Tropical cyclones in the Atlantic Ocean

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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change

The IPCC contends that global warming is likely to increase the frequency and intensity of hurricanes. For example, it states “it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical sea surface temperatures” (IPCC, 2007-I, p. 15). However, numerous peer-reviewed studies suggest otherwise.

Contents

Intensity

Free et al. (2004) write that “increases in hurricane intensity are expected to result from increases in sea surface temperature and decreases in tropopause-level temperature accompanying greenhouse warming (Emanuel, 1987; Henderson-Sellers et al., 1998; Knutson et al., 1998),” but that “because the predicted increase in intensity for doubled CO2 is only 5%-20%, changes over the past 50 years would likely be less than 2%—too small to be detected easily.” They report that “studies of observed frequencies and maximum intensities of tropical cyclones show no consistent upward trend (Landsea et al., 1996; Henderson-Sellers et al., 1998; Solow and Moore, 2002),” and set out to find increases in what they call “potential” hurricane intensity, because, as they describe it, “changes in potential intensity (PI) can be estimated from thermodynamic principles as shown in Emanuel (1986, 1995) given a record of SSTs [sea surface temperatures] and profiles of atmospheric temperature and humidity.” Using radiosonde and SST data from 14 island radiosonde stations in the tropical Atlantic and Pacific Oceans, they compare their results with those of Bister and Emanuel (2002) at grid points near the selected stations. They report that their results “show no significant trend in potential intensity from 1980 to 1995 and no consistent trend from 1975 to 1995.” What is more, they report that between 1975 and 1980, “while SSTs rose, PI decreased, illustrating the hazards of predicting changes in hurricane intensity from projected SST changes alone.”

In the following year, some important new studies once again promoted the IPCC’s claim that warming would enhance tropical cyclone intensity (Emanuel, 2005; Webster et al., 2005), but a new review of the subject once again cast doubt on this contention. Pielke et al. (2005) began their discussion by noting that “globally there has been no increase in tropical cyclone frequency over at least the past several decades,” citing the studies of Lander and Guard (1998), Elsner and Kocher (2000) and Webster et al. (2005). They noted that research on possible future changes in hurricane frequency due to global warming has produced studies that “give such contradictory results as to suggest that the state of understanding of tropical cyclogenesis provides too poor a foundation to base any projections about the future.”

With respect to hurricane intensity, Pielke et al. noted that Emanuel (2005) claimed to have found “a very substantial upward trend in power dissipation (i.e., the sum over the life-time of the storm of the maximum wind speed cubed) in the North Atlantic and western North Pacific.” However, they report that “other studies that have addressed tropical cyclone intensity variations (Landsea et al., 1999; Chan and Liu, 2004) show no significant secular trends during the decades of reliable records.” In addition, they indicate that although early theoretical work by Emanuel (1987) “suggested an increase of about 10% in wind speed for a 2°C increase in tropical sea surface temperature,” more recent work by Knutson and Tuleya (2004) points to only a 5 percent increase in hurricane windspeeds by 2080, and that Michaels et al. (2005) conclude that even this projection is likely twice as great as it should be.

By 2050, Pielke et al. report that “for every additional dollar in damage that the Intergovernmental Panel on Climate Change expects to result from the effects of global warming on tropical cyclones, we should expect between $22 and $60 of increase in damage due to population growth and wealth,” citing the findings of Pielke et al. (2000) in this regard. Based on this evidence, they state without equivocation that “the primary factors that govern the magnitude and patterns of future damages and casualties are how society develops and prepares for storms rather than any presently conceivable future changes in the frequency and intensity of the storms.”

In concluding their review, Pielke et al. note that massive reductions of anthropogenic CO2 emissions “simply will not be effective with respect to addressing future hurricane impacts,” and that “there are much, much better ways to deal with the threat of hurricanes than with energy policies (e.g., Pielke and Pielke, 1997).”

Michaels et al. (2006) subsequently analyzed Emanuel’s (2005) and Webster et al.’s (2005) claims that “rising sea surface temperatures (SSTs) in the North Atlantic hurricane formation region are linked to recent increases in hurricane intensity, and that the trend of rising SSTs during the past 3 to 4 decades bears a strong resemblance to that projected to occur from increasing greenhouse gas concentrations.” The researchers used weekly averaged 1° latitude by 1° longitude SST data together with hurricane track data of the National Hurricane Center that provide hurricane-center locations (latitude and longitude in tenths of a degree) and maximum 1-minute surface wind speeds (both at six-hour intervals) for all tropical storms and hurricanes in the Atlantic basin that occurred between 1982 (when the SST dataset begins) through 2005. Plotting maximum cyclone wind speed against the maximum SST that occurred prior to (or concurrent with) the maximum wind speed of each of the 270 Atlantic tropical cyclones of their study period, they found that for each 1°C increase in SST between 21.5°C and 28.25°C, the maximum wind speed attained by Atlantic basin cyclones rises, in the mean, by 2.8 m/s, and that thereafter, as SSTs rise still further, the first category-3-or-greater storms begin to appear. However, they report “there is no significant relationship between SST and maximum winds at SST exceeding 28.25°C.”

