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Climate change and Hurricane Katrina: what have we learned

Posted on 4 September 2015 by Guest Author

This is a re-post from The Conversation byThe Conversation Kerry Emanuel, Massachusetts Institute of Technology

Three weeks and three days before Hurricane Katrina devastated New Orleans 10 years ago, a paper of mine appeared in the scientific journal Nature showing that North Atlantic hurricane power was strongly correlated with the temperature of the tropical Atlantic during hurricane season, and that both had been increasing rapidly over the previous 30 years or so. It attributed these increases to a combination of natural climate oscillations and to global warming.

Had Katrina not occurred, this paper and another by an independent team would merely have contributed to the slowly accumulating literature on the relationship between climate and hurricanes.

Instead, the two papers inspired a media firestorm, polarizing popular opinion and, to some extent, scientists themselves, on whether global warming was in some way responsible for Katrina. While the firestorm was mostly destructive, benefiting only the media, it had a silver lining in inspiring a much more concerted effort by atmospheric and climate scientists to understand how hurricanes influence and are influenced by climate.

We have learned much in the intervening years.

Sea level and storm surges

An obvious point is that slowly rising sea levels increase the probability of storm-induced surges even when the statistics of the storms, such as top wind speed, themselves remain stable. Storm surges are physically the same thing as tsunamis but driven by wind and atmospheric pressure rather than the shaking seafloor, and they typically arrive near the peak of the storm’s fury.

As with Katrina and Sandy, they are often the most destructive aspects of hurricanes. Had Sandy struck New York a century ago, there would have been substantially less flooding, as sea level was then roughly a foot lower. As sea level increases at an accelerating pace, we can expect more devastating coastal flooding from storms.

A NASA retrospective of Hurricane Katrina build-up done on the five-year anniversary.

Potential intensity

What about the storms themselves? Hurricanes are giant heat engines driven by the thermodynamic disequilibrium between the tropical oceans and atmosphere. This disequilibrium drives a strong flow of heat from the ocean to the atmosphere and is a direct consequence of the greenhouse effect: the tropical atmosphere is so opaque to infrared radiation that the sea surface cannot cool very much by directly radiating heat to space. Instead, it cools mostly by the evaporation of water, the same mechanism by which our sweaty bodies cool on a hot day.

To maintain this evaporation, the sea and atmosphere must be in a state of thermodynamic disequilibrium. As we add greenhouse gases to the atmosphere, this thermodynamic disequilibrium must increase so that cooling by evaporation can compensate the loss of direct infrared cooling to space.

The theory of the hurricane heat engine places an upper limit on hurricane wind speeds. Called the “potential intensity,” it is directly proportional to this disequilibrium. Virtually every study that has been done, dating back to 1987, shows increasing potential intensity in most places as our climate continues to warm; the average trend is about 10 miles per hour (mph) for every degree centigrade of tropical sea surface temperature increase, or roughly 20 mph for each doubling of atmospheric CO2 concentration.

100-year events

Twenty miles per hour might not seem like all that much, but economists and engineers tell us that damage from windstorms increases very rapidly with wind speed.

The actual situation is much more interesting than one might at first suspect. Human society is well adapted to common events. In Boston, a 50 mph wind will not do much damage because it occurs quite frequently and infrastructure is well adapted to it. But a 70 mph wind, which is far rarer, will cause quite a bit of damage.

New Orleans the week after Katrina hit landfall. Liz Roll/FEMA

As a loose rule of thumb, societies are well-adapted to events that occur, on average, once every generation or two. In many places this is codified in building codes, insurance contracts and other policies that are based on or insist on resistance to 100-year events; that is, events with an annual probability of 1%. But to keep costs down, a structure engineered to survive a 100-year wind speed of 100 mph may very well fail at 110 mph.

Typhoon Haiyan is a case in point. The Philippines are regularly hammered by Category 5 typhoons, but it is rare that we hear about these because they seldom do much damage. In the region near Tacloban, the 100-year storm will have a landfalling peak wind speed of about 170 mph.

