Summaries of Recent Hurricane Research

This is a direct copy of the list of papers from this page: http://www.edf.org/page.cfm?tagid=654

I advise you to go there instead of reading here, because it seems as if it is being updated once in a while. It is just here in case the page gets deleted.

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The paper summaries

Emanuel, K. 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436: 686-688.
Abstract | Full text [PDF]

Contribution: Presents data showing that the intensity (or destructive potential) of hurricanes has doubled over the past 30 years and is linked to rising sea surface temperatures (caused by global warming).

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Details: Kerry Emanuel (from the Massachusetts Institute of Technology) examined 55 years of data from the North Atlantic and North Pacific and found a correlation between sea surface temperatures and the destructive potential of hurricanes. His analysis showed that the destructive potential of hurricanes—defined by a storm's wind speed and duration—has approximately doubled over the past 30 years. In both ocean regions, there is a close relationship between water temperatures and hurricane strength. In other words, when sea surface temperatures were cooler, hurricanes had less destructive potential; when sea surface temperatures were warmer, hurricanes had greater destructive potential. Starting in about 1975, sea surface temperatures in the North Atlantic and North Pacific began to increase dramatically, and the destructive potential of hurricanes followed suit.

Webster, PJ, GJ Holland, JA Curry & H-R Chang. 2005. Changes in tropical cyclone number, duration, and intensity in a warming environment. Science 309: 1844-1846.
Abstract | Full text [PDF]

Contribution: Expands analyses to include all of the world's hurricane basins; their results strengthen the link between global warming and hurricane intensity.

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Details: Peter Webster and his colleagues (from the Georgia Institute of Technology) assembled a 35-year record of hurricane intensities and sea surface temperatures in all six hurricane regions of the world (North Atlantic, the western North Pacific, the eastern North Pacific, the South Indian, the North Indian, and the Southwest Pacific). Their analysis showed that, worldwide, the total number of hurricanes per year has not changed. However, there has been a large global increase in the percentage of storms that reached Category 4 or 5. In the early 1970s, 20% of the world’s hurricanes reached Category 4 or 5, but by the 1990s, about 35% of all storms reached these levels. This increase in storm intensity accompanied a steady global rise in sea surface temperatures.

It has been argued that the only reason the U.S. has seen a recent spate of really bad hurricanes is that the North Atlantic is reaching the peak of a normal regional cycle of hurricane intensity. However, this study shows a worldwide increase in storm intensity; the number and percentage of Category 4 and 5 hurricanes has increased in all of the ocean basins around the world. (In fact, of all these regions, the North Atlantic had the smallest increase in intense hurricanes.) This suggests that the trend is global in nature and not part of a multi-decadal natural cycle.

Hoyos, CD, PA Agudelo, PJ Webster & JA Curry. 2006. Deconvolution of the factors contributing to the increase in global hurricane intensity. Science 312: 94-97.
Abstract | Full text [PDF]

Contribution: Brings rigorous statistical analysis to global datasets; their results indicate that rising sea surface temperature is the only environmental factor that explains rising hurricane intensities around the globe.

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Details: The strong relationship between hurricane intensity and sea surface temperature suggests a link between global warming and hurricanes. But is sea surface temperature really the cause, or is the relationship a coincidence? Carlos Hoyos and colleagues (at the Georgia Institute of Technology) compiled 35 years of detailed tropical storm and hurricane data from all six hurricane regions of the world to determine the statistical linkage between the occurrence of the most intense hurricanes (Category 4 or 5) and the relevant environmental factors that are thought to influence hurricane intensity; i.e., sea surface temperature, humidity, wind shear, and zonal stretching deformation. Their results showed that sea surface temperature is the only factor that can explain the global increase in the number of Category 4 and 5 hurricanes over the past 35 years. The other factors contributed to short-term (but not long-term) and regional (but not global) patterns of hurricane intensity. Since increasing sea surface temperatures are driven by global warming, this study adds even more evidence that global warming is causing stronger hurricanes.

Michaels, PJ, PC Knappenberger & RE Davis. 2006. Sea-surface temperatures and tropical cyclones in the Atlantic basin. Geophysical Research Letters 33: L09708.
Abstract | Full text [PDF]

Contribution: Despite statements to the contrary, their study of hurricanes in the Atlantic confirms earlier results—rising sea surface temperatures cause more intense hurricanes.