From these observations, Michaels et al. conclude that “while crossing the 28.25°C threshold is a virtual necessity for attaining category 3 or higher winds, SST greater than 28.25°C does not act to further increase the intensity of tropical cyclones.” The comparison of SSTs actually encountered by individual storms performed by Michaels et al.—as opposed to the comparisons of Emanuel (2005) and Webster et al. (2005), which utilized basin-wide averaged monthly or seasonal SSTs—refutes the idea that anthropogenic activity has detectably influenced the severity of Atlantic basin hurricanes over the past quarter-century.

Simultaneously, Balling and Cerveny (2006) examined temporal patterns in the frequency of intense tropical cyclones (TCs), the rates of rapid intensification of TCs, and the average rate of intensification of hurricanes in the North Atlantic Basin, including the tropical and subtropical North Atlantic, Caribbean Sea, and Gulf of Mexico, where they say there was “a highly statistically significant warming of 0.12°C decade-1 over the period 1970-2003 … based on linear regression analysis and confirmed by a variety of other popular trend identification techniques.” In doing so, they found “no increase in a variety of TC intensification indices,” and that “TC intensification and/or hurricane intensification rates … are not explained by current month or antecedent sea surface temperatures (despite observed surface warming over the study period).” They concluded that “while some researchers have hypothesized that increases in long-term sea surface temperature may lead to marked increases in TC storm intensity, our findings demonstrate that various indicators of TC intensification show no significant trend over the recent three decades.”

Klotzbach and Gray (2006) note that still other papers question the validity of the findings of Emanuel (2005) and Webster et al. (2005) “due to potential bias-correction errors in the earlier part of the data record for the Atlantic basin (Landsea, 2005),” and that “while major hurricane activity in the Atlantic has shown a large increase since 1995, global tropical-cyclone activity, as measured by the accumulated cyclone energy index, has decreased slightly during the past 16 years (Klotzbach, 2006).” And as a result of these and other data and reasoning described in their paper, they “attribute the heightened Atlantic major hurricane activity of the 2004 season as well as the increased Atlantic major hurricane activity of the previous nine years to be a consequence of multidecadal fluctuations in the strength of the Atlantic multidecadal mode and strength of the Atlantic Ocean thermohaline circulation.” In this regard, they say “historical records indicate that positive and negative phases of the Atlantic multidecadal mode and thermohaline circulation last about 25-30 years (typical period ~50-60 years; Gray et al., 1997; Latif et al., 2004),” and “since we have been in this new active thermohaline circulation period for about 11 years, we can likely expect that most of the next 15-20 hurricane seasons will also be active, particularly with regard to increased major hurricane activity.”

Vecchi and Soden (2007a) explored twenty first century projected changes in vertical wind shear (VS) over the tropical Atlantic and its ties to the Pacific Walker circulation via a suite of coupled ocean-atmosphere models forced by emissions scenario A1B (atmospheric CO2 stabilization at 720 ppm by 2100) of the Intergovernmental Panel on Climate Change’s Fourth Assessment Report, where VS is defined as the magnitude of the vector difference between monthly mean winds at 850 and 200 hPa, and where changes are computed between the two 20-year periods 2001-2020 and 2081-2100. The 18-model mean result indicated a prominent increase in VS over the topical Atlantic and East Pacific (10°N-25°N). Noting that “the relative amplitude of the shear increase in these models is comparable to or larger than model-projected changes in other large-scale parameters related to tropical cyclone activity,” the two researchers went on to state that the projected changes “would not suggest a strong anthropogenic increase in tropical Atlantic or Pacific hurricane activity during the 21st Century,” and that “in addition to impacting cyclogenesis, the increase in SER [shear enhancement region] shear could act to inhibit the intensification of tropical cyclones as they traverse from the MDR [main development region] to the Caribbean and North America.” Consequently, and in addition to the growing body of empirical evidence that indicates global warming has little to no impact on the intensity of hurricanes (Donnelly and Woodruff, 2007; Nyberg et al., 2007), there is now considerable up-to-date model-based evidence for the same conclusion.

