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U.S. Hurricanes and Economic Damage: Extreme Value Perspective Daniel Chavas 1 ; Emmi Yonekura 2 ; Christina Karamperidou 3 ; Nicholas Cavanaugh 4 ; and Katherine Seran 5 Abstract: The historical record of U.S. hurricane damage is analyzed using a peaks-over-threshold approach in which the generalized Pareto distribution (GPD) is applied to model excesses above a specied threshold for a given damage metric. In addition to absolute hurricane dam- ages (total damage), this paper denes a damage index as the ratio of base-year economic damages to the available economic value in the af- fected region. The paper then incorporates physical covariates at the individual hurricane level into the GPD model, namely, maximum wind speed and a simple yet novel measure of the mean bathymetric slope at landfall, and applies the analysis to both the total damage and the damage index. The parameters of the GPD models with physical covariates are estimated with maximum-likelihood estimation. The results show that for total damage, the only useful covariate is maximum wind speed. However, for the damage index, both the mean bathymetric slope and the maximum wind speed are found to be useful, with coefcients that are consistent with the known physics of each covariate in causing damage. Moreover, inclusion of covariates in the damage index reduces the maximum-likelihood estimate of the shape parameter to zero, transforming the fat tail for the distribution of total damage to a skinny (exponential) tail for the distribution of the damage index. These results suggest that damage measured as a fraction of estimated potential damage may help to remove the local economic signal from the damage database, leaving a data set that better captures the physical relationship between hurricanes and damage. Finally, as an illustrative example of the potential util- ity of this new methodology within a risk-assessment framework, it is applied to data sets of simulated hurricane tracks corresponding to cur- rent and future Intergovernmental Panel on Climate Change (IPCC) A1B-scenario climate states as modeled by two different climate models. DOI: 10.1061/(ASCE)NH.1527-6996.0000102. © 2013 American Society of Civil Engineers. CE Database subject headings: Hurricanes; Natural disasters; Damage; Risk management; Economic factors; United States. Author keywords: Hurricanes; Natural disasters; Damage; Extreme value theory; Risk management. Introduction Landfalling U.S. hurricanes have been responsible for seven of the top ten costliest insured property losses from natural disasters worldwide since 1980 (Munich Re 2011). Despite a projected de- crease in the overall number of hurricanes globally, the potential for increases in the frequency and intensity of the strongest hurricanes as a result of climate change (Knutson et al. 2010) has raised concerns about similar increases in total economic damage in the future (Mendelsohn et al. 2012; Peduzzi et al. 2012). Moreover, 50% of total economic damage from hurricanes in the United States during the period 18702005, normalized for changes in population, wealth, and ination, was caused by only eight storms (Pielke et al. 2008). Thus, understanding of how damage may change in the future is largely predicated on a proper accounting of the most extreme events in the landfall distribution. These facts have motivated recent work applying extreme value theory to historical hurricane damage with two primary foci: (1) nding appropriate models for total economic damage from hurri- canes and (2) linking total economic damage to large-scale climate signals that are known to affect Atlantic hurricane intensity and frequency. Katz (2002) employed a compound Poisson process for the total economic damage associated with hurricanes and included the state of the El NiñoSouthern Oscillation (ENSO) as a covariate. Jagger et al. (2008, 2010) used hierarchical Bayesian models to generate forecasts of annual insured losses, employing preseason index values of the North Atlantic Oscillation, Atlantic Ocean surface temperature, and ENSO as predictors. Whereas forecasting the distribution of total losses based on climatic precursors is of interest, the sensitivity of annual damage to individual extreme events implies that estimating the risk of extreme losses is more crucial for nancial planning (Jaffee et al. 2008). To that end, Jagger et al. (2008, 2010) demonstrated that the family of generalized Pareto distributions (GPDs) is appropriate for modeling extreme events involving large economic losses such as hurricane landfalls. To date, however, GPD analysis of hurricane damage that incorporates physical variables at the individual hurricane level has not been performed. Hurricanes inict damage primarily via two mechanisms: wind damage to structures, both directly and indirectly as a result of wind-borne debris impact (Lin et al. 2010); and water 1 Researcher, Program in Atmospheres, Oceans, and Climate, Massa- chusetts Institute of Technology, Cambridge, MA 02139 (corresponding author). E-mail: [email protected] 2 Researcher, Dept. of Earth and Environmental Sciences, Columbia Univ., New York, NY 10025. E-mail: [email protected] 3 Researcher, Dept. of Earth and Environmental Engineering, Columbia Univ., New York, NY 10027. E-mail: [email protected] 4 Ph.D. Candidate, Scripps Institution of Oceanography, Univ. of Cali- fornia San Diego, La Jolla, CA 92093. E-mail: [email protected] 5 Ph.D. Candidate, College of Earth, Ocean and Atmospheric Sciences, Oregon State Univ., Corvallis, OR 97331. E-mail: 2ksera[email protected] .edu Note. This manuscript was submitted on December 13, 2011; ap- proved on October 10, 2012; published online on October 12, 2012. Discussion period open until April 1, 2014; separate discussions must be submitted for individual papers. This paper is part of the Natural Hazards Review, Vol. 14, No. 4, November 1, 2013. ©ASCE, ISSN 1527-6988/ 2013/4-237246/$25.00. NATURAL HAZARDS REVIEW © ASCE / NOVEMBER 2013 / 237 Nat. Hazards Rev. 2013.14:237-246. Downloaded from ascelibrary.org by HAWAII,UNIVERSITY OF on 05/19/14. Copyright ASCE. For personal use only; all rights reserved.

