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Posted at 10:30 AM ET, 08/18/2009

Studies Rekindle Hurricane-Climate Change Debate

By Andrew Freedman

* Another Hot One, P.M. Storms? Full Forecast | NatCast *
* Hurricane Camille 40 Years Later | Hurricane Tracking Station *

Satellite image of Hurricane Bill last night. Courtesy NOAA.

Here we go again.

Another hurricane season has swung into gear, bringing with it more discussion about the possible human influence on nature's most powerful storms. The scientific and political rhetoric on this subject that erupted in the wake of the devastating hurricane seasons of 2004 and 2005 has weakened considerably from an initial Category Five monster debate, but every once in a while a new scientific study or weather event emerges that raises the specter of man-made, and not just man-named, storms.

Two new studies on the subject, combined with three named Atlantic tropical storms or hurricanes in the past week alone, have brought the topic to the fore once again in the mainstream media and the blogosphere. The main question continues to be whether greenhouse gas emissions are causing hurricanes to become more frequent and/or powerful. The answer continues to be blurry.

Keep reading for more on these new studies...

Much of what has been said about the new studies has been misleading or difficult to decipher, and it's time to set the spin aside and sift through the research in order to put the work into the broader context of the state of knowledge on this topic.

The new studies by two different research teams both examine the historical record of tropical cyclone (tropical storms and hurricanes) frequency and duration, and together they help answer the question of whether these severe storms are in fact occurring more frequently in recent years, or if scientists are just getting better at observing them.

The verdict? Possibly both.

Satellite image of tropical storm Claudette approaching Florida panhandle. Courtesy NASA.

I know that is not exactly the kind of resolution you were hoping for, and believe me, I'm right there with you, but science rarely if ever offers an open and shut case. That's part of what makes it so fascinating. Here's a brief breakdown of the studies and their conclusions, and what some of the key outstanding questions are at this point.

One of the studies, by a group of four scientists led by Michael E. Mann of Penn State University and published in the journal Nature, examined annual storm counts in the Atlantic basin during the past 1,500 years. Since there were no satellites to keep tabs on storms in say, the 18th Century, the researchers relied on so-called "proxy data," such as sediment records in coastal locations that can reveal evidence of storm surges from land falling hurricanes. They also used statistical techniques to estimate hurricane activity based upon "key large-scale climate factors" related to hurricane frequency, such as sea surface temperatures and El Nino.

Their conclusions: that recent tropical cyclone activity has been abnormally high, but that there was a past period of similarly high activity during medieval times around 1,000 AD. The researchers proposed several factors that might have contributed to the medieval peak, including relatively warm sea surface temperatures in the North Atlantic. Although the study itself did not address the question of whether human activities are behind the recent spike in hurricane activity, it's clear that Mann thinks that may be the case, since the study found a close relationship between sea surface temperatures and storm frequency.

Mann stated the following in a press release from Penn State: "It seems that the paleodata support the contention that greenhouse warming may increase the frequency of Atlantic tropical storms."

"It may not be just that the storms are stronger, but that there are there may be more of them as well," he stated.

The other study [PDF], published in the Journal of Climate, by Christopher S. Landsea of the National Hurricane Center in Miami and three other researchers, questions whether the upswing in hurricane activity in the late 20th Century reflects changing ways of observing storms rather than a real increase in storm activity. Rather than examining proxy data, these scientists took a look at the record of hurricane observations from ship reports, satellite tracking and other methods.

Their conclusions: that scientists are becoming more adept at catching the short-lived, typically weaker tropical cyclones, and that these are boosting annual storm counts. They found that once this and other factors in the record were accounted for, the upward trend in storm counts was greatly reduced.

Although neither study answers what to me are the most intriguing questions in this field, such as whether the strongest hurricanes will occur more frequently and get even stronger from warming seas as climate change progresses, they both offer interesting insight into an area of climate science that has captured the public's attention ever since a hurricane named Katrina tore into the Gulf Coast. One thing is for sure; neither study is going to be the last word on this topic.

Stay tuned to the Capital Weather Gang for continuing coverage of the 2009 Hurricane Season.

By Andrew Freedman  | August 18, 2009; 10:30 AM ET
Categories:  Climate Change, Freedman, Media, Science  
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Chris Landsea, the lead author of the paper in the Journal of Climate, has criticized Mann's paper in Nature. I didn't include the back and forth in the post, since it gets quite technical, but you can find the criticism in the Aug. 12 entry here and Mann's responses here.

Posted by: Andrew-CapitalWeatherGang | August 18, 2009 11:15 AM | Report abuse

The recent spike in tropical activity seems to be a bit odd considering that we're in the beginning of an El Nino which is supposed to dampen tropical development on the Atlantic/Caribbean side of the Central American isthmus.

How do they know that hurricane activity was high around 1000 AD? Tropical cyclones generally don't leave "fossil evidence" the way earthquakes and volcanic eruptions do, though studies of deposited sediments from the Middle Ages might provide a clue on sea surface temperatures at that time.

It's likely that the "Little Ice Age" of the later Middle Ages and Renaissance Age of Discovery had somewhat less tropical activity. We're probably lucky that 1492 AD was a year with few or no tropical cyclones, but Columbus did run into some pretty good hurricanes on his later voyages. In fact he is supposed to have warned the captain of one galleon not to let the fleet leave Santo Domingo on a September day marked by a rather unusual westerly wind. Unfortunately, the captain did not heed Columbus' advice, the fleet left port and immediately ran into a violent hurricane which scattered or wrecked much of the Spanish Plate Fleet that year. This happened some time around 1500 or 1502, as I recall.

Posted by: Bombo47jea | August 18, 2009 12:51 PM | Report abuse

Bombo47jea: tropical cyclones can leave evidence of their impact on coastal locations in the form of sediment deposits, which Michael Mann and his colleagues analyzed for their study. Such deposits can provide a historical record of hurricane landfalls, from which overall hurricane activity can begin to be inferred, with considerable uncertainty. Such techniques are part of a scientific field with a very cool name: "paleotempestology."

Posted by: Andrew-CapitalWeatherGang | August 18, 2009 1:19 PM | Report abuse

It's very difficult trying to tie evidence into the theory that human activity causes an increase in tropical storm frequency. Take 2005, for instance......a record year in the North Atlantic season, including Katrina and Rita. The next three years after that were almost nothing, though, of course, other parts of the world experienced more.

But, the point is, you might as well throw darts at a dartboard than try and predict hurricane seasons in advance.

Posted by: MMCarhelp | August 18, 2009 5:40 PM | Report abuse

There were many insinuations within the MSM during the late 2005 season that the record number of storms were a result of "Global Warming" and that the future would be a cataclysm of frequent and more intense hurricanes.

Of course this was supported by expert scientific opinion.

The consensus was so strong that some insurance companies became reluctant or refused to issue coastal policies.

During the first half of 2006 the three leading modeling firms: Air Worldwide, EQECAT and Risk Management Solutions reevaluated and revised their near term models to incorporate an increased liklihood of hurricanes in the next 5 years.

My,My! How about that!!!!!!!

Posted by: AugustaJim | August 18, 2009 10:14 PM | Report abuse

seems to me like "paleotempestologogical" studies would tend to underestimate the storms from the past. not every hurricane would leave a "mark". and it seems like you'd be likely to miss a lot of the marks that were left. some storms don't even hit land.

obviously these studies about hurricane frequencies have to be taken with a lot of salt.

on another topic, bombo mentioned el nino. isn't a "weak el nino" the best for snow in falls church? is that what we'll have this winter?

Posted by: walter-in-fallschurch | August 19, 2009 3:29 PM | Report abuse

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