In a closely related paper, Vecchi and Soden (2007b) used both climate models and observational reconstructions “to explore the relationship between changes in sea surface temperature and tropical cyclone ‘potential intensity’—a measure that provides an upper bound on cyclone intensity and can also reflect the likelihood of cyclone development.” They found “changes in local sea surface temperature are inadequate for characterizing even the sign [our italics] of changes in potential intensity.” Instead, they report that “long-term changes in potential intensity are closely related to the regional structure of warming,” such that “regions that warm more than the tropical average are characterized by increased potential intensity, and vice versa.” Using this relationship to reconstruct changes in potential intensity over the twentieth century, based on observational reconstructions of sea surface temperature, they further found that “even though tropical Atlantic sea surface temperatures are currently at a historical high, Atlantic potential intensity probably peaked in the 1930s and 1950s,” noting that “recent values are near the historical average.” The two scientists’ conclusion was that the response of tropical cyclone activity to natural climate variations “may be larger than the response to the more uniform patterns of greenhouse-gas-induced warming.”

Also in the year 2007, and at the same time Vecchi and Soden were conducting their studies of the subject, Latif et al. (2007) were analyzing the 1851-2005 history of Accumulated Cyclone Energy (ACE) Index for the Atlantic basin, which parameter, in their words, “takes into account the number, strength and duration of all tropical storms in a season,” after which they “analyzed the results of an atmospheric general circulation model forced by the history of observed global monthly sea surface temperatures for the period 1870-2003.”

With respect to the first part of their study, they report that “the ACE Index shows pronounced multidecadal variability, with enhanced tropical storm activity during the 1890s, 1950s and at present, and mostly reduced activity in between, but no sustained long-term trend,” while with respect to the second part of their study, they report that “a clear warming trend is seen in the tropical North Atlantic sea surface temperature,” but that this warming trend “does not seem to influence the tropical storm activity.”

This state of affairs seemed puzzling at first, because a warming of the tropical North Atlantic is known to reduce vertical wind shear there and thus promote the development of tropical storms. However, Latif et al.’s modeling work revealed that a warming of the tropical Pacific enhances the vertical wind shear over the Atlantic, as does a warming of the tropical Indian Ocean. Consequently, they learned, as they describe it, that “the response of the vertical wind shear over the tropical Atlantic to a warming of all three tropical oceans, as observed during the last decades, will depend on the warming of the Indo-Pacific relative to that of the tropical North Atlantic,” and “apparently,” as they continue, “the warming trends of the three tropical oceans cancel with respect to their effects on the vertical wind shear over the tropical North Atlantic, so that the tropical cyclone activity [has] remained rather stable and mostly within the range of the natural multidecadal variability.”

Nevertheless, a striking exception to this general state of affairs occurred in 2005, when the researchers report that “the tropical North Atlantic warmed more rapidly than the Indo-Pacific,” which reduced vertical wind shear over the North Atlantic, producing the most intense Atlantic hurricane season of the historical record. By contrast, they say that the summer and fall of 2006 were “characterized by El Niño conditions in the Indo-Pacific, leading to a rather small temperature difference between the tropical North Atlantic and the tropical Indian and Pacific Oceans,” and they say that “this explains the weak tropical storm activity [of that year].”

Latif et al. say “the future evolution of Atlantic tropical storm activity will critically depend on the warming of the tropical North Atlantic relative to that in the Indo-Pacific region,” and “changes in the meridianal overturning circulation and their effect on tropical Atlantic sea surface temperatures have to be considered,” and that “changes in ENSO statistics in the tropical Pacific may become important.” Consequently, it is anyone’s guess as to what would actually occur in the real world if the earth were to experience additional substantial warming. However, since the global temperature rise of the twentieth century—which the IPCC contends was unprecedented over the past two millennia—did not lead to a sustained long-term increase in hurricane intensity, there is little reason to believe any further warming would do so.

In one final concurrent study, Scileppi and Donnelly (2007) note that “when a hurricane makes landfall, waves and storm surge can overtop coastal barriers, depositing sandy overwash fans on backbarrier salt marshes and tidal flats,” and that long-term records of hurricane activity are thus formed “as organic-rich sediments accumulate over storm-induced deposits, preserving coarse overwash layers.” Based on this knowledge, they refined and lengthened the hurricane record of the New York City area by first calibrating the sedimentary record of surrounding backbarrier environments to documented hurricanes—including those of 1893, 1821, 1788, and 1693—and then extracting several thousand additional years of hurricane history from this important sedimentary archive.

