Weather and climatic extremes can have serious and damaging effects on human society and infrastructure as well as on ecosystems and wildlife. Thus, they are usually the main focus of attention of the news media in reports on climate. There are some indications from observations concerning how climatic extremes may have changed in the past. Climate models show how they could change in the future either due to natural climate fluctuations or under conditions of greenhouse gas-induced warming. These observed and modeled changes relate directly to the understanding of socioeconomic and ecological impacts related to extremes.Integrative Biolog

Changnon, David

Changnon, Stanley

Easterling, David R.

Evans, Jenni

Groisman, Pavel Ya

Karl, Thomas

Knutson, Thomas R.

Kunkel, Kenneth E.

Mearns, Linda O.

Meehl, Gerald A.

Parmesan, Camille

Pielke, Roger

Pulwarty, Roger

Root, Terry

Sylves, Richard T.

Whetton, Peter

Zwiers, Francis

Bulletin of the American Meteorological Society

English

Texas ScholarWorks

413Bulletin of the American Meteorological SocietyAn Introduction to Trends in ExtremeWeather and Climate Events:Observations, SocioeconomicImpacts, Terrestrial EcologicalImpacts, and Model Projections*Gerald A. Meehl,a Thomas Karl,b David R. Easterling,b Stanley Changnon,cRoger Pielke Jr.,a David Changnon,d Jenni Evans,e Pavel Ya. Groisman,bThomas R. Knutson,f Kenneth E. Kunkel,c Linda O. Mearns,a Camille Parmesan,gRoger Pulwarty,h Terry Root,i Richard T. Sylves,j Peter Whetton,k and Francis Zwiersl*This is the first of five papers in the “Understanding Changes inWeather and Climate Extremes” series.aNational Center for Atmospheric Research, Boulder, Colorado.bNational Climatic Data Center, Asheville, North Carolina.cIllinois State Water Survey, Champaign, Illinois.dNorthern Illinois University, De Kalb, Illinois.eThe Pennsylvania State University, University Park, Pennsyl-vania.fGeophysical Fluid Dynamics Laboratory, Princeton, New Jersey.gUniversity of Texas at Austin, Austin, Texas, and University ofCalifornia, Santa Barbara, Santa Barbara, California.ABSTRACTWeather and climatic extremes can have serious and damaging effects on human society and infrastructure as wellas on ecosystems and wildlife. Thus, they are usually the main focus of attention of the news media in reports on cli-mate. There are some indications from observations concerning how climatic extremes may have changed in the past.Climate models show how they could change in the future either due to natural climate fluctuations or under conditionsof greenhouse gas–induced warming. These observed and modeled changes relate directly to the understanding of so-cioeconomic and ecological impacts related to extremes.hUniversity of Colorado, Boulder, Colorado.iUniversity of Michigan, Ann Arbor, Michigan.jUniversity of Delaware, Newark, Delaware.kCSIRO, Aspendale, Victoria, Australia.lCCCMA, Victoria, British Columbia, Canada.Corresponding author address: Dr. Gerald A. Meehl, Climate andGlobal Dynamics Division, NCAR, P.O. Box 3000, Boulder, CO80307-3000.E-mail: meehl@ncar.ucar.eduIn final form 9 August 1999.©2000 American Meteorological SocietyUnderstanding Changes in Weather and Climate Extremes and Their ImpactsThe following series of five articles was motivated by a need to develop a more comprehensive assess-ment of changes in weather and extreme climate events. We were interested not only in the impact of extremeweather and climate events, but whether these events were changing in frequency or intensity along with theirimpacts. Impacts were viewed in terms of loosely managed ecosystems where wildlife flourishes, as well associoeconomic systems and more heavily managed ecosystems such as agriculture. From a climate perspec-tive, this included a focus both on the historical record and projections for future change.During the summer of 1998 a group of nearly 30 climate scientists, social scientists, and biologists met for10 days at the Aspen Global Change Institute to discuss what we now know, and how we could reduce someof our major uncertainties. These articles summarize much of the work during that meeting and new informa-tion since the meeting.