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Details: Patrick Michaels and his colleagues (at the University of Virginia) focused on the Atlantic basin and used 24 years of data to examine the complex relationship between sea surface temperatures and hurricane wind speeds. They found that warmer waters had hurricanes with greater maximum wind speeds, although the maximum winds speeds did not seem to increase any further at the highest temperatures observed. They concluded that "rising sea surface temperature will act to increase the percentage of major hurricanes but not the ultimate intensity of these storms." Their analyses also suggested that there may be a temperature tipping point for major hurricane development—that is, water temperatures must be above 83 degrees Fahrenheit for hurricanes to jump to Category 3 or higher. If true, this result adds further evidence that global warming is likely to affect hurricane intensity.

Although the authors state that their results "weaken the notion of a simple cause-and-effect relationship between rising sea surface temperatures and stronger Atlantic hurricanes," their study in fact concludes that there is a direct linkage between the two and, thus, that global warming acts to increase the intensity of future hurricanes. Furthermore, while Michaels et al. argue that the impact on hurricane intensity from the current amount of global warming should be "too small to reliably measure," their approach, based on an analysis of individual storms (instead of annual averages), is not well-suited to identifying long-term trends.

Klotzbach, PJ. 2006. Trends in global tropical cyclone activity over the past twenty years (1986-2005). Geophysical Research Letters 33: L10805.
Abstract | Full text [PDF]

Contribution: Despite statements to the contrary, results are largely consistent with those of Webster et al.—the number and percentage of the most intense storms have increased.

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Details: Phillip Klotzbach (Colorado State University) revisited the question of whether hurricanes are becoming more intense. He restricted his study to a 20-year period—divided into two 10-year blocks (1986-1995 and 1996-2005)—and then compared hurricane activity between blocks. Even with these limitations, his result—that the total number of Category 4 and 5 hurricanes increased 10% between time periods—is not inconsistent with the results of previous studies (e.g., Webster, et al.). When Webster et al.’s more detailed data are grouped in the same way (i.e., lumped into two 10-year blocks), the resulting increase in hurricane intensity is the same as in Klotzbach’s study. The percent increase is smaller using the Klotzbach approach because the coarse 10-year resolution obscures the large increase in hurricane intensity that occurred in the late 1980s and early 1990s. This and the fact that Klotzbach chose to omit 15 years of data (from 1970-1985) obscure the size of the long-term trend. Klotzbach’s other result, that there has been no change in the total number of storms, is consistent with earlier studies.

Sriver, R & M Huber. 2006. Low frequency variability in globally integrated tropical cyclone power dissipation. Geophysical Research Letters 33: L11705, doi:10.1029/2006GL026167.
Abstract | Article (subscription required)

Contribution: Using a different dataset to examine global hurricane intensity, the researchers confirm earlier findings that storms are stronger.

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Details: Ryan Sriver and Matthew Huber of Purdue University examined over 40 years of storm wind speed data. (This information came from the European Centre for Medium-Range Weather Forecasts Reanalysis Project, not the Joint Typhoon Warning Center and the National Hurricane Center, which Kerry Emanuel, Peter Webster and colleagues used for their earlier studies.) Sriver and Huber used the wind speeds to calculate the power dissipation (strength) of storms in all of the world’s hurricane basins (not just the North Atlantic and North Pacific as in Emanuel’s study). Even though the data for the studies came from different sources, the results were essentially the same: hurricane intensity has increased over the past few decades, globally as well as in the Atlantic and Pacific basins. In fact, Sriver and Huber's work suggests that Emanuel may have underestimated the size of the increase.

Mann, ME & KA Emanuel. 2006. Atlantic hurricane trends linked to climate change. EOS 87 (24): 233-244. 
Full text [PDF]

Contribution: Finds that global warming and aerosol pollution, not natural regional cycles, have been responsible for the long-term sea surface temperature variations in the Atlantic hurricane region.

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Details: Rising sea surface temperatures are increasing hurricane intensity. How much of this effect is due to global warming and how much of it is due to natural variability? Mann and Emanuel (from Penn State and MIT, respectively) tackled this question with a statistical analysis of sea surface temperature data from the past 130 years, focusing on the Atlantic hurricane region. They first compared sea surface temperatures in the Atlantic to the global average sea surface temperature and found that Atlantic temperatures closely followed the global trend. The match wasn’t perfect, however, meaning that global warming had most, but not all, of the influence on rising sea surface temperatures in the Atlantic. The remaining influence—particularly a slight Atlantic cooling between about 1950 and 1980—appeared initially to belong to the Atlantic Multi-decadal Oscillation (AMO), a long-term sea surface temperature cycle that has been blamed for recent increases in Atlantic hurricane intensity. However, when Mann and Emanuel added to their calculations the known cooling effect of aerosol pollution during the late 20th century, the effect of the AMO virtually disappeared. Mann and Emanuel concluded that "there is no evidence that a natural climate oscillation such as the AMO contributes to long-term tropical North Atlantic sea surface temperature variations."