But Haiyan, probably the strongest hurricane or typhoon ever to be recorded at landfall, had wind speeds upwards of 190 mph, accompanied by a phenomenal storm surge. The difference between 170 mph and 190 mph in this case was more than 6,300 deaths and almost total devastation. This is what happens when events start to fall outside generational experience.

Theory and computer models show that the incidence of the strongest hurricanes – those that come closest to achieving their potential intensity – will increase as the climate warms, and there is some indication that this is happening. But these most destructive, high-category storms constitute only around 12% of the world’s tropical cyclones; the great majority do little damage but occur far more often.

Storm surge during Hurricane Irene in Greenwich, Connecticut in 2011. Cyclonebiskit, CC BY-SA

Both theory and most models predict that, ironically, the frequency of such weaker storms should decline as the climate warms. Satellite data also show that storms are reaching their peak at higher latitudes, consistent with theories and models. This may portend reduced risk in some of the deep tropics but increased risk in middle latitudes.

In general, systematic changes in hurricane formation regions and tracks are of as much concern to us as changes in the overall statistics of storm frequency and intensity. So too is the expected large increase in hurricane rainfall, which drives hurricane freshwater flooding, the second deadliest consequence of these storms after flooding from storm surges.

Little time to adapt

Global warming is occurring far too fast for effective human adaptation. The next ice age, like the last one, may very well put a mile of ice on top of New York City, but it will take so long for that to happen that most of us will not even notice our collective adaptation.

In contrast, adapting to the myriad changes expected over the next 100 years is such a horrendous prospect that otherwise intelligent people rebel against the idea even to the extent of denying the very existence of the risk. This recalcitrance, coupled with rising sea levels, subsiding land and increased incidence of strong hurricanes, all but guarantees that New Orleans will have moved or have been abandoned by the next century.

Kerry Emanuel is Professor of Atmospheric Science at Massachusetts Institute of Technology

This article was originally published on The Conversation. Read the original article.

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Comments

Comments 1 to 10:

  1. "such a horrendous prospect that otherwise intelligent people rebel against the idea even to the extent of denying the very existence of the risk."

    Whoa...the Christchurch city council is requiring new homes built near the sea to allow for future flooding with other mitigations and are at risk of being sued by outraged home-owners. The pity of it all is that they are pinning their colours to a very conservative sea-level rise estimate that is sure to be exceeded. 

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  2. There is another side to the hurricane story that suggests natural variability has a significant effect on hurricanes and may play a bigger role than global warming.  Christopher Landsea, a scientist at the US NAtional Hurricane Center is not as certain as scientific organisations, such as the UK Royal Society and the US National Academies  of Sciences, that climate change is worsening hurricanes.  He considers hurricanes occur in cycles " "The late nineteenth century was a very busy period," he explains. "Then from the 1900s until about 1925, it was very quiet. The late 20s to the 60s were very busy. The 1970s to the mid-90s were quiet again, and then from the late 90s onward, it's been generally very busy.".  (http://tinyurl.com/pnqcdb3)

    The article gives a comprehensive discussion of hurricanes and expresses views some of which  do not entirely support those in the piece by Kerry Emanuel.

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  3. Re: ryland #2

    To quote the article linked:

    Although Landsea doesn't dispute that humans are causing climate change, he does doubt the assertion that global warming has significantly increased hurricane activity. To explain his doubts, he points to the imperfections in hurricane records.

    Above, Professor Emanuel says:

    Theory and computer models show that the incidence of the strongest hurricanes – those that come closest to achieving their potential intensity – will increase as the climate warms, and there is some indication that this is happening. But these most destructive, high-category storms constitute only around 12% of the world’s tropical cyclones; the great majority do little damage but occur far more often.

    These are not mutually contradictory - Emanuel is saying that there will an increase in the "most destructive, high category" storms, while Landsea is talking about ALL hurricanes.