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  • U.S. Hurricanes and Economic Damage:Extreme Value Perspective

    Daniel Chavas1; Emmi Yonekura2; Christina Karamperidou3; Nicholas Cavanaugh4; andKatherine Serafin5

    Abstract: The historical record of U.S. hurricane damage is analyzed using a peaks-over-threshold approach in which the generalized Paretodistribution (GPD) is applied to model excesses above a specified threshold for a given damage metric. In addition to absolute hurricane dam-ages (total damage), this paper defines a damage index as the ratio of base-year economic damages to the available economic value in the af-fected region. The paper then incorporates physical covariates at the individual hurricane level into the GPD model, namely, maximum windspeed and a simple yet novelmeasure of themean bathymetric slope at landfall, and applies the analysis to both the total damage and the damageindex. The parameters of the GPD models with physical covariates are estimated with maximum-likelihood estimation. The results show thatfor total damage, the only useful covariate is maximum wind speed. However, for the damage index, both the mean bathymetric slope and themaximumwind speed are found to be useful, with coefficients that are consistent with the known physics of each covariate in causing damage.Moreover, inclusion of covariates in the damage index reduces the maximum-likelihood estimate of the shape parameter to zero, transformingthe fat tail for the distribution of total damage to a skinny (exponential) tail for the distribution of the damage index. These results suggest thatdamage measured as a fraction of estimated potential damage may help to remove the local economic signal from the damage database, leavinga data set that better captures the physical relationship between hurricanes and damage. Finally, as an illustrative example of the potential util-ity of this new methodology within a risk-assessment framework, it is applied to data sets of simulated hurricane tracks corresponding to cur-rent and future Intergovernmental Panel on Climate Change (IPCC) A1B-scenario climate states as modeled by two different climate models.DOI: 10.1061/(ASCE)NH.1527-6996.0000102. © 2013 American Society of Civil Engineers.

    CE Database subject headings: Hurricanes; Natural disasters; Damage; Risk management; Economic factors; United States.

    Author keywords: Hurricanes; Natural disasters; Damage; Extreme value theory; Risk management.

    Introduction

    Landfalling U.S. hurricanes have been responsible for seven of thetop ten costliest insured property losses from natural disastersworldwide since 1980 (Munich Re 2011). Despite a projected de-crease in the overall number of hurricanes globally, the potential forincreases in the frequency and intensity of the strongest hurricanes asa result of climate change (Knutson et al. 2010) has raised concernsabout similar increases in total economic damage in the future(Mendelsohn et al. 2012; Peduzzi et al. 2012). Moreover, 50% oftotal economic damage from hurricanes in the United States during

    the period 1870–2005, normalized for changes in population,wealth, and inflation, was caused by only eight storms (Pielke et al.2008). Thus, understanding of howdamagemay change in the futureis largely predicated on a proper accounting of the most extremeevents in the landfall distribution.

    These facts have motivated recent work applying extreme valuetheory to historical hurricane damage with two primary foci: (1)finding appropriate models for total economic damage from hurri-canes and (2) linking total economic damage to large-scale climatesignals that are known to affect Atlantic hurricane intensity andfrequency. Katz (2002) employed a compound Poisson process forthe total economic damage associated with hurricanes and includedthe state of the El Niño–Southern Oscillation (ENSO) as a covariate.Jagger et al. (2008, 2010) used hierarchical Bayesian models togenerate forecasts of annual insured losses, employing preseasonindex values of the North Atlantic Oscillation, Atlantic Oceansurface temperature, and ENSO as predictors. Whereas forecastingthe distribution of total losses based on climatic precursors is ofinterest, the sensitivity of annual damage to individual extremeevents implies that estimating the risk of extreme losses is morecrucial for financial planning (Jaffee et al. 2008). To that end, Jaggeret al. (2008, 2010) demonstrated that the family of generalizedPareto distributions (GPDs) is appropriate for modeling extremeevents involving large economic losses such as hurricane landfalls.

    To date, however, GPD analysis of hurricane damage thatincorporates physical variables at the individual hurricane level hasnot been performed. Hurricanes inflict damage primarily via twomechanisms: wind damage to structures, both directly and indirectlyas a result of wind-borne debris impact (Lin et al. 2010); and water

    1Researcher, Program in Atmospheres, Oceans, and Climate, Massa-chusetts Institute of Technology, Cambridge, MA 02139 (correspondingauthor). E-mail: [email protected]

    2Researcher,Dept. ofEarth andEnvironmental Sciences,ColumbiaUniv.,New York, NY 10025. E-mail: [email protected]

    3Researcher, Dept. of Earth and Environmental Engineering, ColumbiaUniv., New York, NY 10027. E-mail: [email protected]

    4Ph.D. Candidate, Scripps Institution of Oceanography, Univ. of Cali-fornia San Diego, La Jolla, CA 92093. E-mail: [email protected]

    5Ph.D. Candidate, College of Earth, Ocean and Atmospheric Sciences,Oregon State Univ., Corvallis, OR 97331. E-mail: [email protected]

    Note. This manuscript was submitted on December 13, 2011; ap-proved on October 10, 2012; published online on October 12, 2012.Discussion period open until April 1, 2014; separate discussions must besubmitted for individual papers. This paper is part of theNatural HazardsReview, Vol. 14, No. 4, November 1, 2013. ©ASCE, ISSN 1527-6988/2013/4-237–246/$25.00.

    NATURAL HAZARDS REVIEW © ASCE / NOVEMBER 2013 / 237

    Nat. Hazards Rev. 2013.14:237-246.

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    http://dx.doi.org/10.1061/(ASCE)NH.1527-6996.0000102mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]

  • damage from storm surge and precipitation (Iman et al. 2005; Rogerset al. 2009). The former is a function of the wind field, whereas thelatter is a more complex function of the wind and pressure fields aswell as near-coast bathymetry, storm translation speed, storm track,and landfall angle (Irish et al. 2008; Tuleya et al. 2007). Inlandflooding is additionally sensitive to both inland topography and themagnitude and direction of vertical wind shear in the vicinity ofthe storm (Lonfat et al. 2007).