Trenberth, KE & DJ Shea. 2006. Atlantic hurricanes and natural variability in 2005. Geophysical Research Letters 33: L12704.
Abstract | Full text [PDF]

Contribution: Shows that half of the unusual warmth in sea surface temperatures during the record 2005 Atlantic hurricane season was due to global warming.

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Details: Kevin Trenberth and Dennis Shea (from the National Center for Atmospheric Research) rendered the debate on whether hurricane intensity has increased or not largely moot from a policy perspective by showing that global warming played a major role in fomenting the record-breaking hurricane season of 2005. They did this by identifying the importance of the various factors that could have contributed to the Atlantic’s extraordinary warmth in 2005, that in turn led to the worst Atlantic hurricane season on record. They took 130 years of sea surface temperature data (both North Atlantic and global average temperatures) and subtracted the global trend from the Atlantic temperature record to reveal the Atlantic Multi-decadal Oscillation (AMO). The amount of warming in 2005 that was due to the AMO alone was very small—10% or less—whereas global warming contributed approximately half of the extra heat. Trenberth and Shea also used previously described relationships between Pacific and Atlantic sea surface temperatures during El Niño years to determine that 25% of the warming in 2005 was due to heat associated with the 2004-2005 El Niño. (The remainder of the warming is associated with random year-to-year variability.) In their conclusion, the authors point out that global warming "is guaranteed to continue" and that, even though not every season will be as extreme as 2005, global warming "provides a new background level that increases the risk of future enhanced [hurricane] activity."

Landsea, CW, BA Harper, K Hoarau & JA Knaff. 2006. Can we detect trends in extreme tropical cyclones? Science 313: 452-454.
Abstract | Full text [PDF]

Contribution: Questions the validity of the work of Emanuel and Webster et al. by arguing that the pre-1990 hurricane database is not sufficiently reliable to infer multi-decadal trends in hurricane intensity.

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Details: In a Science Perspectives piece, Christopher Landsea and his co-authors reviewed the methodologies and protocols used to monitor and document hurricane characteristics over each of the major ocean basins since the early 1970’s. They argue that because of uncertainties in the methodologies used in the early part of the record as well as operational changes on the methods used, the hurricane database is not reliable enough to accurately discern a trend in hurricane intensity over this period. Regions primarily dependent upon satellite imagery for their hurricane data are likely, they argue, to have an artificial upward trend in hurricane intensity.  The authors suggest that efforts to reanalyze the data from earlier measurements may remove some artifacts and make for more reliable trend analysis. (Read Environmental Defense commentary on this paper.)

Elsner, JB. 2006. Evidence in support of the climate change-Atlantic hurricane hypothesis. Geophysical Research Letters 33: L16705.
Full text [PDF]

Contribution: Shows that global air temperatures predict Atlantic sea surface temperatures after a lag time of one to nine years.

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Details: James Elsner, from Florida State University, applied a statistical causality analysis to a data set consisting of global mean air temperatures, Atlantic sea surface temperatures and hurricane positions and winds from 135 consecutive hurricane seasons from 1871-2005. He wanted to test two competing hypotheses:

  1. Rising air temperatures due to global warming cause increased Atlantic sea surface temperatures, which in turn contribute to more intense hurricanes; or
  2. Both global air temperatures and increasing hurricane intensity are caused by natural, cyclical fluctuations in Atlantic sea surface temperatures related to the Atlantic Multidecadal Oscillation.

Elsner’s analysis indicated that the first hypothesis held true. Specifically, he found that global warming-caused increases in global air temperatures have played a direct role in increasing Atlantic sea surface temperatures (with a lag time of one to nine years) and that increases in Atlantic sea surface temperatures play a direct role in increasing hurricane intensity.

Curry, JA, PJ Webster & GJ Holland. 2006. Mixing politics and science in testing the hypothesis that greenhouse warming is causing a global increase in hurricane intensity. BAMS (August 2006): 1025-1037.
Full text [PDF]

Contribution: Judith Curry and her colleagues at Georgia Tech and the National Center for Atmospheric Research review the current scientific debate about the link between hurricanes and global warming.