As a result of these efforts, the two researchers determined that “alternating periods of quiescent conditions and frequent hurricane landfall are recorded in the sedimentary record and likely indicate that climate conditions may have modulated hurricane activity on millennial timescales.” Of special interest in this regard, as they describe it, is the fact that “several major hurricanes occur in the western Long Island record during the latter part of the Little Ice Age (~1550-1850 AD) when sea surface temperatures were generally colder than present,” but that “no major hurricanes have impacted this area since 1893,” when the earth experienced the warming that took it from the Little Ice Age to the Current Warm Period.

Noting that Emanuel (2005) and Webster et al. (2005) had produced analyses that suggest that “cooler climate conditions in the past may have resulted in fewer strong hurricanes,” but that their own findings suggest just the opposite, Scileppe and Donnelly concluded that “other climate phenomena, such as atmospheric circulation, may have been favorable for intense hurricane development despite lower sea surface temperatures” prior to the development of the Current Warm Period.

Last, William M. Briggs (2008) developed Bayesian statistical models for the number of tropical cyclones, the rate at which these cyclones became hurricanes, and the rate at which the hurricanes became category 4+ storms in the North Atlantic, based on data from 1966 to 2006; this work led him to conclude that there is “no evidence that the distributional mean of individual storm intensity, measured by storm days, track length, or individual storm power dissipation index, has changed (increased or decreased) through time.”

In light of the many real-world observations (as well as certain modeling work) discussed above, it would appear that even the supposedly unprecedented global warming of the past century or more has not led to an increase in the intensity of Atlantic hurricanes.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/h/hurratlanintensity.php.

Frequency in the Past Few Millennia

Has the warming of the past century increased the yearly number of intense Atlantic Basin hurricanes? We offer a brief review of some studies that have explored this question via thousand-year reconstructions of the region’s intense hurricane activity.

Liu and Fearn (1993) analyzed sediment cores retrieved from the center of Lake Shelby in Alabama (USA) to determine the history of intense (category 4 and 5) hurricane activity there over the past 3,500 years. This work revealed that over the period of their study, “major hurricanes of category 4 or 5 intensity directly struck the Alabama coast … with an average recurrence interval of ~600 years.” They also report that the last of these hurricane strikes occurred about 700 years ago. Hence, it would appear that twentieth century global warming has not accelerated the occurrence of such severe storm activity.

Seven years later, Liu and Fearn (2000) conducted a similar study based on 16 sediment cores retrieved from Western Lake, Florida (USA), which they used to produce a proxy record of intense hurricane strikes for this region of the Gulf of Mexico that covered the past 7,000 years. In this study, 12 major hurricanes of category 4 or 5 intensity were found to have struck the Western Lake region. Nearly all of these events were centered on a 2,400-year period between 1,000 and 3,400 years ago, when 11 of the 12 events were recorded. In contrast, between 0 to 1,000 and 3,400 to 7,000 years ago, only one and zero major hurricane strikes were recorded, respectively. According to the two researchers, a probable explanation for the “remarkable increase in hurricane frequency and intensity” that affected the Florida Panhandle and the Gulf Coast after 1400 BC would have been a continental-scale shift in circulation patterns that caused the jet stream to shift south and the Bermuda High southwest of their earlier Holocene positions, such as would be expected with global cooling, giving strength to their contention that “paleohurricane records from the past century or even the past millennium are not long enough to capture the full range of variability of catastrophic hurricane activities inherent in the Holocene climatic regime.”

Last, we have the study of Donnelly and Woodruff (2007), who state that “it has been proposed that an increase in sea surface temperatures caused by anthropogenic climate change has led to an increase in the frequency of intense tropical cyclones,” citing the studies of Emanuel (2005) and Webster et al. (2005). Donnelly and Woodruff developed “a record of intense [category 4 and greater] hurricane activity in the western North Atlantic Ocean over the past 5,000 years based on sediment cores from a Caribbean lagoon [Laguna Playa Grande on the island of Vieques, Puerto Rico] that contains coarse-grained deposits associated with intense hurricane landfalls.”

Based on this work, the two researchers from the Woods Hole Oceanographic Institution detected three major intervals of intense hurricane strikes: one between 5,400 and 3,600 calendar years before present (yr BP, where “present” is AD 1950), one between 2,500 and 1,000 yr BP, and one after 250 yr BP. They also report that coral-based sea surface temperature (SST) data from Puerto Rico “indicate that mean annual Little Ice Age (250-135 yr BP or AD 1700-1815) SSTs were 2-3°C cooler than they are now,” and they say that “an analysis of Caribbean hurricanes documented in Spanish archives indicates that 1766-1780 was one of the most active intervals in the period between 1500 and 1800 (Garcia-Herrera et al., 2005), when tree-ring-based reconstructions indicate a negative (cooler) phase of the Atlantic Multidecadal Oscillation (Gray et al., 2004).”