    Previous research has analyzed absolute damage normalized to2005 dollars, as calculated in Pielke et al. (2008) (e.g., Nordhaus2010; Bouwer and Botzen 2011;Mendelsohn et al. 2011). However,given the large variation in economic value along the coast, analysisof absolute damage data may confound the relative roles of physicalstorm characteristics and economic value at landfall in damageattribution, particularly at the extremes. As a remedy, one mayinstead analyze the ratio of the base-year (i.e., nonnormalized)damages to an estimate of the total economic value of the regionaffected by the hurricane, i.e., the fraction of available economicvalue that is destroyed by the storm. This quantity, defined herein asthe damage index and described in more detail in subsequentparagraphs, is a measure of the physical damage capacity of a stormthat seeks to remove variability in economic value at the landfalllocation from the damage database. Such an approach was firstintroduced in the context of all natural disasters at a global scale byNeumayer and Barthel (2011) and preliminarily in the context ofhurricanes in an alternate form by Maina (2010).

    This work seeks to fit GPD models to both absolute damage(hereafter total damage) and the damage index using the maximumhurricane wind speed at landfall Vmax and the mean slope of thecontinental shelf s as covariates. Storm size, as measured by theradius of 34-knot winds r34, was also tested but lacks sufficient datain the historical record to warrant inclusion in the final analysis. Thismethodology is applied to both total damage and the damage index,and the results are compared. Based on this comparison, the authorsargue that for the purposes of GPD analysis of hurricane damage, thedamage index is a more suitable metric for the relationship betweenthe physical characteristics of a given landfalling hurricane and thedamage that it produces. The authors then illustrate the potentialutility of this approach to assess the probability of extreme lossesgiven historical data, as well as synthetic data based on climatemodel projections.

    This paper is organized as follows. After an account of the datasets used and their limitations, the paper develops the economicdamage index approach and discusses its relation to total damage.The paper fits GPD models of the damage index with and withoutphysical covariates and compares the results with the corre-sponding models of total damage. Finally, as an illustration of thepotential utility of this approach, the paper applies the damageindex model to synthetic hurricane data in current and future cli-mate conditions from two climate models. Summary and con-clusions close the paper.

    Data

    Economic damage data (in both base-year and 2005 dollars nor-malized based on population), household data in affected coastalcounties for U.S. landfalling hurricanes (1900–2004), and annualnationalwealth data are taken from the analysis of Pielke et al. (2008)and disaggregated into individual landfall events. Economic damageis defined as the direct losses as determined in the weeks or monthsafter a hurricane landfall and is often estimated based on the reportedinsured damage (typically taken as a 2:1 ratio since at least 1987).These damage estimates may therefore be attributed to wind, storm

    surge, and inland flooding. To the authors’ knowledge, a data set ofdamage disaggregated by hazard type is not publicly available.Annual county household data are updated from the work of CollinsandLowe (2001) and are taken from theU.S. CensusBureau decadalsurvey database (U.S. Census Bureau 2000, 2002) linearly in-terpolated to the year of interest.

    Given the lack of a single unambiguous landfall, the followingstorms are excluded: 1909 Storm 10, 1944 Storm 7, 1962 Daisy,1965 Debbie, 1988 Beryl, and 1988 Gilbert. Given a lack of dis-aggregated economic data, the following landfalls are excluded:1960 Donna Landfalls 2 and 3, 1979 David Landfalls 1 and 2, and1985 Gloria Landfalls 1 and 2. Of these 12 excluded landfalls, threehave normalized damage exceeding $1 billion, although only oneexceeds $3 billion (1944 Storm 7: $13 billion; rank 5 21). Addi-tionally, household data for affected counties have not been com-piled from 2005 onward, and thus damage index values cannot becomputed after 2004. The result is a final data set of 202 individuallandfall events during the period 1900–2004.

    Bathymetric data are extracted as grids with 30-arc-second res-olution from the SRTM30_PLUS version 6.0 data of the ScrippsInstitution of Oceanography’s Satellite Geodesy Research Groupand are based on a gravity-to-topography ratio calibrated using theSmith and Sandwell global 1-min grid altimetry data and 298millionedited soundings (Becker et al. 2009). Landfall locations are extractedfrom the Hurricane Database (HURDAT) Best Track data set(Jarvinen et al. 1984), modified slightly to account for a significantchange in the wind-pressure relationship used by the NationalHurricane Center in 1970 (Landsea 1993); data and documentationfrom Kerry Emanuel (ftp://texmex.mit.edu/pub/emanuel/HURR/tracks/ ). An automated algorithm combines HURDAT Best Trackdata with a coastal map database to objectively determine landfalllocations, and each location is subsequently verified manually.Maximum 1-min near-surface hurricane wind speed at landfall istaken to be the final 6-hourly value prior to landfall. Current andfuture climate hurricane track data sets are generated by a statistical-deterministic hurricane model (Emanuel et al. 2006) based on thecurrent (1981–2000) and future (2081–2100) IPCC AR4 A1Bscenario (Naki�cenovi�c et al. 2000) climates as simulated by theGeophysical Fluid Dynamics Laboratory (GFDL) CM2.0 Model(Delworth et al. 2006) and the European Center/Hamburg Model 5(ECHAM5) (Roeckner et al. 2003).

    Limitations of the Data

    There are a number of important limitations to these economic datasets, as discussed in both Pielke et al. (2008) and Collins and Lowe(2001). For the damage data, the true relationship between economicand insured damage is uncertain andwill vary across time because ofchanges in insurance use and practice, aswell as from storm to storm,particularly in cases of extensive flood damage, which is oftenexcluded from insurance policies. Moreover, data on insured losseswere not collected prior to 1949, so damage estimates during thisperiod were produced by the National Oceanic and AtmosphericAdministration (NOAA) based on local postevent surveys, whichmay be prone to larger errors without private estimates availablefor reference.