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Details: They discuss the appropriate procedures for testing the complex "causal chain" linking global warming to hurricane intensity. They also highlight the valid scientific questions that have been raised concerning recent research on this topic. Finally, Curry et al. identify the criticisms that are scientifically and/or logically flawed and thus should be excluded from the on-going debate.

Kafatos, M, D Sun, R Gautam, Z Boybeyi, R Yang & G Cervone. 2006. Role of anomalous warm gulf waters in the intensification of Hurricane Katrina. Geophysical Research Letters 33: L1780.
Abstract | Article (subscription required)

Contribution: Finds that the intensity of Hurricane Katrina was correlated to the difference between sea surface and air temperatures; because of its short-term, small-scale focus, this paper is likely to be of more interest to hurricane forecasters than to climate scientists.

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Details: Although hurricane forecasters are very good at predicting storm tracks, they are less skilled at predicting a hurricane’s intensity along that track. In this study, Menas Kafatos and his colleagues at George Mason University focused on Hurricane Katrina to better understand the factors that were involved in this storm's rapid intensification over the Gulf of Mexico. They compiled sea surface temperature, air temperature, wind speed and dew point data from a NASA satellite and NOAA buoys in the Gulf of Mexico. They found that Katrina reached peak intensity when the difference between sea surface temperatures and air temperatures was greatest. Their data also suggested that Katrina may have been particularly sensitive to sea surface temperatures in the northeast quadrant of the storm's track.

Santer, BD, TML Wigley, PJ Gleckler, C Bonfils, MF Wehner, K AchutaRao, TP Barnett, JS Boyle, W Brüggemann, M Fiorino, N Gillett, JE Hansen, PD Jones, SA Klein, GA Meehl, SCB Raper, RW Reynolds, KE Taylor, and WM Washington. 2006. Forced and unforced ocean temperature changes in Atlantic and Pacific tropical cyclogenesis regions. PNAS (Published online before print September 12, 2006. 10.1073/pnas.0602861103).
Abstract | Article (subscription required)

Contribution: Finds that human-caused global warming is primarily responsible for the recent increase in sea surface temperatures in the Atlantic and Pacific hurricane regions.

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Details: Scientists know that hurricane intensity is closely related to sea surface temperatures: the warmer the water, the stronger the storm. Scientists also know that sea surface temperatures in the world’s hurricane basins (where storms first develop) have been increasing and hurricanes have been getting stronger. What is causing the seas to warm, and therefore the storms to intensify? A group of scientists led by Ben Santer at Lawrence Livermore National Laboratory used 22 climate models to determine what factors can explain the observed warming in the Atlantic and Pacific hurricane basins. They ran the models over the period 1906-2005. One set of simulations included just natural factors—essentially, what the climate would have been like had the Industrial Revolution never occurred. A second set of simulations included both natural factors and human-caused factors like increases in greenhouse gases. The models did a good job reproducing observed temperature changes only when they included increases in greenhouse gases. Santer and his colleagues calculated that there is an 84% chance that 67% of the recent ocean warming is due to rising levels of greenhouse gases.

Slott, JM, AB Murray, AD Ashton & TJ Crowley. 2006. Coastline responses to changing storm patterns. Geophysical Research Letters 33: L18404.
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Contribution: Jordan Slott and his colleagues at Duke University and Woods Hole Oceanographic Institute modeled how coastlines in the Carolinas could change if tropical storms become stronger.

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Details: Their results indicate that even small increases in storm wind speeds are likely to have large impacts. Particularly along coastlines with complex shapes, some shoreline segments are likely to erode much more quickly than would be expected from sea level rise alone, while other areas could grow as sediments build up. The research reveals shortcomings in current coastal management plans that assume the effects of climate change—primarily sea level rise— will affect coasts uniformly.

Evan, AT, J Dunion, JA Foley, AK Heidinger & CS Velden. 2006. New evidence for a relationship between Atlantic tropical cyclone activity and African dust outbreaks. Geophysical Research Letters 33: L19813.
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Contribution: Reports that the presence of dry, dusty air over the tropical Atlantic tends to suppress Atlantic hurricane activity.

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Details: The link between hurricanes and global warming has been the subject of several recent scientific studies, but scientists are also studying the effects of other environmental variables on storm development. One of these is the Saharan Air Layer (SAL), a warm, dry and dusty air mass that can form above northern Africa and travel across the tropical Atlantic Ocean. Hurricane researchers have hypothesized that the SAL could disrupt storm formation. To test this theory, Amato Evan and his colleagues at the University of Wisconsin-Madison used satellite data to look at the correlation between dust cover (a proxy for the SAL) and hurricane activity, which they defined as the number of days in which at least one named tropical storm was in a particular study region.

