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Tropical Cyclones (Atlantic Ocean - Global Warming Effects: Frequency, The Past Few Centuries) -- Summary
Has the warming of the past century, which rescued the world from the extreme cold of the Little Ice Age, led to the yearly formation of more numerous Atlantic Basin tropical storms and hurricanes? This question is investigated here via a brief review of several studies that have broached this question with sufficiently-long databases to provide reliable answers.

Elsner et al. (2000) provided a statistical and physical basis for understanding regional variations in major hurricane activity along the U.S. coastline on long timescales; and in doing so, they presented data on major hurricane occurrences in 50-year intervals for Bermuda, Jamaica and Puerto Rico. These data revealed that hurricanes occurred at far lower frequencies in the last half of the 20th century than they did in the preceding five 50-year periods, and at all three of the locations studied. From 1701 to 1850, for example, when the Earth was locked in the icy grip of the Little Ice Age, major hurricane frequency was 2.77 times greater at Bermuda, Jamaica and Puerto Rico than it was from 1951 to 1998. And from 1851 to 1950, when the planet was in transition from Little Ice Age to current conditions, the three locations experienced a mean hurricane frequency that was 2.15 times greater than what they experienced from 1951 to 1998.

Such findings for the Caribbean Sea were echoed by Elsner (2008), who wrote in his summary of the International Summit on Hurricanes and Climate Change that was held in May of 2007, wherein he states that paleotempestology -- which he defines as the study of prehistoric storms based on geological and biological evidence -- indicates the presence of more hurricanes in the northeastern Caribbean Sea "during the second half of the Little Ice Age when sea temperatures near Puerto Rico were a few degrees (Celsius) cooler than today," which he goes on to say provides evidence that "today's warmth is not needed for increased storminess."

In another multi-century study, Boose et al. (2001) used historical records to reconstruct hurricane damage regimes for an area composed of the six New England states plus adjoining New York City and Long Island for the period 1620-1997. In describing their findings, they wrote that "there was no clear century-scale trend in the number of major hurricanes." At lower damage levels, however, fewer hurricanes were recorded in the 17th and 18th centuries than in the 19th and 20th centuries; but the three researchers concluded that "this difference is probably the result of improvements in meteorological observations and records since the early 19th century." When focusing in on the better records of the last 200 years, however, it can be determined that the cooler 19th century had five of the highest-damage storms, while the warmer 20th century had only one such storm.

Nyberg et al. (2007) developed a history of major (category 3-5) Atlantic hurricanes over the past 270 years based on proxy records of vertical wind shear and sea surface temperature that they derived from corals and a marine sediment core. These parameters are the primary controlling forces that set the stage for the formation of major hurricanes in the main development region westward of Africa across the tropical Atlantic and Caribbean Sea between latitudes 10 and 20°N, where 85% of all major hurricanes and 60% of all non-major hurricanes and tropical storms of the Atlantic are formed. This effort resulted in their discovering that the average frequency of major Atlantic hurricanes "decreased gradually from the 1760s until the early 1990s, reaching anomalously low values during the 1970s and 1980s." More specifically, they note that "a gradual downward trend is evident from an average of ~4.1 (1775-1785) to ~1.5 major hurricanes [per year] during the late 1960s to early 1990s," and that "the current active phase (1995-2005) is unexceptional compared to the other high-activity periods of ~1756-1774, 1780-1785, 1801-1812, 1840-1850, 1873-1890 and 1928-1933." In light of such findings, they concluded that the recent ratcheting up of Atlantic major hurricane activity appears to be simply "a recovery to normal hurricane activity." And in a commentary on Nyberg et al.'s paper, Elsner (2007) states that "the assumption that hurricanes are simply passive responders to climate change should be challenged," which is what Nyberg et al. do in a very convincing manner.

Also noting that "global warming is postulated by some researchers to increase hurricane intensity in the north basin of the Atlantic Ocean," with the implication that "a warming ocean may increase the frequency, intensity, or timing of storms of tropical origin that reach New York State," Vermette (2007) employed the Historical Hurricane Tracks tool of the National Oceanic and Atmospheric Administration's Coastal Service Center to document all Atlantic Basin tropical cyclones that reached New York State between 1851 and 2005, in order to assess the degree of likelihood that 20th-century global warming might have influenced these storms -- as climate alarmists suggest it should -- particularly for hurricanes but also for tropical storms, tropical depressions and extratropical storms.

This work revealed, in Vermette's words, that "a total of 76 storms of tropical origin passed over New York State between 1851 and 2005," and that of these storms, 14 were hurricanes, 27 were tropical storms, 7 were tropical depressions and 28 were extratropical storms." For Long Island, he further reports that "the average frequency of hurricanes and storms of tropical origin (all types) is one in every 11 years and one in every 2 years, respectively." Also of note is his finding that storm activity was greatest in both the late 19th century and the late 20th century, and the fact that "the frequency and intensity of storms in the late 20th century are similar to those of the late 19th century." As a result, Vermette concludes that "rather than a linear change, that may be associated with a global warming, the changes in recent time are following a multidecadal cycle and returning to conditions of the latter half of the 19th century."

