Carbon Dioxide and Earth's Future Pursuing the ... - Magazooms

www.co2science.org
P a g e | 35
distribution and grain size of storm over-wash sediments contained within four of those cores
from two transects -- which were most ideally positioned -- they were able to construct a
detailed history of intense hurricane strikes from 5300 to 900 years before present (BP). Based
on their analyses, the two scientists determined that “there has been no notable variation in
intense storm impacts across the northwestern Gulf of Mexico coast during this time interval,”
i.e., 5300-900 yr BP, “implying no direct link between changing climate conditions and annual
hurricane impact probability.” In addition, they report that “there have been no significant
differences in the landfall probabilities of storms between the eastern and western Gulf of
Mexico during the late Holocene, suggesting that storm steering mechanisms have not varied
during this time.”
In discussing their findings -- as well as the similar results obtained by others for Western Lake,
Florida (USA), and Lake Shelby, Alabama (USA) -- the two researchers concluded that current
rates of intense hurricane impacts “do not seem unprecedented when compared to intense
strikes over the past 5000 years,” while noting that “similar probabilities in high-intensity
hurricane strikes for the eastern and western Gulf of Mexico do not show any clear-cut out-ofphase
relationship that would enlighten us as to climate controls on storm pathways.” Thus,
they reiterated their conclusion that “in the northern Gulf of Mexico, there have been no
significant variations in storm impact probabilities and/or storm steering mechanisms from ca.
5300-900 yr BP.”
With respect to hurricanes occurring over the Pacific Ocean, there are a number of recent
studies, including that of Chan (2008), who investigated possible causes of the multi-decadal
variability in intense TC (category 4 and 5) occurrence in the western North Pacific (WNP),
which basin generally has the largest number of TCs every year. And based on data for the
period 1960-2005, the Hong Kong researcher determined that decadal variations in intense
typhoon activity largely result from a combination of the behavior of the El Niño-Southern
Oscillation (ENSO) and Pacific Decadal Oscillation (PDO).
In discussing this finding, Chan said that “the view that global warming would lead to more
intense TCs owing to the enhancement of thermodynamic factors ignores the fact that for TCs
to intensify significantly, the dynamic factors must ‘cooperate’,” and he added that “the latter
have not been demonstrated to be enhanced basin wide.” Thus, he suggested “the more likely
conclusion is that the major low-frequency variations in the frequency of intense TC occurrence
is probably a multi-decadal one in response to similar variations in the factors that govern the
formation, intensification and movement of TCs,” and he noted that “such variations largely
result from modifications of the atmospheric and oceanographic conditions in response to
ENSO and PDO.” Thus, and “at least for the WNP,” Chan stated “it is not possible to conclude
that the variations in intense typhoon activity are attributable to the effect of global warming.”
Defining rapid intensification (RI) of a tropical cyclone as occurring when the maximum wind
speed of a TC reaches at least 5 knots in the first 6 hours, 10 knots in the first 12 hours, and 30
knots in 24 hours, Wang and Zhou (2008) determined that “all category 4 and 5 hurricanes in
the Atlantic basin and 90% of the equivalent-strength typhoons in the western North Pacific
[ search engine powered by magazooms.com ]