In light of these findings, Donnelly and Woodruff concluded that “the information available suggests that tropical Atlantic SSTs were probably not the principal driver of intense hurricane activity over the past several millennia.” Indeed, there is no compelling reason to believe that the current level of intense hurricane activity is in any way unprecedented or that it has been caused by global warming. Quite to the contrary, the two researchers write that “studies relying on recent climatology indicate that North Atlantic hurricane activity is greater during [cooler] La Niña years and suppressed during [warmer] El Niño years (Gray, 1984; Bove et al., 1998), due primarily to increased vertical wind shear in strong El Niño years hindering hurricane development.”

In summary, millennial-scale reconstructions of intense hurricane activity within the Atlantic Basin provide no support for the claim that global warming will lead to the creation of more intense Atlantic hurricanes that will batter the east, southeast, and southern coasts of the United States. In fact, they suggest just the opposite.

Frequency in the Past Few Centuries

Has the warming of the past century, which rescued the world from the extreme cold of the Little Ice Age, led to the formation of more numerous Atlantic Basin tropical storms and hurricanes? We review 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; 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 lower frequencies in the last half of the twentieth century than they did in the preceding five 50-year periods, 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; 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.

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 seventeenth and eighteenth centuries than in the nineteenth and twentieth 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.” Confining ourselves to the better records of the past 200 years, we note that the cooler nineteenth century had five of the highest-damage storms, while the warmer twentieth 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 percent of all major hurricanes and 60 percent 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.” They conclude that the recent ratcheting up of Atlantic major hurricane activity appears to be simply “a recovery to normal hurricane activity.” 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.”

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 twentieth century global warming might be influencing these storms, 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, seven 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 nineteenth century and the late twentieth 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.” He also concludes that “yet unanswered is whether a warmer global climate of the future will take hurricane activity beyond what has been experienced in the observed record.”

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.” We also observe that the new record clearly indicates that the planet’s current high levels of air temperature and CO2 concentration cannot be blamed for the 2005 Katrina catastrophe, as both parameters were much lower when tropical cyclone and hurricane activity in that region were much higher in the early- to mid-1800s.

Around the same time, 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.”

Vecchi and Knutson (2008) write in the introduction to their study of the subject 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.” In further exploring this question, 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. This 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, 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.”

Elsner (2008), in his summary of the International Summit on Hurricanes and Climate Change held in May 2007 on the Greek island of Crete, said 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” provides evidence that “today’s warmth is not needed for increased storminess.”

In conclusion, the bulk of the evidence that has been accumulated to date over multi-century timescales indicates that late twentieth century yearly hurricane numbers were considerably lower than those observed in colder prior centuries. It is by no means clear that further global warming, due to any cause, would lead to an increase or decrease in U.S. landfalling hurricanes. All we can say is that up to this point in time, global warming appears to have had a weak negative impact on their numbers.

Frequency in the Past Century

Have tropical storms and hurricanes of the Atlantic Ocean become more numerous over the past century, in response to what the IPCC describes as unprecedented global warming? This became a matter of intense speculation following a spike of storm occurrences in 2004-2005, but once again it is instructive to approach the question by starting with the findings of earlier research.

Bove et al. (1998) examined the characteristics of all recorded landfalling U.S. Gulf Coast hurricanes—defined as those whose eyes made landfall between Cape Sable, Florida and Brownsville, Texas—from 1896 to 1995. They found that the first half of this period saw considerably more hurricanes than the last half: 11.8 per decade vs. 9.4 per decade. The same was true for intense hurricanes of category 3 or more on the Saffir-Simpson storm scale: 4.8 vs. 3.6. The numbers of all hurricanes and the numbers of intense hurricanes both tended downward from 1966 to the end of the period investigated, with the decade 1986-1995 exhibiting the fewest intense hurricanes of the entire century. The three researchers concluded that “fears of increased hurricane activity in the Gulf of Mexico are premature.”

Noting that the 1995 Atlantic hurricane season was one of near-record tropical storm and hurricane activity, but that during the preceding four years (1991-94) such activity over the Atlantic basin was the lowest since the keeping of reliable records began in the mid-1940s, Landsea et al. (1998) studied the meteorological characteristics of the two periods to determine what might have caused the remarkable upswing in storm activity in 1995. In doing so, they found that “perhaps the primary factor for the increased hurricane activity during 1995 can be attributed to a favorable large-scale pattern of extremely low vertical wind shear throughout the main development region.” They also noted that “in addition to changes in the large-scale flow fields, the enhanced Atlantic hurricane activity has also been linked to below-normal sea-level pressure, abnormally warm ocean waters, and very humid values of total precipitable water.”