    If Emanuel is right, we better get ready. If we delay in the hope that Landsea is also correct on extreme storms, and if he is wrong, then we (or a lot of people, somewhere) are screwed.

    Incidentally, if you read father down your link, you will find NASA scientists who support the view of Professor Emanuel's article.

    Storms

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  4. Ryland, you are confusing saying a) there will be more powerful hurricanes, with b) there will be more hurricanes period. Emanuel does not say the latter, he says the former.

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  5. Katrina came ashore in Louisiana as a Category 3 hurricane with sustained winds of 125 mph, and a two part storm surge that was not particularly impressive (12-16ft). It has been well documented by civil authorities and the Army Corp of Engineers that the hurricane was not the main cause of the devastation visited on New Orleans. The cause of the devastatoin was poor levee design and construction by local and federal government authorities. Although the Army Corp of Engineers initially asserted that the levees would have failed even if the design flaws had not been present, on April 5, 2006, months after independent investigators had demonstrated that levee failures were not caused by natural forces beyond intended design strength, Lieutenant General Carl Strock, Chief of Engineers and Commander of the Corps of Engineers, testified before the United States Senate Subcommittee on Energy and Water that "We have now concluded we had problems with the design of the structure." He also testified that the U.S. Army Corps of Engineers did not know of this mechanism of failure prior to August 29, 2005. The claim of ignorance is refuted, however, by the National Science Foundation investigators hired by the Corps of Engineers, who point to a 1986 study by the Corps itself that such breaches were possible in the I-wall design.

    Amazingly, however, the word 'levee' does not even appear once in this post. It's more than a bit disingenuous for Kerry Emanuel to use Katrina, and government incompetence, as an example of AGW effects. 

    In the case of Typhoon Haiyan, it is also well documented that, a) category 5 typoons are not historically unusual in that part of the world, b) the effects of Haiyan were amplified by the fact that it funnelled straight into Leyte Gulf and San Pablo In addition, it was not the most devesating typoon to hit the Philipines. That distinction belongs to a typoon which hit the islands in 1881, well before, pardon the pun, global warming was a gleam in anyone's eye.

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  6. Ignaz,

    This article describes the strength of hurricanes and sea level rise and how heey affect damage.  The levees in New Orleans are not germaine to the discussion.  Your comment is off topic.

    Please provide a citation for your claim that the 1881 hurricane was stronger than Haiyan.  In any case, Haiyan was the strongest hurricane recorded at landfall in the history of carefully recorded data.  You are trying to minimize record strength at landfall.  That is small consolation for the people who have to weather stronger and stronger hurricanes at landfall.  The fact that in the past occasional hurricanes were as strong as current ones does not compensate for the increase, caused by AGW,  in the strongest hurricanes.

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  7. First, Ignaz @5 has plagiarized from wikipedia, which states:

    "On April 5, 2006, months after independent investigators had demonstrated that the levee failures were not due to natural forces beyond intended design strength, Lt. Gen. Carl Strock testified before the U.S. Senate Subcommittee on Energy and Water that, "We have now concluded we had problems with the design of the structure." He also testified that the U.S. Army Corps of Engineers did not know of this mechanism of failure prior to August 29, 2005. The claim of ignorance is refuted, however, by the National Science Foundation investigators hired by the Army Corps of Engineers, who point to a 1986 study by the corps itself that such separations were possible in the I-wall design."

    You will notice that his wording is exactly copied by Ignaz without attribution, following his clause, "Although the Army Corp of Engineers initially asserted that the levees would have failed even if the design flaws had not been present ...".

    Second,  despite Ignaz's willingness to quote (without attribution) from wikipedia, he neglects to quote wikipedia when it says:

    "According to Professor Raymond Seed of the University of California, Berkeley, a surge of water estimated at 24 feet (7 m), about 10 feet (3 m) higher than the height of the levees along the city's eastern flank, swept into New Orleans from the Gulf of Mexico, causing most of the flooding in the city. He said that storm surge from Lake Borgne travelling up the Intracoastal Waterway caused the breaches on the Industrial Canal."