In a similar study, Mock (2008) developed a "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 record -- which was derived from daily newspaper accounts, private diaries, plantation diaries, journals, letters and ship records, and which was augmented "with the North Atlantic hurricane database as it pertains to all Louisiana tropical cyclones up through 2007" -- is, in Mock's words, "the longest continuous tropical cyclone reconstruction conducted to date for the United States Gulf Coast." And this record reveals that "the 1820s/early 1830s and the early 1860s are the most active periods for the entire record."

In discussing his findings, the University of South Carolina researcher says 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." In addition, he states that "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, [they] would have devastating consequences for New Orleans, perhaps equaling or exceeding the impacts such as in hurricane Katrina in 2005." And, of course, the new record clearly indicates that the planet's current high levels of both air temperature and CO2 concentration cannot be blamed for the 2005 Katrina catastrophe, as both parameters were much lower than they are currently when tropical cyclone and hurricane activity in that region were much higher than they are now back in the early to mid-1800s.

Also working in 2008, Chenoweth and Divine (2008) examined newspaper accounts, ships' logbooks, meteorological journals and other documents in order to reconstruct a history of tropical cyclones passing through the 61.5°W meridian between the coast of South America (~9.7°N) and 25.0°N over the period 1690-2007, which they describe as "the longest and most complete record for any area of the world." In doing so, the two authors report that they could find "no evidence of statistically significant trend in the number of tropical cyclones passing through the region on any time scale," but they note that "hurricane frequency is down about 20% in the 20th century compared to earlier centuries," and that "this decline is consistent with the 20th century observed record of decreasing hurricane landfall rates in the U.S. (Landsea et al., 1999; Elsner et al., 2004) and proxy reconstruction of higher tropical cyclone frequency in Puerto Rico before the 20th century (Nyberg et al., 2007), as well as model-simulated small changes in Atlantic basin tropical cyclone numbers in a doubled CO2 environment (Emanuel et al., 2008; Knutson et al., 2008)." In addition, they report that "the period 1968-1977 was probably the most inactive period since the islands were settled in the 1620s and 1630s," which finding, in their words, "supports the results of Nyberg et al. (2007) of unprecedented low frequency of major hurricanes in the 1970s and 1980s."

Following up on their work four years later, Chenoweth and Divine (2012) examined the records employed in their earlier paper in somewhat more detail, determining "the maximum estimated wind speed for each tropical cyclone for each hurricane season to produce a seasonal value of the total cyclone energy of each storm along various transects that pass through the 61.5°W meridian." And somewhat analogous to accumulated cyclone energy (ACE), they calculated Lesser Antilles Cyclone Energy (LACE) along a fixed spatial domain (10-25°N, 61.5°W) at any time a tropical cyclone passed through it, after which they performed spectral and wavelet analysis on the LACE time series and tested it for statistical significance of trends.

Based on their analysis, Chenoweth and Divine report that their record of tropical cyclone activity "reveals no trends in LACE in the best-sampled regions for the past 320 years," and that "even in the incompletely sampled region north of the Lesser Antilles there is no trend in either numbers or LACE," noting that these results are similar to those reported earlier by them (Chenoweth and Divine, 2008) on tropical cyclone counts. In addition, they indicate that LACE along the 61.5°W meridian is "highly correlated" with Atlantic-Basin-wide ACE.

In another study, Wang and Lee (2008) used the "improved extended reconstructed" sea surface temperature (SST) data described by Smith and Reynolds (2004) for the period 1854-2006 to examine historical temperature changes over the global ocean, after which they regressed vertical wind shear -- "calculated as the magnitude of the vector difference between winds at 200 mb and 850 mb during the Atlantic hurricane season (June to November), using NCEP-NCAR reanalysis data" -- onto a temporal variation of global warming defined by the SST data. This work led to their discovery that warming of the surface of the global ocean is typically associated with a secular increase of tropospheric vertical wind shear in the main development region (MDR) for Atlantic hurricanes, and that the long-term increased wind shear of that region has coincided with a weak but robust downward trend in U.S. landfalling hurricanes. However, this relationship has a pattern to it, whereby local ocean warming in the Atlantic MDR actually reduces the vertical wind shear there, while "warmings in the tropical Pacific and Indian Oceans produce an opposite effect, i.e., they increase the vertical wind shear in the MDR for Atlantic hurricanes."

In light of these findings, the two researchers conclude that "the tropical oceans compete with one another for their impacts on the vertical wind shear over the MDR for Atlantic hurricanes," and they say that to this point in time, "warmings in the tropical Pacific and Indian Oceans win the competition and produce increased wind shear which reduces U.S. landfalling hurricanes." As for the years and decades ahead, they write that "whether future global warming increases the vertical wind shear in the MDR for Atlantic hurricanes will depend on the relative role induced by secular warmings over the tropical oceans." Thus, it is by no means clear whether further global warming, due to any cause, will lead to an increase or decrease in U.S. landfalling hurricanes. All that can be said is that up to this point in time, global warming appears to have had a weak negative impact on their numbers.