An additional factor that may have contributed to the enhanced activity of the 1995 Atlantic hurricane season was the westerly phase of the stratospheric quasi-biennial oscillation, which is known to enhance Atlantic basin storm activity. Possibly the most important factor of all, however, was what Landsea et al. called the “dramatic transition from the prolonged late 1991-early 1995 warm episode (El Niño) to cold episode (La Niña) conditions,” which contributed to what they described as “the dramatic reversal” of weather characteristics “which dominated during the [prior] four hurricane seasons.”

“Some have asked,” in the words of the four researchers, “whether the increase in hurricanes during 1995 is related to the global surface temperature increases that have been observed over the last century, some contribution of which is often ascribed to increases in anthropogenic ‘greenhouse’ gases.” In reply, they stated that “such an interpretation is not warranted,” because the various factors noted above seem sufficient to explain the observations. “Additionally,” as they further wrote, “Atlantic hurricane activity has actually decreased significantly in both frequency of intense hurricanes and mean intensity of all named storms over the past few decades,” and “this holds true even with the inclusion of 1995’s Atlantic hurricane season.”

In a major synthesis of Atlantic basin hurricane indices published the following year, Landsea et al. (1999) reported long-term variations in tropical cyclone activity for this region (North Atlantic Ocean, Gulf of Mexico, and Caribbean Sea). Over the period 1944-1996, decreasing trends were found for (1) the total number of hurricanes, (2) the number of intense hurricanes, (3) the annual number of hurricane days, (4) the maximum attained wind speed of all hurricane storms averaged over the course of a year, and (5) the highest wind speed associated with the strongest hurricane recorded in each year. In addition, they reported that the total number of Atlantic hurricanes making landfall in the United States had decreased over the 1899-1996 time period, and that normalized trends in hurricane damage in the United States between 1925 and 1996 revealed such damage to be decreasing at a rate of $728 million per decade.

In a similar study that included a slightly longer period of record (1935-1998), Parisi and Lund (2000) conducted a number of statistical tests on all Atlantic Basin hurricanes that made landfall in the contiguous United States, finding that “a simple linear regression of the yearly number of landfalling hurricanes on the years of study produces a trend slope estimate of -0.011 ± 0.0086 storms per year.” To drive home the significance of that result, they expressly called attention to the fact that “the estimated trend slope is negative,” which means, of course, that the yearly number of such storms is decreasing, which is just the opposite of what they described as the “frequent hypothesis … that global warming is causing increased storm activity.” Their statistical analysis indicates that “the trend slope is not significantly different from zero.”

Contemporaneously, Easterling et al. (2000) noted that the mean temperature of the globe rose by about 0.6°C over the past century, and they thus looked for possible impacts of this phenomenon on extreme weather events, which if found to be increasing, as they describe it, “would add to the body of evidence that there is a discernable human affect on the climate.” Their search, however, revealed few changes of significance, although they did determine that “the number of intense and landfalling Atlantic hurricanes has declined.”

Lupo and Johnston (2000) found “there has been relatively little trend in the overall occurrence of hurricanes within the Atlantic Ocean Basin (62 year period),” reflecting an upward trend in category 1 hurricanes which is countered by downward or weak trends in the occurrence of category 2-5 hurricanes. Stratifying by hurricane genesis region indicated the tendency for more hurricanes to form in La Niña years during PDO1 (1977-1999) was strongly influenced by more storms being generated in the Caribbean and Eastern Atlantic. Only two storms formed in these regions during El Niño years. During PDO2 (1947-1976) there was a weak tendency for more (fewer) storms forming in the Gulf and Caribbean (West and East Atlantic) sub-regions during La Niña years, while the reverse occurred for El Niño years.

Three years later, Balling and Cerveny (2003) wrote that “many numerical modeling papers have appeared showing that a warmer world with higher sea surface temperatures and elevated atmospheric moisture levels could increase the frequency, intensity, or duration of future tropical cyclones,” but that empirical studies had failed to reveal any such relationships. They also noted that “some scientists have suggested that the buildup of greenhouse gases can ultimately alter other characteristics of tropical cyclones, ranging from timing of the active season to the location of the events,” and that these relationships have not been thoroughly studied with historical real-world data. They proceeded to fill this void by conducting such a study for tropical storms in the Caribbean Sea, the Gulf of Mexico, and the western North Atlantic Ocean.

More specifically, the two Arizona State University climatologists constructed a daily database of tropical storms that occurred within their study area over the period 1950-2002, generating “a variety of parameters dealing with duration, timing, and location of storm season,” after which they tested for trends in these characteristics, attempting to explain the observed variances in the variables using regional, hemispheric, and global temperatures. In doing so, they “found no trends related to timing and duration of the hurricane season and geographic position of storms in the Caribbean Sea, Gulf of Mexico and tropical sector of the western North Atlantic Ocean.” Likewise, they said they “could find no significant trends in these variables and generally no association with them and the local ocean, hemispheric, and global temperatures.”