The research covered a southerly area of the Atlantic Ocean (between northwest Africa and the Carribbean) and a six-week period (August 20-September 30) each year between 1982 and 2004. They found that hurricane activity was generally lower in years with more dust and that the correlation was strongest when El Niño and La Niña years were excluded from the analysis. Evan and his colleagues found that dust cover could account for about 25% of the variance in hurricane activity.

(Read Environmental Defense commentary on this paper.)


Keim, BD & KD Robbins. 2006. Occurrence dates of North Atlantic tropical storms and hurricanes: 2005 in perspective. Geophysical Research Letters 33: L21706.

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Contribution: Shows that every storm in the 2005 hurricane season occurred earlier than comparable storms in previous seasons; confirms that hurricane seasons are more severe when sea surface temperatures are high.

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Details: Barry Keim and Kevin Robbins of Louisiana State University compared the timing of storm development in the 2005 Atlantic hurricane season to that of other seasons for each year since 1851. Looking at storms in chronological order within seasons, they found that every named nth tropical storm in 2005 occurred earlier than the median date for corresponding nth storms in other years. For example, the fifth named storm of 2005 occurred on July 23, 47 days earlier than the median date of occurrence for fifth storms in all seasons since 1851. In fact, several storms in the 2005 season set records for earliest nth storm ever.

The researchers' data reinforce the important point that hurricane seasons tend to be more severe when sea surface temperatures are high. Keim and Robbins conclude that "exceptionally warm sea surface temperatures" played an important role in the length and severity of the 2005 season.

Fasullo, J. 2006. Assessing tropical cyclone trends in the context of potential sampling biases. Geophysical Research Letters 33: L24808.

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Contribution: Demonstrates that the reported increase in hurricane intensity is unlikely to have been caused by hypothetical biases in the pre-1990 hurricane dataset arising from categorizing storms incorrectly (so-called “subclassification”).

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Details: Some scientists are concerned that early hurricane records (particularly those from 1970 to 1990) consistently "subclassified" the most intense storms — meaning, for example, that hurricanes of Category 4 strength were erroneously categorized as Category 3s. This sort of systematic bias could artificially create the appearance of an increase in hurricane strength even if in reality storm intensity had not changed over time.

To quantify the potential effect of systematic bias, John Fasullo (from the National Center for Atmospheric Research) created 10,000 artificial storm records, each 35 years long with the same number of storms (both total number and per intensity category). Each storm in each record was randomly selected so there was no net trend in hurricane intensity. Fasullo measured storm intensity from this trendless record with a systematic subclassification bias, reasoning that if such a bias exists in real-world records, his study would find the same trends that have been reported in recent analyses of storm data.

He found that systematic measurement bias could produce an artificial apparent increase in Category 4 and 5 hurricanes over the 35-year period. However, the bias failed to reproduce reported trends in Category 1, 2 and 3 storms. Fasullo concluded that although there is still important work to be done in refining the long-term tropical cyclone record, "the influence of subclassification on the data record must be minimal."

Kossin, JP, KR Knapp, DJ Vimont, RJ Murnane & BA Harper. 2007. A globally consistent reanalysis of hurricane variability and trends. Geophysical Research Letters 34: L04815. doi: 10.1029/2006GL028836

Abstract | Article [PDF]

Contribution: Uses a new algorithm to analyze satellite records of hurricane intensity; confirms a “very real and dangerous” increase in hurricane strength in the North Atlantic but finds no significant global trend.

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Details: Methods of measuring hurricane intensity vary from region to region. Jim Kossin and his colleagues at the University of Wisconsin-Madison tried to standardize the data with an algorithm that estimates intensity based on original satellite images. After testing the algorithm using data from the North Atlantic, they compiled a record of hurricane intensity for all six hurricane basins. They found “very real and dangerous increases in recent Atlantic hurricane activity” but no significant global trends.

Kossin et al.'s analyses came with two important caveats:

1) Their analysis is only a first step in a larger effort — another technique is considered more accurate than their algorithm, but that method takes more time to implement;

2) Kossin's algorithm was refined using data from the National Hurricane Center. However, for four of the six ocean basins, they applied the algorithm to data from another agency, the Joint Typhoon Warning Center (JTWC), which, as Kossin et al. acknowledge, warns that "comparisons between these data and … data from other agencies should be made 'with extreme caution.'"