—Tom Karl, Stan Changnon, Dave Easterling, Jerry Meehl, and Camille Parmesan414 Vol. 81, No. 3, March 20001. IntroductionThough weather and climate extremes can havenegative effects on society and ecosystems in manyobvious ways (floods, droughts, damaging high winds,extreme heat, and cold, etc.), for some systems in someareas, extreme events are beneficial. Bird breeding inwetlands in the arid zone of Australia may only occurafter, say, a one in 5-yr rainfall event (i.e., the biotacould be poorer without sporadic extreme rainfallevents and perhaps even better off with more of them).The record warm and unusually dry winter in thenorthern United States resulting from the influence ofEl Niño 1997–98 brought major gains in reduced en-ergy costs ($5.6 billion), record retail and home sales($5 billion), and a reduction of about 800 lives nor-mally lost to winter conditions. Conversely, lossesin the southern and western states amounted to over$4 billion with 200 lives lost due to storm activity.Parts of the northwest coast of Australia receive mostof their rainfall from sporadically occurring tropicalcyclones, and even human systems (e.g., water stor-age) would be damaged without them. Therefore, weneed a comprehensive understanding not only of whathas happened and what may happen with changes inweather and climate extremes, but also what thosechanges could mean in a variety of different contextsin human and natural systems.2. Defining changes of extremesTo understand how changes in weather and climateextremes could influence society and ecosystems, itis useful first to conceptually address how such ex-tremes could change in a statistical sense. Figure 1presents a typical distribution of a climate variable thatis normally distributed, such as temperature. The solidcurve represents the present-day frequency distribu-tion of a weather phenomenon (such as the daily maxi-mum temperature). Shading indicates the extremeparts of the distribution, representing events in the tailsof the distribution that occur infrequently (i.e., valuesthat are far from the mean or median value of the dis-tribution). If there is a simple shift of the distributionin a future climate, there will be an increase in extremeevents on one end and a decrease at the other (Fig. 1a).This can occur through a change of the mean where,for example, if the temperature at a location warms bya certain amount, this will almost certainly produce anincrease in the number of extreme hot days and a de-FIG. 1. Schematic diagram depicting how changes in mean andvariance can affect extreme weather and climate events.415Bulletin of the American Meteorological Societycrease in the number of extreme cold days. It is im-portant to note that the frequency of extremes changesnonlinearly with the change in the mean of a distribu-tion, that is, a small change in the mean can result in alarge change in the frequency of extremes (Mearnset al. 1984).Other aspects of the distribution may also change.For example, the standard deviation in a future climatemay increase, producing changes in extreme events atboth ends of the frequency distribution (Fig. 1b). Achange in the variance of a distribution will have alarger effect on the frequency of extremes than achange in the mean (Katz and Brown 1992), thoughthese events must be “extreme” enough (i.e., more thanone standard deviation from the mean) for this resultto hold. Relatively speaking, a 1°C change in the stan-dard deviation of the distribution will have a greaterimpact on the frequency of an extreme temperaturethan a 1°C change in the mean of the distribution. Tocomplicate matters, the mean, standard deviation, andeven the symmetry of the distribution may change atthe same time, consequently altering the occurrenceof extremes in several different ways (Fig. 1c, show-ing changes in both mean and variance). Not only canthe parameters of the distribution change as notedabove (mean, variance, etc.) but for variables like pre-cipitation, which is not normally distributed but bet-ter represented by a gamma distribution, a change inthe mean also causes a change in the variance(Groisman et al. 1999). This helps to explain why in-creases in total precipitation are disproportionatelyexpressed in the extremes as discussed by theEasterling et al. paper in this series (Easterling et al.2000). However, the relationship between the meanand variance arises because each statistic depends onboth the shape and scale parameters of the gamma dis-tribution. It is possible to change those parameters ina way that adjusts the mean while holding the varianceconstant.Fortunately, it is often possible to estimate changesin infrequent extremes, such as those that might oc-cur once every 10–100 years, without detailed knowl-edge of the parent distribution. This is becausestatistical science provides a well-developed asymp-totic theory for extreme values (see, e.g., Leadbetteret al. 1983), which predicts that the largest observa-tion in a large sample, such as the annual maximumtemperature or 24-h precipitation amount, will tend tohave one of only three extreme value distributions de-pending only upon the shape of the upper tail of theparent distribution. One of these three distributions isthe familiar Gumbel distribution (Gumbel 1958). Thisapproach to extreme value analysis, together with re-lated techniques based on the study of the crossing ofhigh thresholds (e.g., Davison and Smith 1990), hasbeen used extensively and