Elsner et al. (2004) conducted a changepoint analysis of time series of annual major North Atlantic hurricane counts and annual major U.S. hurricane counts for the twentieth century, which technique, in their words, “quantitatively identifies temporal shifts in the mean value of the observations.” This work revealed that “major North Atlantic hurricanes have become more frequent since 1995,” but at “a level reminiscent of the 1940s and 1950s.” In actuality, however, they had not quite reached that level, nor had they maintained it for as long a time. Their data indicate that the mean annual hurricane count for the seven-year period 1995-2001 was 3.86, while the mean count for the 14-year period 1948-1961 was 4.14. They also reported that, “in general, twentieth-century U.S. hurricane activity shows no abrupt shifts,” noting, however, that there was an exception over Florida, “where activity decreased during the early 1950s and again during the late 1960s.” Last, they found that “El Niño events tend to suppress hurricane activity along the entire coast with the most pronounced effects over Florida.”

In contradiction of the IPCC’s claim that global warming leads to more intense hurricane activity, the results of Elsner et al.’s study found that not only did North Atlantic hurricane activity not increase over the entire twentieth century, hurricane activity also did not increase in response to the more sporadic warming associated with periodic El Niño conditions.

Two years later, things got a bit more interesting. “The 2005 hurricane season,” in the words of Virmani and Weisberg (2006), “saw an unprecedented number of named tropical storms since records began in 1851.” Moreover, they said it followed “on the heels of the unusual 2004 hurricane season when, in addition to the first South Atlantic hurricane, a record-breaking number of major hurricanes made landfall in the United States, also causing destruction on the Caribbean islands in their path.” The question they thus posed was whether these things occurred in response to recent global warming or if they bore sufficient similarities with hurricane seasons of years past to preclude such an attribution.

The two researchers determined that “latent heat loss from the tropical Atlantic and Caribbean was less in late spring and early summer 2005 than preceding years due to anomalously weak trade winds associated with weaker sea-level pressure,” which phenomenon “resulted in anomalously high sea surface temperatures” that “contributed to earlier and more intense hurricanes in 2005.” However, they went on to note that “these conditions in the Atlantic and Caribbean during 2004 and 2005 were not unprecedented and were equally favorable during the active hurricane seasons of 1958, 1969, 1980, 1995 and 1998.” In addition, they said there was “not a clear link between the Atlantic Multidecadal Oscillation or the long term trend [of temperature] and individual active hurricane years, confirming the importance of other factors in hurricane formation.”

The following year, Mann and Emanuel (2006) used quantitative records stretching back to the mid-nineteenth century to develop a positive correlation between sea surface temperatures and Atlantic basin tropical cyclone frequency for the period 1871-2005, while Holland and Webster (2007) had analyzed Atlantic tropical cyclone frequency back to 1855 and found a doubling of the number of tropical cyclones over the past 100 years. Both of these papers linked these changes to anthropogenic greenhouse warming. In a compelling rebuttal of those conclusions, however, Landsea (2007) cited a number of possible biases that may exist in the cyclone frequency trends derived in the two studies, concluding that “improved monitoring in recent years is responsible for most, if not all, of the observed trend in increasing frequency of tropical cyclones.”

Parisi and Lund (2008) calculated return periods of Atlantic-basin U.S. landfalling hurricanes based on “historical data from the 1900 to 2006 period via extreme value methods and Poisson regression techniques” for each of the categories (1-5) of the Saffir-Simpson Hurricane Scale. This work revealed that return periods (in years) for these hurricanes were, in ascending Saffir-Simpson Scale category order: (1) 0.9, (2) 1.3, (3) 2.0, (4) 4.7, and (5) 23.1. In addition, the two researchers reported that corresponding non-encounter probabilities in any one hurricane season were calculated to be (1) 0.17, (2) 0.37, (3) 0.55, (4) 0.78, and (5) 0.95. They stated that the hypothesis that U.S. hurricane strike frequencies are “increasing in time” is “statistically rejected.”

Lupo et al. (2008) added data for seven more years to the data originally analyzed by Lupo and Johnston (2000) and found it “did not change the major findings.” The authors hypothesized that the Atlantic hurricane season of 2005 was so active, not only because of the recent increase in hurricane activity which may be associated with the PDO, but also possibly due to decreased upper tropospheric shear over the Atlantic which may have been associated with a stronger easterly phase of the quasi-biennial oscillation along with warmer-than-normal SSTs.

