June 26, 2008

Let’s Revisit Katrina, Again

Filed under: Climate Extremes, Hurricanes

Are we ever going to put Katrina to bed? We have covered no end of articles clearly showing that hurricane activity is not increasing and likely will not increase in frequency or intensity due to the ongoing buildup of greenhouse gases. Virtually every prominent scientist involved in hurricane research agrees that it is brutally unfair to blame any one event on global warming, and yet to this day, almost every global warming presenter hints around that we caused Katrina, or at least we substantially added to its strength. As time passes, you would think this storyline would die. However, the recent tragedy in Myanmar associated with Cyclone Nargis left tens of thousands dead and reinvigorated the “global warming equals bigger hurricane” crusade.

Incredibly, more than a million websites come up for a search of Myanmar and Katrina! Countless titles appear such as “Cyclone’s path through Myanmar resembled Katrina’s wrath” or “Myanmar cyclone, Katrina, People in Glass Houses” or “ABC Calls Myanmar Cyclone ‘Asia’s Katrina’” or “Are devastating storms like Hurricane Katrina and Myanmar Cyclone a sure sign of global warming?” Throw in some pictures of Al Gore, and the connection is made loud and clear between global warming and hurricanes all over the world.

Yet another article has appeared in a major journal (Geochemistry, Geophysics, Geosystems) entitled “Tropical cyclone variations in Louisiana, U.S.A., since the late eighteenth century.” The work was done by Cary Mock of the University of South Carolina and the research was funded by the National Science Foundation. Mock notes that “the current Atlantic Basin record is too short to encompass the full range of temporal variability needed to calculate accurate probabilities and recurrence intervals essential for long-range hurricane prediction and hazard assessment. A longer temporal perspective of hurricane activity would be quite reassuring, particularly since the characteristics of climatic forcing mechanisms of the previous centuries, as well as the last few decades, are different, and because increased coastal development and population is likely to continue in conjunction with anticipated future climate change.” We doubt anyone would argue that longer term records of hurricane activity would be very useful at this point.

In the case of Louisiana, people have been living there many centuries, and for a variety of reasons, they have keep records on unusual weather phenomena, especially hurricanes. Mock states “This paper presents a new unique documentary reconstruction of tropical cyclones for Louisiana, U.S.A. that extends continuously back to 1799 for tropical cyclones, and to 1779 for hurricanes. This is clearly the longest continuous tropical cyclone reconstruction conducted to date for the United States Gulf Coast, with the reconstruction focusing on the addition of newly documented storms from 1799 to 1871.” We always welcome more data, and we suspected all along that we would welcome results from longer-term records from Louisiana.

Mock explains “Detailed weather descriptions of Louisiana tropical cyclones date back to the late eighteenth century. New Orleans served as the state’s main center of maritime, urban, and economic activity, enabling the buildup and archival preservation of colonial documentary data. About two dozen different newspapers provide some detailed information of tropical cyclones, as at times they record the exact hourly timing of storm impact, wind direction, wind intensity, rainfall, storm surge, damage to buildings and trees, specifics on geographic extent of damage, ship disasters, and deaths.” Mock further states “Generally, newspapers contained more information concerning stronger storms and their impacts. Newspapers at times contained ‘‘contributed’’ letters, appearing in print up to several weeks after the occurrence of a storm from localities spread throughout Louisiana. Most newspapers prior to the mid-1830s appeared in print on a weekly basis, and newspapers thereafter were published on a daily basis, with some even providing two newspaper editions per day. Private diaries, journals, and letters provide the majority of land-based documentary manuscripts from the early 1820s up through the American Civil War. Most of these manuscripts were in the form of plantation diaries, as plantations were a prominent commercial enterprise that required meticulous record-keeping.”

There are also records from the sea; Mock notes “Also of valuable use, a few hundred ship protest records from the New Orleans Notarial Archives provide unique abundant documentary data on storm impacts in the Mississippi Delta and along the Louisiana coast, dating back to the late eighteenth century. Many contained valuable information on weak tropical cyclones not documented in newspapers and diaries.” We love it – more data from the real world!

If you prefer only observations made from meteorological instruments, Mock explains “Occasional instrumental weather records in Louisiana and nearby Mississippi and Alabama, mostly dating after the late 1830s, provided important quantitative weather data that occasionally included barometric pressure. Other meteorological variables observed include temperature, precipitation, wind speed, wind strength, cloudiness, and written remarks were often provided as well. These instrumental data were recorded by weather enthusiasts from personal diaries, the U.S. Army Surgeon General, and the Smithsonian Institution, U.S. Navy and British logbooks, and the U.S. Coastal Survey.”

OK – we get the message … there are many ways to gather information about hurricanes that occurred long ago. Figure 1 below represents the bottom line, and as noted in the case of both tropical cyclones and hurricanes, there are no long-term upward trends whatsoever, and in fact, the overall trends are downward. Mock states “The Louisiana tropical cyclone and hurricane reconstructions presented here indicate that the modern records which cover just a little over a hundred years is too short to provide a full spectrum of tropical cyclone variability, both in terms of frequency and magnitude. Some active periods in the nineteenth century have not occurred during the recent rise of coastal population and development over the past several decades.” Mock warns “If a higher frequency of major hurricanes occurred in the near future in a similar manner as the early 1800s or in single years such as in 1812, 1831, and 1860, would have devastating consequences for New Orleans, perhaps equaling or exceeding the impacts such as in hurricane Katrina in 2005.”

Enough said!


Figure 1. (Top) Time series of Louisiana tropical cyclones along with centered 10-year running sums from 1799 to
2007. (Bottom) Time series of Louisiana hurricanes along with centered 10-year running sums from 1799 to 2007. Major hurricanes are listed as green circles. Additional Louisiana hurricanes are also known for 1722, 1779 (two storms), 1780, 1794 (two storms), 1795, and 1796 (from Mock, 2008)

Reference:

Mock, C.J. 2008. Tropical cyclone variations in Louisiana, U.S.A., since the late eighteenth century. Geochemistry, Geophysics, Geosystems, 9, Q05V02, doi:10.1029/2007GC001846.




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