    The county-level economic data also contain important uncer-tainties. First, U.S. Census data were not collected for populationand households prior to 1925 and 1940, respectively, and thereforedata from this period are calculated via extrapolation and thus havemuch greater uncertainty. Second, as originally described in Jarrellet al. (1992), the determination of which counties were affected bythe storm was based primarily on whether the county experiencedhurricane-force winds and/or a storm surge at least 4 ft above

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    ftp://texmex.mit.edu/pub/emanuel/HURR/tracks/&tnqh_x3009;ftp://texmex.mit.edu/pub/emanuel/HURR/tracks/&tnqh_x3009;ftp://texmex.mit.edu/pub/emanuel/HURR/tracks/&tnqh_x3009;

  • climatologic tide, but because of observational limitations, thisendeavor ultimately was highly subjective, particularly earlier in thehistorical record. Third, variability in the size, shape, and orientationof county borders along the U.S. Atlantic and Gulf coasts will resultin modest variability in estimates of affected households and po-pulation along the coast, although large urban areas in coastal statestypically lie on the coast itself, suggesting that this uncertainty maybe relatively modest. Lastly, only coastal counties are included asaffected by the storm, with no account given of inland counties thatmay have been subjected to significant losses, particularly in the caseof inland flooding. The authors are unaware of any comprehensiveestimate of the fraction of damage that occurs in coastal versus inlandcounties for a typical storm. Thus the model results presented insubsequent sections are focused primarily on damage caused bywind and storm surge.

    Economic Damage Index

    Theory

    Conventional hurricane damage models use absolute economic dam-age as the dependent variable (Nordhaus 2010; Bouwer and Botzen2011; Mendelsohn et al. 2011). Such models suffer from the need tonormalize damage to a base year and to adjust for populationchanges. In addition, absolute economic damage is highly sensitiveto land-use specifics for each event, most important of which is theeconomic value of the affected region, thereby potentially conflatingthe relative roles of variations in physical storm characteristics andcoastal economic value in damage model predictions.

    Here the authors suggest a model that assumes that the physicalcharacteristics of the hurricane and the economic value at the landfalllocation are independent. Following the approach of Neumayer andBarthel (2011), the authors decompose the total economic damageinto the product of the damage capacity of the hurricane, denoted asthe damage index, and the economic value at the landfall location

    Dyi ¼ Ii � Eyi (1)

    where Dyi 5 economic damage for storm i reported in base-year ydollars; Ii 5 damage index of storm i; andEyi5 economic value of thelandfall region of storm i. This equation can be rearranged to give thedefinition of the damage index

    Ii ¼ DyiEyi (2)

    Conveniently, this approach can be applied directly to base-yeardata and thus obviates the need to normalize economic values toa common year. The authors calculate the damage index using theraw data employed by Pielke et al. (2008). The numerator is definedas base-year (i.e., nonnormalized) economic damage and is dis-aggregated into individual landfalls where applicable. The de-nominator is given by

    Eyi ¼ Hyi �Wy (3)

    whereHyi 5 number of households in the counties deemed affectedby storm I; and Wy 5 national average wealth per household, cal-culated as the ratio of the aggregate base-year national wealth to theaggregate number of households nationally.

    In theory, the damage index depends only on the physical char-acteristics of the hazards that cause or enhance damage, such asmaximum wind speed, storm size, bathymetry, translation speed,landfall angle, and event duration, and is boundedwithin the interval

    [0, 1], where a value of 1 indicates total economic loss. In practice,though, the damage index has a few caveats. First, the numerousuncertainties discussed earlier in estimates of economic value, par-ticularly the uninsured, in both the numerator and the denominator,as well as the exclusion of inland counties in the denominator,muddy the strict upper bound. Second, calculating local economicvalue from the national average for household wealth is likely tounderestimate the true economic value along the coast, particularlyin localized affluent regions (e.g., vacation homes), given thatcoastal properties tend to be valued higher than elsewhere in thecountry (Collins and Lowe 2001). Finally, as noted inNeumayer andBarthel (2011), the damage index implicitly includes changes invulnerability over time as a result of changes in building codes orother adaptive measures taken by rational individuals and gov-ernments. Despite these uncertainties, the damage index still shouldcapture the broad variability of coastal development in both spaceand time.

    Damage versus Damage Index: Basic Statistics andthe Example of Hurricane Bret (1999)

    Fig. 1 shows the time series of total damage and damage index. Themean values (SDs) of the total damage and damage index are $4.73billion ($14.1 billion) and 0.05 (0.11), respectively. An example thatelucidates the stark contrast between total damage and damage indexis Hurricane Bret (1999), which was a large Category 4 hurricane thatmade landfall in Texas in a region with a gradually sloping conti-nental shelf favorable for high storm surges. Bret ranks first indamage index, with a value of 0.89, indicating that it destroyed 89%of the total wealth in the affected counties. However, the absolutedamage was only $76 million—well below the absolute damagethreshold—because the landfall region was sparsely populated.

    The linear correlation coefficient between damage index and theeconomic value at landfall is r5 20:1, confirming that the twoquantities are nearly independent. The authors argue on this basisthat it is more appropriate to fit a model that reflects the true intensityand destructive capacity of a hurricane such as Bret rather than theabsolute economic damage that it inflicted. This is especially im-portant when one endeavors to make future projections on hurricanedamage because such projections are independently influenced bypotential changes in the physical characteristics of hurricanes andby changes in the economic value along the coast (Pielke et al. 2008).In the following section, the authors fit GPDs to the total damage andthe damage index, with and without physical covariates, to capturethe aforementioned characteristics of damage caused by landfallinghurricanes.