    To illustrate this point, consider the following map of part of the flooded areas in New Orleans following Katrina:

    The map comes via a USGS article, that states:

    "The storm surges produced by Hurricane Katrina on August 29, 2005, breached the levees protecting New Orleans in numerous places, flooding approximately 75 percent of the metropolitan area. Most of the levee failures were caused by overtopping, as the storm surge rose over the top of a levee and scoured out the base of the landward embankment or floodwall. Three major and costly breaches appear to have been caused by failure of the soils underlying the levees or failure of the earthen levee embankments themselves; in several places, levee foundations failed when water levels were below the tops of the levees."

    (My emphasis)

    The blue stars on the map indicate locations where the levies were overtopped, and the redstars locations of structural failure in the levies.  As can be seen, most of the flooding in this restricted area came from levies being overtopped, and even in the locations where structural failure was a significant factor (New Orleans East Bank and New Orleans East), there was also flooding due to overtopping of levies.  In the areas effected by structural failures, therefore, those failures made flooding worse - but flooding would still have occured without the failures.

    As this larger map of the flooding (below) indicates, the area in the map above represents only a subsection of New Orleans and of the flooding.  The flooding outside of the area of the map above is all due overtopping of levies (as I understand it).

    So, in addition to a bout of plagiarism, Ignaz has taken the summary of reports that show the majority of the flooding and much of the damage to have been due to overtopping of the levies and interpreted them to indicate that effectively all of the damage was due to structural failure.  That is, he has egregiously misrepresented the findings of the various inquiries into the flooding resulting from Katrina.

    It may be noted that his failure to properly note and cite his quotation also served the purpose of (temporarilly) hiding his misrepresentation.

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  8. Tom Curtis, your response to Ignaz was terrific!  I know that the moderators might delete my post if I say that  Ignaz is  intellectually dishonest, so I won't.

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  9. Elsewhere, Ignaz has again asserted that "As per the Army Corp of Engineers, New Orleans flooded because of flawed levee design".  I think his refusal to discuss the topic here, where my response to his nonsense is immediately available is telling.  Typically for Ignaz, he provides no citation and no link for his claim.  I presume, therefore, that he is again rellying on the testimony of Lt General Karl Strock that he reffers to above.  The only direct report of that testimony that I can find states:


    "In the closest thing yet to a mea culpa, the commander of the Army Corps of Engineers acknowledged Wednesday that a "design failure" led to the breach of the 17th Street Canal levee that flooded much of the city during Hurricane Katrina.  Lt. Gen. Carl Strock told a Senate committee that the corps neglected to consider the possibility that floodwalls atop the 17th Street Canal levee would lurch away from their footings under significant water pressure and eat away at the earthen barriers below.  "We did not account for that occurring," Strock said after the Senate Appropriations subcommittee hearing. "It could be called a design failure.""


    The report makes it very clear that Strock reffers only to the 17th street canal failures, not to all levee failures.

    His restricted admission is appropriate, as the USGS discussion of the levee failures makes quite clear.  That is because the majority of levee "failures" were the result of the levees being overtopped - ie, of "storm induced" failures in the wording of the legend of the first map @7 above.

    Yet again it is very plain that Ignaz is taking restricted evidence applicable to only a few of the levee failures, and explicitly stated in connection to the 17th street Canal failures only, and treating them as an admission regarding all failures, contrary to the facts.

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    Moderator Response:

    [RH] Yes. Ignaz is quickly running out of rope and is trying the patience of moderators.

  10. The agenda is clear.  Ignaz, in a monumentally simplistic move, reveals that government is evil because of the flawed design of one section of a Corps levee project.  I'm surprised Ignaz hasn't mentioned Obama.

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    Moderator Response:

    [DB] Ignaz has recused himself from further participation in this venue, finding compliance with this site's Comments Policy a too-onerous burden.

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