The JTWC records also did not include data from the 2005 season. Kossin et al. conclude that if their results are correct — that there is no global trend in hurricane activity — it will be important to understand "why hurricane activity in the Atlantic basin is varying in a fundamentally different way than the rest of the world despite similar upward trends of [sea surface temperature] in each basin."

Vecchi, G. A., and B. J. Soden (2007) Increased tropical Atlantic wind shear in model projections of global warming. Geophys. Res. Lett. 34: L08702. doi:10.1029/2006GL028905.

Abstract | Article [PDF] (subscription required)

Contribution: Examines model projections of a variety of environmental factors known to affect hurricane development and shows that in most hurricane basins, simulated environmental changes consistently favor hurricane formation. Most models predict that wind shear, which can inhibit storm development, will increase in the Atlantic hurricane basin over the 21st century.

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Details: High wind shear, in which different air layers move at different speeds, can inhibit storm development.

Gabriel Vecchi and Brian Soden, of the National Oceanic and Atmospheric Administration and University of Miami, respectively, showed that 18 different climate models predict that global warming will increase wind shear in the north Atlantic hurricane basin. They also observed that the models project “consistent changes towards more hurricane-favorable conditions” in the Indian and western Pacific hurricane basins.

Christopher Landsea, of the National Hurricane Center, has interpreted the paper’s main result — an increase in Atlantic wind shear — to mean that global warming’s effects on wind shear will cancel out the effects of increased sea surface temperatures on hurricane intensity. However, this interpretation is not part of the researchers' findings. In fact, as Hoyos et al. showed in their 2006 analysis of actual data, sea surface temperature has been far more influential than wind shear in large-scale, long-term increases in hurricane intensity.

Landsea, CW. 2007. Counting Atlantic tropical cyclones back to 1900. Eos 88 (1 May 2007).
Article [PDF]

Contribution: Shows that the apparent increase in tropical storm frequency (not intensity) over the 20th century is likely to be an artifact of improved storm detection methods.

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Details: Hurricane records for the Atlantic basin go back to the mid-19th century. In this paper, Christopher Landsea (from NOAA’s National Hurricane Center) explained that early records are likely to be incomplete because detecting tropical storms was harder to do before satellite technology was available. He warned that studies reporting a link between sea surface temperatures and overall storm frequency are "not reasonable" because the apparent increase in tropical storms over time is an artifact of spotty records from the late 19th and early 20th centuries.

Environmental Defense commentary: Landsea's focus is on overall tropical storm frequency, not intensity. But storm strength, not frequency, has been the focus of studies showing the probable link between global warming and hurricanes. For example, the 2005 papers by Emanuel and Webster et al. (at the top of this page) looked only at the satellite era and found that while the overall frequency of tropical storms has not changed, storms have become more ferocious as sea surface temperatures have increased.

Donnelly, JP & JD Woodruff. 2007. Intense hurricane activity over the past 5,000 years controlled by El Niño and the West African monsoon. Nature 447: 465-468.

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Contribution: Demonstrates that El Niño and the West African monsoon influence Atlantic hurricane intensity over time scales of centuries to millennia.

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Details: Though hot ocean waters fuel stronger hurricanes, sea surface temperature (SST) is not the only variable that contributes to hurricane intensity. Jeffrey Donnelly and Jonathan Woodruff, from the Woods Hole Oceanographic Institution, compiled a 5,000-year record of intense Atlantic storms to look at a range of factors that affect storm intensity. They examined sediment cores from Puerto Rican lagoons (strong storms deposit a layer of coarse sand on the lagoons’ otherwise fine sediments).

Previous research by other scientists indicated that SSTs are higher now than at any other time in the past five millennia. The authors concluded that “tropical SSTs as warm as at present are apparently not a requisite condition for increased intense hurricane activity," although they noted that the available SST records are coarse: "more high-resolution records...are required to address the role of SSTs on intense hurricane activity over this period adequately."  They also found that, on a timescale of hundreds to thousands of years, periods of intense Atlantic hurricanes tended to coincide with El Ninos (anomalously warm waters in the Eastern Pacific that change weather patterns around the world) and periods of high rainfall in tropical West Africa.

Nyberg, J, BA Malmgren, A Winter, MR Jury, KH Kilbourne &TM Quinn (2007) Low Atlantic hurricane activity in the 1970s and 1980s compared to the past 270 years. Nature 447:  698-702.

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Contribution: Shows that wind shear, which has been falling since the late 1980s, is an important determinant of Atlantic hurricane intensity over the long-term.