    Generalized Pareto Distributions

    Theory

    Extreme value analysis is performed using a peaks-over-thresholdapproach (Coles 2001), which models all excesses above a speci-fied threshold value with a GPD whose probability-density functionis given by

    Pðxjx. uÞ ¼

    8>><>>:

    1s

    �1þ j x2 u

    s

    �212ð1=jÞif j� 0

    1sexp

    �2x2 u

    s

    �if j ¼ 0

    (4)

    where u 5 constant threshold; x 5 total damage or damage index;s 5 scale parameter; and j 5 shape parameter. The latter twoparameters may be taken as constant or as a function of one or more

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  • covariates. The authors first present the GPD fit without covariatesand then proceed to include two physical covariates in the scale andshape parameters, namely, themaximumwind speed at landfallVmaxand the mean bathymetric slope s. In both cases, the input data set isconverted to standardized anomalies with zero mean and unitvariance. Total damage data are first divided by a factor of 109 forease of comprehension.

    For the analysis, following Coles (2001), the authors select thethreshold for each data set as the lowest value above which the de-pendence of the GPD parameters on this threshold is small in order tomaximize the size of the subset of data used for the GPDmodel whilestill maintaining stable parameter estimates. Based on this analysis(not shown), the authors set the thresholds at $5 billion (N5 39) and0.05 (N5 45), respectively, although the results are found to be robustto variations in these values over a reasonably large range ofthresholds ($2 billion–$6 billion and 0.02–0.06 for total damage anddamage index, respectively). All parameter estimates and errors arebased on a maximum-likelihood estimation (MLE) approach calcu-lated using the extRemes package (Gilleland and Katz 2011) in the Rstatistical software interface (R Development Core Team 2010).

    Damage versus Damage Index: GPD Fitwithout Covariates

    Table 1 displays the MLE-based parameter values and standarderrors for the GPD fit without covariates. The GPD of total damagehas a statistically significant positive shape parameter, with a MLEestimate of 0.58 and standard error (SE) of 0.27, which indicates thepresence of a fat upper tail. The same is not true for the GPD of the

    damage index, whose MLE-estimated shape parameter is 0.14 with aSE of 0.17, although the error range of this estimate indicates that thevalue is still likely positive. The respective quantile-quantile plots aredisplayed in Fig. 2. In both cases, the model fits the data qualitativelywell except for the most extreme cases. Quantitatively, for totaldamage, the model data match well for D2005 , $30 billion. For $30billion , D2005 , $80 billion, the model diverges slightly with anaverage error of $7.6 billion, and for the most extreme case (GreatMiami 1926), the model error is $44 billion. Nevertheless, this errorlies well within the 95% confidence intervals, and given the numerousuncertainties in estimating damage and normalizing to 2005 dollars fora storm this early in the historical database, such an error may well bewithin the bounds of observational uncertainty. For the damage index,the model error is small over a larger fraction of the data set than fortotal damage (Ii , 0:4), but the model fit exhibits a more pronounceddivergence for the three most extreme cases, with error maximizing at0.2 for the most extreme case of Hurricane Bret (1999).

    Because the damage index is intended to capture the effect ofthe physical characteristics of the storms, one might expect the tailof its distribution to exhibit skinny (j5 0) or bounded (j, 0) tail

    Table 1. Maximum-Likelihood Estimation Parameter Values (StandardError) for GPD Fit with No Covariates

    Variable s j

    Total damage 7.79 (2.34) 0.58 (0.27)Damage index 0.13 (0.03) 0.14 (0.17)

    Note: Thresholds for total damage and damage index are $5 billion(N5 39) and 0.05 (N5 45), respectively.

    Fig. 1. (Color) Time series of damages caused by U.S. landfalling hurricanes as measured by (a) total damage in 2005 dollars and (b) damage index(dashed red lines denote thresholds of $5 billion and 0.05, respectively); storms with the three highest values of each are labeled (LF denotes landfallnumber for multilandfall storms)

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  • behavior given the bounds on physical quantities such as stormintensity, wind speed, shelf slope, and so forth. However, it is nota requirement that a physically bounded geophysical phenomenonproduce bounded tail behavior (Katz 2013). Nonetheless, this initialstatistical result provides evidence that the heavy-tailed distributionthat has been linked traditionally to economic damage by hurricanesmay be amanifestation primarily of the strong variation in economicvalue along the coast rather than the physical characteristics ofstorms themselves.

    DamagesversusDamage Index:GPDFitwithCovariates

    The authors then add two covariates, maximumwind speedVmax andmean bathymetric slope s at landfall, to the scale s and shapej parameters in the following form:

    lnðsÞ ¼ s0 þ s1Vmax þ s2s (5a)

    j ¼ j0 þ j1Vmax þ j2s (5b)

    Recall that each input data set is first normalized to have zero meanand unit variance. The logarithm is used on the left-hand side ofEq. (5a) to ensure that the scale parameter remains positive and thusthe GPD well defined. Covariate values for landfall intensity Vmaxare taken as the values at the final HURDAT Best Track 6-hourlyvalue prior to landfall.

    To incorporate the complex effect of bathymetry on storm surgein a simple manner, the authors calculate as a covariate a meanbathymetric slope s based on the distance r along the shore-normalvector extending to a particular isobath of depth h. Themean slope istherefore defined as

    s ¼ hr

    (6)

    Themean slope is calculated at 65 points along theU.S.mainlandcoast from Texas to Maine (Fig. 3). The 65 coastal points wereselected to remove the effect of small-scale coastal features that maynot be characteristic of the region as a whole. For each storm, thecovariate value is taken to be the slope at the point nearest to thelandfall location. Note that a low slope indicates a wide, shallowshelf, which favors a higher storm surge (Irish andResio 2010). Irishand Resio (2010) found that the distance to the h5 30m isobath isan optimal characteristic length scale for storm-surge generation.Indeed, across a variety of metrics for mean slope based on distance

    to a given isobath, water depth at a given distance from shore, andintegrated depth over either distance measure, the authors found thatthe distance to theh5 30 m isobath performedbest andwill henceforthbe the metric for r used in this work.