Debate over an anthropogenic influence on Atlantic basin hurricane frequency has nevertheless continued. Acknowledging such, Vecchi and Knutson (2008) write that "there is currently disagreement within the hurricane/climate community on whether anthropogenic forcing (greenhouse gases, aerosols, ozone depletion, etc.) has caused an increase in Atlantic tropical storm or hurricane frequency." As their contribution to the subject, they derived an estimate of the expected number of North Atlantic tropical cyclones (TCs) that were missed by the observing system in the pre-satellite era (1878-1965), after which they analyzed trends of both reconstructed TC numbers and duration over various time periods and looked at how they may or may not have been related to trends in sea surface temperature over the main development region of North Atlantic TCs. The work revealed, in their words, that "the estimated trend for 1900-2006 is highly significant (+~4.2 storms century-1)," but they say that the trend "is strongly influenced by a minimum in 1910-30, perhaps artificially enhancing significance." When using their base case adjustment for missed TCs and considering the entire 1878-2006 record, for example, they find that the trend in the number of TCs is only "weakly positive" and "not statistically significant," while they note that the trend in average TC duration over the 1878-2006 period "is negative and highly significant."

Similar shortcomings in the observational record have been reported by other researchers. Writing as background for their study, for example, Landsea et al. (2010) note that "records of Atlantic basin tropical cyclones (TCs) since the late nineteenth century indicate a very large upward trend in storm frequency," and they say that this increase in documented TCs "has been previously interpreted as resulting from anthropogenic climate change." However, they go on to state that "improvements in observing and recording practices provide an alternative interpretation for these changes," and they report that "recent studies suggest that the number of potentially missed TCs is sufficient to explain a large part of the recorded increase in TC counts."

Against this backdrop, Landsea et al. explored the influence of another factor -- TC duration -- on observed changes in TC frequency, working with the widely-used Atlantic hurricane database known as HURDAT; and in doing so, they found that the occurrence of short-lived storms of two days duration or less had increased dramatically, from less than one per year in the late 19th and early 20th centuries to about five per year since about AD 2000, while medium- to long-lived storms had increased "little, if at all." Based on such findings, they concluded that the previously documented increase in total TC frequency since the late nineteenth century in the database was "primarily due to an increase in very short-lived TCs."

Shifting their focus to moderate and long-lived TCs, Landsea et al. next conducted a sampling study based on the distribution of ship observations, which provided quantitative estimates of the frequency of missed TCs with durations exceeding two days. And upon adding the estimated numbers of missed TCs to the time series of moderate and long-lived Atlantic TCs, they found that "neither time series exhibits a significant trend since the late nineteenth century." In fact, they report there was a nominal decrease in the adjusted time series.

In light of these several findings, Landsea et al. conclude that sub-sampling of TCs back in time will artificially introduce increases in a wide array of TC characteristics, including "frequency of hurricanes and major hurricanes, duration of TCs, length of season, peak intensity, and integrated TC measures [like Accumulated Cyclone Energy (ACE) and Power Dissipation Index (PDI)]," which they say "should not be used directly from HURDAT for climate variability and change studies without consideration of, or quantitatively accounting for, how observational network alterations are affecting these statistics."

In one final paper, Vecchi and Knutson (2011) conducted a new analysis of the characteristics of Atlantic hurricanes (tropical cyclones or TCs, whose peak winds exceeded 33 m/s) for the period 1878-2008, based on the widely-used HURDAT database, developing a new estimate of the number of hurricanes that occurred in the pre-satellite era (1878-1965), based on analyses of TC storm tracks and the geographical distribution of the tracks of the ships that reported TC encounters. In doing so, the two researchers report that "both the adjusted and unadjusted basin-wide hurricane data indicate the existence of strong interannual and decadal swings," and although they say that "existing records of Atlantic hurricanes show a substantial increase since the late 1800s," their analysis suggests that "this increase could have been due to increased observational capability." In fact, they say that "after adjusting for an estimated number of 'missed' hurricanes (including hurricanes that likely would have been miss-classified as tropical storms), the secular change since the late-nineteenth century in Atlantic hurricane frequency is nominally negative -- though not statistically significant." And given such findings, the two researchers from NOAA's Geophysical Fluid Dynamics Laboratory say that their results "do not support the hypothesis that the warming of the tropical North Atlantic due to anthropogenic greenhouse gas emissions has caused Atlantic hurricane frequency to increase."

As things stand currently, therefore, the search for global warming effects on Atlantic Ocean tropical cyclone frequency remains elusive, and it does so in spite of the fact that climate alarmists typically contend that 20th-century global warming was unprecedented over the past millennium or two.

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Last updated 30 January 2013