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Details: Johan Nyberg and several colleagues at the Geological Survey of Sweden used sediment and coral samples to estimate wind shear and Atlantic hurricane activity over the past 270 years. Their reconstructed dataset indicated that wind shear has been a key factor in regional hurricane activity over the last three centuries and that hurricane activity was anomalously low in the 1970s and 1980s, possibly because of higher wind shear caused by greater warming in the atmosphere relative to the ocean.

They note, however, that wind shear has been falling since the late 1980s. The authors conclude that the sharp increase in hurricane intensity since 1995 is a return to "normal hurricane activity," but they warn that if wind shear continues to decrease while sea surface temperatures keep rising, it "may result in longer storm lifetimes, … higher hurricane frequencies and greater storm intensities."

Chang, EKM & Y Guo. 2007 Is the number of North Atlantic tropical cyclones significantly underestimated prior to the availability of satellite observations? Geophysical Research Letters 34: L14801, doi:10.1029/2007GL030169.

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Contribution: Estimates that early 20th century records of Atlantic hurricanes missed up to 2.1 storms per year from 1900 to 1913, and 1 or less per year during the 1920s and beyond.

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Details: Atlantic hurricane records go back more than 150 years, extending well before satellites allowed scientists to track every storm. How many storms did the early records miss? Edmund Chang and Yanjuan Guo, of Stony Brook University, compared satellite records of Atlantic hurricanes to ship records over the satellite era to determine the probability that a ship could detect a hurricane, given the storm’s wind speed and distance from the ship. Then they took all the storm tracks (the location and strength of a storm over its lifetime) from 1996 to 2005 and compared them to ship observations for the same date and time in each year between 1900 and 1965. This allowed them to estimate the probability of a storm being detected.

Their results, which rest on the assumption that storm tracks have not altered over the past century, indicate that:

  • pre-World War I records missed an average of 2.1 storms per year;
  • starting in the 1920s, storm records probably missed one storm per year, with this number decreasing over time.

Chang and Guo also note that since not all ship records have been digitized, their results may overestimate the number of missing storms.

Manzello, DP, M Brandt, TB Smith, D Lirman, JC Hendee & RS Nemeth. 2007. Hurricanes benefit bleached corals. Proceedings of the National Academy of Sciences Published online before print July 2, 2007, 10.1073/pnas.0701194104

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Contribution: Shows that heat-stressed corals can benefit from the cool wake that follows a hurricane.

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Details: There may be at least one minor silver lining to hurricanes. It has long been known that hot ocean water that fuels a strong storm also leads to coral bleaching, in which coral reefs lose their colorful symbiotic algae; sustained bleaching is fatal.

Hurricanes can cause severe physical damage to coral reefs, but they also leave a wake of cool water behind them, so researchers have hypothesized that the passage of a hurricane might alleviate heat stress of bleached corals. To test this hypothesis, Derek Manzello (National Oceanic and Atmospheric Administration) and his colleagues analyzed records of Caribbean coral bleaching and water temperature before and after tropical storms in the 1998, 1999, 2001, 2004 and 2005 seasons. They found large, sustained decreases in sea temperature near reefs after storms, even when storm tracks were hundreds of kilometers away. The authors were able to compare these temperature changes to surveys of coral bleaching in 2005. They found that coral bleaching was less prevalent in areas cooled by hurricanes and concluded that "these results are the only known scenario where the effects of a hurricane can benefit a stressed marine community."

Holland, GJ and PJ Webster. 2007. Heightened tropical cyclone activity in the North Atlantic: natural variability or climate trend? Philosophical Transactions of the Royal Society A doi:10.1098/rsta.2007.2083

Full article

Contribution: Finds a strong relationship between rising Atlantic sea surface temperature and increased hurricane frequency over the past century.

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Details: Most research on hurricanes and global warming has focused on the link between hurricane intensity and sea surface temperatures from the 1970s to the present. More recently scientists have begun to focus on hurricane frequency over longer time periods.

In a 2007 Eos paper, Landsea found an increase in Atlantic hurricane frequency over the past century, but he argued that the trend was an artifact of incomplete early records. In this paper, Greg Holland (National Center for Atmospheric Research) and Peter Webster (Georgia Tech) concluded that the 100-year trend is real and that there is a substantial influence from human-caused global warming. They found that the increase disappeared only if they assumed an implausibly high number of missing storms in the early 20th century.