    The authors fit GPD models to total damage and damage indexand then apply a deviance test to determine whether the succes-sive addition of a given covariate to the scale or shape parameterimproves the model fit. The covariates are tested in the followingorder:1. Individual covariates in the scale parameter;2. Multiple covariates in the scale parameter for those found

    individually useful; and3. Single covariates in the shape parameter for those found useful

    in the scale parameter.A value of p, 0:05 indicates that the parameter coefficient is

    statistically significantly different from zero at the 95% confidenceinterval. For total damage, Vmax is found to be a marginally usefulcovariate in the scale parameter ( p5 0:06), whereas s is not( p5 0:19). On the other hand, for the damage index, s is the mostuseful individual covariate in the scale parameter ( p5 0:014),whereas Vmax is marginally useful ( p5 0:07); indeed, adding Vmaxas a second covariate with s also is marginally useful ( p5 0:07).

    Fig. 2. Quantile-quantile plot for (a) total damage and (b) damage index without covariates (gray lines represent the 95% confidence band) (fromDoksum and Sievers 1976)

    Fig. 3. (Color)Distribution of the logarithmof themean slope along theU.S. coast (see text for details); regions with low slope values (bluecolors) are more vulnerable to damage from storm surges)

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  • The p-value for both covariates compared with the covariate-freecase is p5 0:009. The available data set is insufficient to determinestatistically significant utility of covariates in the shape parameterin either case and therefore it is left constant.

    The resulting covariate coefficients with SEs are displayed inTable 2. TheGPDof total damage again has a statistically significantpositive shape parameter, with a MLE estimate of 0.54 and a SE of0.26. As for theGPD of the damage index, theMLE-estimated shapeparameter is now reduced to zero (20:003) with a SE of 0.15. Therespective residual quantile-quantile plots (Ben and Yohai 2004) aredisplayed in Fig. 4. For total damage, the distribution of modeledresiduals fits well for the lower quantiles but then slightly falls shortof the observed residuals over the remaining quantiles, although themodel does not exhibit any pronounced divergence from the data.For the damage index, the model distribution of residuals also fitswell over the lower quantiles, but the remainder of the residualdistribution is skewed to the right relative to the model, as indicatedby themodel first overestimating and then underestimating residualsfor higher quantiles.

    Thus, in the case of the damage index with covariates, the MLErange for the constant shape parameter is shifted downward signif-icantly such that it is nowcentered at approximately zero. The damageindex appears capable of transforming the damage distribution fromfat tailed to skinny tailed, albeit with significant lingering un-certainty. This result provides further evidence that the fat tail in thedistribution of economic damage may be primarily a result of thedistribution of economic value along the coast.

    Importantly, for the damage index, not only are the two covariatesfounduseful, but the signs of their respective coefficients—positive forVmax and negative for s—are also consistent with physical intuition:damage should be directly proportional to Vmax and inverselyproportional to s, all else being equal. Moreover, the magnitude of

    the coefficient of s is approximately twice that of Vmax, indicatingthat the capacity of a given storm to cause damage is more sensitiveto changes in (normalized) slope compared with maximum windspeed (although they are by no means independent for storm-surgegeneration). These relationships are found to be largely insensitiveto variations in the threshold.

    Overall, in contrast to total damage, the authors find that thedamage index is sensitive to both maximum wind speed and localbathymetry at landfall, giving statistical confirmation to what wouldbe expected of the underlying physics of a landfalling hurricane.Surprisingly, the analysis suggests that among the most destructivestorms, the capacity for a storm to cause damagemaybemost stronglylinked to modulation of the storm surge by the local bathymetry, aresult that is absent in the analysis of total damage.

    Potential Application to Hurricane Climate

    Finally, the authors provide an illustrative example of how this ap-proach could potentially be used to assess changes in the probabilitydistribution of the most damaging hurricanes in a future climatestate. To that end, the GPD model with covariates calculated fromthe historical data is applied to two sets of approximately 5,000synthetic landfalling tracks, one representative of the current climate(1981–2000) and one representative of the future climate (2081–2100) under the IPCC A1B scenario as simulated by two climatemodels: GFDL CM2.0 and ECHAM5. Comparison of the spatialand seasonal variability of the synthetic storms with the historicalrecord is provided by Emanuel et al. (2008) as a means of assessingthe quality of each model’s synthetic storm data set. The authorsemphasize here that given the large range of projected changes inhurricane intensity and frequency at the regional scale across models(Knutson et al. 2010), proper assessment of changing risk profilesdemands analysis of output from an ensemble of climate models;indeed, the GFDL model lies on the upper end of the model rangefor projected increases in Atlantic hurricane activity in the futureclimate (Villarini et al. 2011), although this is largely a consequenceof a significant increase in its projection of landfall frequency ratherthan changes in the characteristics of individual storms. Moreover,it is implicitly assumed that the estimated model parameters will notchange in the future because a lack of data prohibits a robust as-sessment of nonstationarity. Thus the results presented in subsequentparagraphs should not be interpreted as a quantitative projection forchanges in hurricane risk but rather as a demonstration of the potential

    Table 2. Maximum-Likelihood Estimation Parameter Coefficient Values(Standard Error) for GPD Fit with Covariates Following Eqs. (5a) and(5b) for Scale and Shape Parameters

    Variable s0 s1 s2 j0

    Total damage 1.67 (0.38) 0.43 (0.23) N=A 0.54 (0.26)Damage index 22:24 (0.24) 0.28 (0.15) 20:55 (0.17) 20:003 (0.15)

    Note:TotaldamageincludesVmax (p5 0:06), whereas damage index includesVmax and s ( p5 0:009). Thresholds for total damage and damage index are$5 billion (N5 39) and 0.05 (N5 45), respectively.

    Fig. 4. Residual quantile-quantile plot (exponential scale) for (a) total damage and (b) damage index with covariates (total damage includes Vmax andthe damage index includes Vmax and s; gray lines are as in Fig. 2)

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  • use of the statistical model presented here within the framework ofa more comprehensive risk assessment.