In light of the long history of multi-decadal to century-scale analyses that have come to the same conclusion, we must reject the oft-heard claim that Atlantic hurricanes have increased in frequency in response to twentieth century global warming. In fact, since 2005, a major hurricane has not directly struck the U.S., despite the IPCC’s prediction that CO2-induced global warming will cause more frequent and severe hurricanes to strike America’s Atlantic coast. NOAA researchers Wang et al. (2011) analyzed the Atlantic Warming Pool (AWP) history and found that, while hurricanes are more frequent when the AWP is larger, these hurricanes are generally formed farther from shore and move northward into the open Atlantic, avoiding the U.S. mainland.

The El Niño Effect

How does the frequency of Atlantic basin hurricanes respond to increases in ocean temperature? In exploring this important question one has to look not only at Atlantic Ocean temperatures, but also those in the eastern tropical Pacific, in particular during La Niña and El Niño conditions. Wilson (1999) utilized data from the last half of the twentieth century to determine that the probability of having three or more intense Atlantic hurricanes was only 14 percent during an El Niño year (warm temperatures in the eastern tropical Pacific), but fully 53 percent during a La Niña year (cold ocean temperatures in the eastern tropical Pacific). When ocean temperatures warm in the eastern tropical Pacific, they cause stronger upper level winds in the tropical Atlantic and a greater likelihood that storms would become sheared, and hence weaker. The opposite (weaker upper level winds) occurs during La Niña years.

Muller and Stone (2001) conducted a similar study of tropical storm and hurricane strikes along the southeast U.S. coast from South Padre Island (Texas) to Cape Hatteras (North Carolina), using data from the entire past century. For tropical storms and hurricanes together, they found an average of 3.3 strikes per La Niña season, 2.6 strikes per neutral season, and 1.7 strikes per El Niño season. For hurricanes alone, the average rate of strike occurrence ranged from 1.7 per La Niña season to 0.5 per El Niño season, which represents a frequency-of-occurrence decline of fully 70 percent in going from cooler La Niña conditions to warmer El Niño conditions. Likewise, Elsner et al. (2001)—who also worked with data from the entire past century—found that when there are below normal sea surface temperatures in the equatorial Pacific, “the probability of a U.S. hurricane increases.”

Lyons (2004) also conducted a number of analyses of U.S. landfalling tropical storms and hurricanes, dividing them into three different groupings: the 10 highest storm and hurricane landfall years, the nine lowest such years, and all other years. These groupings revealed, in Lyons’ words, that “La Niña conditions occurred 19% more often during high U.S. landfall years than during remaining years,” and that “El Niño conditions occurred 10% more often during low U.S. landfall years than during remaining years.” In addition, it was determined that “La Niña (El Niño) conditions were 18% (25%) more frequent during high (low) U.S. landfall years than during low (high) U.S. landfall years.”

An analogous approach was used by Pielke and Landsea (1999) to study the effect of warming on the intensity of Atlantic basin hurricanes, using data from the period 1925 to 1997. In their analysis, they first determined that 22 years of this period were El Niño years, 22 were La Niña years, and 29 were neither El Niño nor La Niña years. Then, they compared the average hurricane wind speed of the cooler La Niña years with that of the warmer El Niño years, finding that in going from the cooler climatic state to the warmer climatic state, average hurricane wind speed dropped by about 6 meters per second.

Independent confirmation of these findings was provided by Pielke and Landsea’s assessment of concurrent hurricane damage in the United States: El Niño years experienced only half the damage of La Niña years. And in a 10-year study of a Mediterranean waterbird (Cory’s Shearwater) carried out on the other side of the Atlantic, Brichetti et al. (2000) determined—contrary to their own expectation—that survival rates during warmer El Niño years were greater than during cooler La Niña years.

In another pertinent study, Landsea et al. (1998) analyzed the meteorological circumstances associated with the development of the 1995 Atlantic hurricane season, which was characterized by near-record tropical storm and hurricane activity after four years (1991-94) that had exhibited the lowest such activity since the keeping of reliable records began. They determined that the most important factor behind this dramatic transition from extreme low to extreme high tropical storm and hurricane activity was what they called the “dramatic transition from the prolonged late 1991-early 1995 warm episode (El Niño) to cold episode (La Niña) conditions.”

Last, in a twentieth century changepoint analysis of time series of major North Atlantic and U.S. annual hurricane counts, which in the words of its authors, “quantitatively identifies temporal shifts in the mean value of the observations,” Elsner et al. (2004) found that “El Niño events tend to suppress hurricane activity along the entire coast with the most pronounced effects over Florida.”

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