March 6, 2007

A 220-Year Hurricane Record?

Filed under: Climate Extremes, Hurricanes

A very interesting article was published in a recent issue of the prestigious Proceedings of the National Academy of Sciences, and the results are of considerable interest in the debate about tropical cyclones and global warming.

Recall that global warming alarmists want us to believe that all the ups and downs in the frequency and intensity of Atlantic hurricanes during the better part of the 20th century have been caused by mankind’s enhancement of the atmospheric concentration of various gas and aerosol species, while climate moderates believe that while anthropogenic activities may very well have had some impact on Atlantic tropical storms, natural variations probably have played a much larger role in storm behavior.

This new paper sheds some light on this issue.

A team of scientists from the Universities of South Carolina, Tennessee, and New Mexico began their article by noting that “Hurricane frequency is related to multidecadal-scale variations in sea surface temperatures, vertical wind shear, and the coupled ocean–atmosphere climate modes that influence these factors. The relatively short instrumental record of meteorological observations makes it difficult to discern long-term (i.e., multidecadal) trends and fluctuations in tropical cyclone activity or to differentiate natural versus anthropogenic components of these trends.” We all agree it would be wonderful to find a long-term record of tropical cyclones on the order of hundreds of years, but until now, no such record exists. The record exists, but it had to be discovered by the Miller et al. team.

Here is the trick. Miller et al. note that “Well organized tropical cyclones, such as major hurricanes, produce large amounts of precipitation with distinctly lower (by as much as 10‰) oxygen isotope compositions than typical low-latitude thunderstorms. Evidence of isotopically depleted precipitation may persist in soil waters for several weeks after a large event and will be incorporated into the cellulose as the tree utilizes the soil water, capturing an isotopic record of tropical cyclone activity.” Basically, the rain from large tropical cyclones has a unique oxygen isotope signature, trees build mass using that water, and in theory, the tree rings should preserve the oxygen isotope signature for a given growth period.

This story gets even more amazing given longleaf pine trees growing in southern Georgia and northern Florida. These trees “preserve distinct earlywood (growth in the early portion of the growing season; approximately April to mid-June) and latewood (growth in the later portion of the growing season; approximately mid-June to November) components that can be separately analyzed to obtain seasonally resolved isotope compositions.” The hurricane season in the southeastern United States extends from August to October which beautifully overlaps the latewood season for the longleaf pines.

Miller et al. examined trees that had been cut on the Valdosta State University campus and measured the oxygen isotope levels in the earlywood and latewood sections of each annual tree ring. They used some statistical wizardry comparing the difference between the isotopes in the earlywood and latewood, and bang, the years with tropical cyclones stand out magnificently (see below). They could calibrate the relationship during the most recent half century when both historical records and isotope measurements are available. The tree rings extend over 200 years providing an incredible record of tropical cyclone activity on the order of centuries.


Figure 1. These plots reveal the difference between earlywood and latewood oxygen isotope levels such that negative values are associated with tropical cyclones.

The team writes that the proxy record “shows close agreement with instrumental records that the 1950 decade was the busiest for hurricane activity in the 20th century. The proxy record further supports historical records that suggest significant tropical cyclone activity for the southeastern United States between 1865–1880. The isotope proxy detects six storms in the 1870 decade, although only one (1871; the largest 1870 decade anomaly) appears to have made direct landfall on the Georgia coast. Other decades of apparent activity include the 1840 and 1850 decades, 1800–1820 decades, and 1770s decade. Periods of relative quiescence in Georgia appear to be the 1781–1805 (except 1793 and 1795) and the 1970 decade”. They go on to describe ‘‘Great Hurricanes’’ of 1780, 1847, and 1857.

Furthermore, they note that “Over the period 1855–1940, the isotope proxy indicates 22 years with tropical cyclones, 21 of which are reported in the historical record to have affected the general study area. For the period 1770–1855, the proxy suggests many more years (25 years) affected by one (or more) tropical cyclones.” There were 2.9 storms per decade from 1770-1885 but only 2.5 storms per decade from 1855-1940.

There are many lessons from this incredible reconstruction. First, it is obvious that large hurricanes have impacted southern Georgia throughout the past 220 years, and some of the storms were larger than any storm in recent years. But more importantly, the record shows that some periods are active, others are quiet, and that this has been the case for a long time into the past (i.e. prior to any large-scale anthropogenic climate influences). This means that there is now more reason to believe that variations during the 20th century in the frequency and intensity of Atlantic tropical cyclones are very likely to have a significant natural component to them.

Reference:

Miller, D.L., C.I. Mora, H.D. Grissino-Mayer, C.J. Mock, M.E. Uhle, and Z. Sharp, 2006. Tree-ring isotope records of tropical cyclone activity. Proceedings of the National Academy of Sciences, 103, 14,294-14,297.




No Comments

No comments yet.

RSS feed for comments on this post.

Sorry, the comment form is closed at this time.

Powered by WordPress