    For each synthetic landfall event, the authors compute the cova-riates Vmax and s and apply them to Eq. (5a) to estimate scaleparameters for each storm. The resulting GPD probability-densityfunction (PDF) for each storm is summed together and renormalizedto a single standard PDF (i.e., integrates to unity), the result of whichthen represents the aggregate PDF for all landfalling storms, irre-spective of landfall frequency. Importantly, although this implicitlyand incorrectly assumes that every storm is above threshold, identicalapplication of this methodology to both current and future hurricanedata sets still permits an apples-to-apples comparison that capturesshifts in the probability distribution of the most extreme events.

    To extend this approach to changes in actual economic damage,though, reliable information on changes in landfall frequency (bothspatially and temporally) and economic value along the coast isneeded. For the former, there is little agreement across models even

    on the sign of the change in Atlantic hurricane frequency, much lesson landfall frequency (e.g., Knutson et al. 2010). Similarly, for thelatter, regional economic change is highly uncertain (Pielke et al.2000), although recent history indicates that development in vul-nerable coastal areas will continue to proceed at rates much higherthan those of much of the rest of the country (van der Vink et al.1998). Thus the authors elect to apply the methodology solely toexplore changes in the PDF of the damage index, leaving the issuesof frequency and economic value to future work.

    Fig. 5(a) displays the aggregate GPD PDFs for current and thefuture GFDL climates as well as for the historical record. The currentGFDL climate matches the historical PDF reasonably well, althoughwith a slight skew to the left [historical pðIi . 0:2Þ5 0:27 versusGFDL pðIi . 0:2Þ5 0:24], lending some confidence in the capacityof this model to project changes in the most damaging landfallinghurricanes into the future. Note that shifts in the PDF result fromchanges in the scale parameter because the shape parameter does

    Fig. 5. (Color) (a)Aggregate PDFs of the damage index from theGPDfit for the historical record (green dashed line) aswell as for the current and IPCCA1B future climates in the GFDL model; red (blue) area indicates rightward (leftward) shift in the distribution in the future climate; (b) change in themean scale parameter of theGPDfit with covariates of the damage index along theU.S. coast between the current and future climate simulations (markersize indicates relative spatial frequency of landfall for the combined data sets; the displayed changes do not exceed the standard errors in the parameterestimates themselves and thus are likely not statistically significant)

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  • not have covariates and thus remains constant. A rightward shift inthe PDF, indicative of an increased probability that a given land-falling storm will be more damaging, is evident under the A1Bscenario; this is independent of landfall frequency. The mean in-crease in the scale parameter is 0.01, or approximately 10% of itsmean value, and the probability of a stormwith a damage index valueexceeding 0.2 increases by 3.6%. The cause of this is likely theoverall upward shift in the distribution of maximum wind speedVmax under this scenario but also may be the result of shifts in thefrequency of landfalls at locations with different s. Importantly,though, the increase in the scale parameter lieswithin the range givenby the SEs for the parameter estimates [for average values of Vmaxand s, s5 0:106 with an error range of (0.084, 0.135)], indicatingthat the shift in the PDF is not statistically significant.

    Shifts in the distribution are disaggregated into coastal segmentsand are also shown inFig. 5(b), which displays the distribution of thechange in the mean value of the scale parameter between the currentand the future GFDL climates along the U.S. coastline. The largestincreases occur along the northern Gulf Coast (maximum increaseis 1 0:034), where the relative landfall frequency is the highest andwhere coastal slopes tend to be shallow (Fig. 3), making the regionconducive to large storm surges. Meanwhile, decreases are foundalong the mid-Atlantic and Northeast coasts (maximum decrease is20:028 on northernNorth Carolina coast). Again, given the SE rangefor the parameters and the reduced sample size that accompanies

    a regional disaggregation, even the largest increases in s are likelynot statistically significant.

    Given that the GFDL model lies at the upper end of the range ofprojections for changes in hurricane activity (although primarilyresulting from increases in landfall frequency), the authors per-formed an identical analysis for output of the ECHAM5 model,which lies on the lower end of this range. The ECHAM5 modeloutput exhibits virtually no shift in the aggregate GPD PDFs be-tween the current and future climate states (Fig. 6), with a spatialdistribution of changes in the scale parameter that is broadly similarto that of the GFDL model but with magnitudes that do not exceed0.013. Notably, ECHAM5 does a poorer job of reproducing thehistorical GPD PDF, with a PDF that is skewed farther to the leftthan GFDL [ECHAM5 pðIi 5 0:2Þ5 0:21]. The ECHAM5 andGFDLA1Bmodel results taken in combination help to elucidate thedisagreement in model projections of changes in the most damaginglandfalling hurricanes and underscores the need for a multimodelensemble approach that also accounts for changes in landfall fre-quency to assess changes in hurricane damage risk with greaterfidelity.

    Discussion and Conclusion

    This study employs a peaks-over-threshold approach to the analysisof hurricane damage that enables use of the generalized Pareto

    Fig. 6. (Color) As in Fig. 5, but for the ECHAM5 model

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  • distribution (GPD) tomodel excesses above a specified threshold. Inaddition to the historical database of total damage given in Pielkeet al. (2008), the authors define a damage index as the ratio of base-year economic damage to the available economic value in the af-fected region. The authors then incorporate physical covariates at theindividual hurricane level into the GPD, namely, maximum windspeed and a simple yet novel measure of the mean bathymetric slopeat landfall, and apply the analysis to both the total damage and thedamage index for the purposes of direct comparison.

    The authors find, based onmaximum-likelihood estimation (MLE),that while both total damage and the damage index are sensitive tomaximum wind speed, the damage index is additionally sensitive tothe local bathymetric slope and has covariate coefficients that areconsistent with the known physics of each in causing damage. Fur-thermore, the tail behavior of the modeled distribution of the damageindex, particularly with covariates, suggests that the fat tail associatedwith the distribution of total damage may primarily be a consequenceof the large variation in economic value along the coast.