Moreover, the increase in hurricane frequency occurred in two abrupt changes, first in 1931 and then in 1995, with approximately 50 percent storms and hurricanes each time. These abrupt increases did not coincide with changes in hurricane observation techniques, but they did coincide with jumps in sea surface temperatures (SSTs) in the eastern Atlantic. Holland and Webster also noted that although the proportion of tropical storms becoming major hurricanes has increased since the 1970s (as described in earlier papers), over the past century the proportion has fluctuated with no long-term trend. These fluctuations are highly correlated with SSTs in the Gulf of Mexico. Given the strong relationship between hurricane frequency and SSTs, which are increasing due to human-caused global warming, the authors "are led to the confident conclusion that the recent upsurge in tropical cyclone frequency is due in part to greenhouse warming, and this is most likely the dominant effect."

Mann, ME, KA Emanuel, GJ Holland & PJ Webster. 2007. Atlantic tropical cyclones revisited. Eos 88 (36): 349-350.
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Contribution: Shows that even with uncertainties in early Atlantic hurricane records, there is a connection between sea surface warming and Atlantic hurricane activity.

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Details: Michael Mann (of Penn State) and his colleagues critiqued Landsea’s conclusion (2007) that the apparent increase in Atlantic tropical storms is an artifact of detection methods.

First, they point out that the two biggest increases in storm counts did not coincide with changes in observing methods.

Second, Landsea (2007) assumed that the percentage of all storms to make landfall has remained constant over time, even though storm tracks are known to vary.

Third, other analyses have concluded that early hurricane records missed fewer storms than Landsea suggested. Mann et al. then use Landsea’s own numbers to show that, even if his conclusions are correct, there is still a strong correlation between Atlantic sea surface temperature and hurricane activity.

Wang, C and S-K Lee. 2008. Global warming and United States landfalling hurricanes. Geophysical Research Letters 35: L02708.

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Contribution: Reports a weak downward trend in the number of hurricanes that make landfall in the U.S., but concludes future trends will depend on the spatial distribution of ocean warming.

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Details: Recent studies have begun to focus on the numbers of storms over long time periods. In this paper, Chunzai Wang and Sang-Ki Lee, of the National Oceanic and Atmospheric Association and University of Miami, respectively, examined whether global warming has influenced the number of Atlantic hurricanes that hit the United States. First, they used regression analysis to show that wind shear generally increases with global sea surface temperature (SST). However, they also found competing effects of different ocean basins: Atlantic wind shear tends to increase with warming in the Pacific and Indian Oceans, but decrease with warming in the Atlantic. Next, they showed a negative relationship between Atlantic wind shear and the number of landfalling U.S. hurricanes.

Given that global warming has increased SST and Atlantic wind shear tends to increase with global SST, Wang and Lee then asked whether the number of hurricanes hitting the U.S. has actually decreased over time. They found that since 1854, the number of landfalling storms has decreased slightly, although the trend is not statistically significant. They concluded that future trends in landfalling storms will depend in part on the spatial distribution of warming: "If the effects of warmings in the tropical Pacific and Indian Oceans cannot overcome that of Atlantic warming, global warming may favor landfall incidence for the United States."

Saunders, MA & AS Lea. 2008. Large contribution of sea surface warming to recent increase in Atlantic hurricane activity. Nature 451: 557-561.
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Contribution: Quantifies the contribution of sea surface temperature and wind patterns to Atlantic hurricane activity between 1965 and 2005; finds that hurricane frequency and activity increase 40 percent with a 0.5 °C rise in sea surface temperature.

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Details: Mark Saunders and Adam Lea, of University College London, used a statistical model to determine the influence of local sea surface temperature (SST) and wind on Atlantic hurricane frequency (the number of tropical storms, hurricanes and intense hurricanes) and activity (Accumulated Cyclone Energy, an index of storm energy based on wind speed).

The two variables together explain up to 81 percent of hurricane activity, with wind explaining more variance than SST. When they removed the influence of wind, they found that warming in the main development region for Atlantic hurricanes has had a strong influence on hurricane frequency and activity since 1965. Hurricane frequency and activity increased approximately 40 percent for each 0.5 °C of sea surface warming.

They also determined that, between 1996 and 2005, "local sea surface warming was responsible for ~40 percent of the increase in hurricane activity".

Knutson, TR, JJ Sirutis, ST Garner, GA Vecchi & IM Held. 2008. Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience 1: 359 – 364.
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Contribution: Describes model results projecting fewer but stronger Atlantic hurricanes due to future global warming; concludes that future trends will depend on the spatial distribution of warming among the world’s ocean basins.

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