyclonicVoyage:No doom and gloom Aqua, it's real facts.where is jeffstradamas when we need him. people are panicking and we need his soothing balm.
1481. aquak9
 TWH- we have on here in Jacksonville, too. It's Tim "HeadForTheHillsWe'reAllGonnaDie" Deegan. Of course he brings in the ratings...seems that's all weather is anymore on the mass media.
Member Since: August 13, 2005 Posts: 192 Comments: 27857
1480. twhcracker
 and omg, all those people at tyndall afb from other places. they get so scared that if a tropical storm is coming they buy out every battery, loaf of bread and jug of water in a hundred mile radius, clog up all the evacuation routes for a solid week so nobody else can get out, board up their windows for a 50 mph tropical blow... but i reckon by nature all hurricane prone areas are touristy prone and subject to nutty people.
1479. twhcracker
 we used to have this weatherman on channel 13 in panama city. He was from up north in iowa or somewhere, and he would get so freaked out by a hurricane. his name was bob peterman and we used to call him Bob "we are all gonna die" Peterman haha. i just wish he was still around for this season :P)
1477. Skyepony (Mod)
 Report: At least 200 buried in Rio mudslideRIO DE JANEIRO (AP) - A top Rio de Janeiro state security official says at least 200 people have been buried in the latest landslide to hit the area.Civil Defense subsecretary Pedro Machado tells Globo television that the slide plowed into a slum in the city of Niteroi.Machado said Thursday that anyone there at the time was likely killed instantly.Record rains since Monday have triggered mudslides across the Rio region.The official death toll currently stands at 153 but is expected to rise sharply after Wednesday night's slide in Niteroi.
1476. presslord
 tank you berry much...
1474. WxLogic
 Quoting StormW:Our Annual AMS Banquet went well last night. Dr. Jamie Rhome, Storm Surge Specialist from NHC was our guest speaker...great info!I was elected as webmaster for another term. Busy times ahead!Good morning...Congratulations... I hope... hehe
1473. Chicklit
 Another beautiful morning in ECFl...pretty turbulent elsewhere!Loop
1472. presslord
 OK gang...I need some help...the Schooner Halie and Mathew is underway on the Food Voyage to Haiti...you can follow their progress at this link....I need some weather input...especially windage...for their present location near Grand Bahama....Anybody here care to take a WAG?Link
1470. CyclonicVoyage
 No doom and gloom Aqua, it's real facts.
1469. CyclonicVoyage
 Maybe that thin layer of dry air will be present at the mid levels again?Not likely as El Nino usually leaves a pretty moist environment in it's wake.
1468. aquak9
 Quoting CyclonicVoyage:Not much, if anything, on the nay side for an active season. Not much in the long term indicators for lack of landfalls either.AAAUURRGGHHH!!! can we lay offa the DoomAndGloom for a little while? Please?
Member Since: August 13, 2005 Posts: 192 Comments: 27857
1467. CyclonicVoyage
 Quoting CycloneOz:What a difference a day makes. El Nino is going away quickly now and the setup is like 2005.Uh...2005? Pretty scary stuff!Not much, if anything, on the nay side for an active season. Not much in the long term indicators for lack of landfalls either.
1466. Orcasystems
 Still not much going on... but at least its going to rain in Macon GA (like thats a surprise?)AOIAOIAOIAOI
1463. biff4ugo
 Good Morning,Thanks Aqua. 1024, Indianrivguy. Great Post. I look for environmental cues before a storm too.I didn't see a post on cloud cover, but came across an ISSCP study that looks pretty good.http://isccp.giss.nasa.gov/projects/flux.htmlCheers, Looks like I may be seeing some of you in the EOC this year if the activity level comes true.
1461. weathermanwannabe
 1457. RTLSNK 7:29 AM EDT on April 08, 2010Good Morning from Tallahassee; I was thinking the exact same thing (at least for a day) so my Wife will stop sneezing for a few hours. It's a very narrow and quick moving front so will hopefully not cause much damage in these parts in spite of passage, in the Big Bend region of Florida, in the late afternoon when temps are at maximum.
1458. unclemush
 HAZARDOUS WEATHER OUTLOOKNATIONAL WEATHER SERVICE MARQUETTE MI430 AM EDT THU APR 8 2010MIZ005>007-011>014-085-091200-MARQUETTE-ALGER-LUCE-DICKINSON-MENOMINEE-DELTA-SOUTHERN SCHOOLCRAFT-NORTHERN SCHOOLCRAFT-430 AM EDT THU APR 8 2010 /330 AM CDT THU APR 8 2010/THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL UPPERMICHIGAN..DAY ONE...TODAY AND TONIGHTSNOW WILL CONTINUE TO FALL ACROSS THE AREA. WHILE 1 TO LOCALLY 4INCHES OF SNOW CAN BE EXPECTED THROUGH THIS EVENING...MUCH OF THISWILL BE OVER GRASSY SURFACES. THE HIGHEST AMOUNTS ARE FIGURED EASTOF A LINE FROM MARQUETTE TO MENOMINEE.
1457. RTLSNK
 Well, this should take care of the pollen here in Macon Georgia today:
1456. SWFLgazer
 'Mornin' Cane. Not much traffic.

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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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