    The authors provide an illustrative example of the potential use ofthis new methodology by applying it to data sets containing ap-proximately 5,000 synthetic landfalling tracks for current and futureA1B-scenario climate states, as simulated by two climate models:GFDL CM2.0 and ECHAM5. Climate models disagree stronglyabout how Atlantic hurricane activity will change in the future; theGFDL model lies on the upper end of this model range, primarilybecause of changes in landfall frequency, whereas the ECHAM5model lies toward the lower end. Therefore, this application shouldnot be interpreted as an assessment of hurricane risk. Comprehen-sive assessment of changes in the total economic damage requiresanalysis of output from multiple climate models that accounts forchanges in both landfall intensity and frequency, as well as changesin economic value, at all points along the coast, a worthy task that isleft to future work.

    Nonetheless, this work makes three novel contributions to thefield of economic damage and hurricanes:1. Incorporation of physical quantities of individual storms as

    covariates in an extreme value theory framework;2. Inclusion of a simple but useful representation of the effects of

    local bathymetry on the damage capacity of a storm; and3. Calculation of a damage index and statistical comparison

    between total damage and the damage index as metrics forthe relationship between economic damage and landfallinghurricanes.

    The results lend support to the ideas put forth in a global contextby Neumayer and Barthel (2011): if there exists significant spatialvariation in economic value that is susceptible to damage froma natural hazard such as a landfalling hurricane, efforts to attributedamage to the physical components of the natural hazard usingabsolute economic damage data for that disaster may confoundeconomic and hazard variability. In its place, the authors find thatdamage measured as a fraction of potential damage can help toremove the spatial economic variability from the damage database,leaving a signal that may be better representative of the physicalrelationship between the hazard and the damage it causes. A keylimitation to this approach, though, is the availability of sufficientdata that can capture the spatial distribution of economic valueaffected by hazard events. Moreover, it is unclear whether sucha damage index is as easily defined in the context of more spatiallyand temporally complex hazards such as droughts and heat waves,particularly in developing countries, where adequate observations ofboth physical storm characteristics and local economic data may notbe available (United Nations 2011; Klose 2011).

    Nevertheless, this work provides an appealing alternative frame-work for assessing the relationship between the physical characteristics

    of landfalling hurricanes and the economic damage they cause, andit illustrates the potential applicability of this framework in assessingeconomic risk in both current and future climates. Given the sta-tistically sparse historical record for landfalling hurricanes, choos-ing a proper framework for analysis is important both for optimizingthe information extracted from the available data and for interpretingthe results. Subsequently, these results serve as the bases for riskassessments. Thus the approach outlined here may provide a stepforward in the interpretation of and adaptive response to the his-torical record of hurricanes and economic damage.

    Given the uncertainties in the input data highlighted earlier, it isimportant to consider the potential effects of these uncertainties onthe GPD parameter estimates. In particular, the inability to accountfor damage resulting from inland flooding may result in an over-estimation of the damage index in cases where inland floodingaccounts for a significant portion of a storm’s total damage becausethe economic value of the affected inland counties is not accountedfor in the calculation of Eq. (3). The effect on the statistical modeltherefore would be a GPD PDF that skews toward higher damageindex values—i.e., overestimates of s, j, or both. If these over-estimates are significant, this could potentially explain some of thediscrepancy between the climate models’GPD PDFs and that of thehistorical record. Additionally, the decrease in data quality earlier inthe twentieth century poses challenges given that the behavior of theupper tail is sensitive to the few storms that reside in that region ofthe PDF. This is particularly important for total damage, for whichthe top three landfalls all occurred prior to 1930. On the other hand,for the damage index, although the third-largest value is associatedwith a storm that occurred in 1918, the top two cases and, moregenerally, most of the highest values (e.g., 80% for Ii . 0:2) oc-curred after 1950, when both meteorologic observations and eco-nomic data were more precise. This suggests that the results for totaldamage may be less certain than for the damage index; it is unclearwhether this decreased data quality would cause a high or low biasrelative to the true value.

    Future work seeks to extend the application of this analysis toprojections of future hurricane activity across the full suite of climatemodels to gain a better quantitative assessment of how the distri-bution of extreme hurricane damage events may shift in the futureunder alternative climate states. A more rigorous accounting of thespatial distribution of economic damage (i.e., coastal versus inland)would serve to greatly enhance the precision of this approach. Fi-nally, coupling of this work with models of landfall frequency andcoastal economic vulnerability may enable quantitative evaluationsof absolute economic risk from landfalling hurricanes both todayand in the future, which may be of greater interest to stakeholdersand policy makers alike.

    Acknowledgments

    The authors thank Rick Katz, Eric Gilleland, and Asuka Suzuki-Parker for their valuable help and guidance as part of the NationalCenter for Atmospheric Research (NCAR) 2011 Advanced StudyProgram (ASP) Summer Colloquium on the Statistical Assessmentof ExtremeWeather Phenomena under Climate Change. The authorsalso thank the NCAR ASP for inviting them to the workshop thatafforded them the opportunity to meet one another and to learn thetechniques applied in this work. NCAR is sponsored by the NationalScience Foundation. The authors also gratefully acknowledge Profes-sor Roger Pielke Jr., who provided the economic damage and coastaleconomic value data sets, and Professor Kerry Emanuel and his com-pany, WindRiskTech, LLC, who provided hurricane tracks from hismodel. NCAR is sponsored by the National Science Foundation.The authors thank three anonymous reviewers for providing valuable

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  • feedback that significantly improved this work. The authors alsothank Kevin Sharp at ICAT Managers, LLC, for help obtainingdamage data and both ICAT and StormPulse.org for their usefulweb interfaces for exploring hurricane landfall data.

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