reliably in meteorology, cli-matology, and hydrology to predict the rare extremesof phenomena such as precipitation, streamflow, tem-perature, and wind speed. Statistical science continuesto make advances in extreme value theory (e.g., Smithet al. 1997; Dupuis and Field 1998).3. ConsequencesThe frequency and/or intensity of extremes canchange, and both can cause major problems. For ex-ample, during the 1990s the number of insured catas-trophes (weather events causing more than $5 millionin losses) in the United States doubled over prior fre-quencies. Losses soared, but the average loss per eventdid not increase.When we discuss extremes we must consider themfrom the point of view of the statistical characteristicsdescribed above and from the socioeconomic or eco-logical effects of the event. The latter can be thoughtof in terms of thresholds of the physical systems be-yond which serious impacts occur. For example, thereis an effect on human mortality and morbidity if thereis an occurrence of a series of days in summer whenminimum temperatures exceed 30°C (Kalkstein andSmoyer 1993). This relationship can also apply todomesticated animals. For cattle in the central GreatPlains, THI values (a combined temperature and hu-midity index) greater than 84 for a series of three daysresults in significant cattle deaths from heat stress(Hahn and Mader 1997).These are seemingly straightforward conse-quences, but often the relationship between humansociety, natural ecosystems, domesticated animals,wildlife, and weather and climate extremes is not al-ways linear and not intuitive. Thus, the effects of thesethresholds are not universal since the vulnerability ofthe human and natural systems contributes to howsevere the impact will be, independent of the physi-cal system itself. In this sense, the impact of climateon society and ecosystems could change due tochanges in the physical climate system (including bothnatural and anthropogenic causes) or due to changesin the vulnerability of society and ecosystems (evenif the climate does not change; e.g., Kunkel et al.1999).416 Vol. 81, No. 3, March 2000For example, if the level of hurricane activity ofthe more active hurricane seasons in the 1940s and1950s were to occur today, societal impacts would besubstantially greater than in earlier decades. The im-pact of a higher tropical storm frequency could bemade worse by population increases and density, morepeople living near the coast, greater wealth, and otherfactors (Pielke and Pielke 1997; Diaz and Pulwarty1997), though human choices have produced this situ-ation. Thus, it is worth noting in this context that thevulnerability of systems to particular extremes alsomay change as a result of adaptation to changing cli-matic means. Consider the following case: reducedfrost frequency may be interpreted as meaning a re-duced risk of frost damage to crops. But farmers sim-ply may use the opportunity this presents to plantearlier in the season, and effectively maintain theircurrent or even greater risk of a damaging frost. Onecould also imagine some similar adaptations in natu-ral ecosystems that would have the effect of reducingthe impact of the change in the frequency of extremesdue to the change in the mean.4. Aspects of weather and climateextremesThe series of papers that follows describes the cur-rent state of our knowledge concerning the effects ofweather and climate extremes from several points ofview, all of which are interrelated. First we examinestatistical changes in extremes already observed. Thenwe address the socioeconomic and ecological impactsthat are closely tied to changes in extremes, and lookat projections from climate model experiments con-cerning how weather and climate extremes couldchange in the future.Acknowledgments. We would like to thank the National Sci-ence Foundation, National Oceanic and Atmospheric Adminis-tration, National Aeronautics and Space Administration, and theU.S. Department of Agriculture/Forest Service for providing sup-port to the Aspen Global Change Institute to host the Climate Ex-tremes Workshop, August 1998. This workshop provided theimpetus for this and the accompanying papers.ReferencesDavison, A. C., and R. L. Smith, 1990: Models for exceedancesover high thresholds. J. Roy. Stat. Soc., 52B, 393–442.Diaz, H. F., and R. S. Pulwarty, 1997: Hurricanes, Climate andSocioeconomic Impacts. Springer-Verlag, 292 pp.Dupuis, D. J., and C. A. Field, 1998: Robust estimation of ex-tremes. Can. J. Stat., 26, 199–216.Easterling, D. R., J. L. Evans, P. Ya. Groisman, T. R. Karl, K. E.Kunkel, and P. Ambenje, 2000: Observed variability andtrends in extreme climate events: A brief review. Bull. Amer.Meteor. Soc., 81, 417–425.Groisman, P. Ya., and Coauthors, 1999: Changes in the probabil-ity of heavy precipitation: Important indicators of climaticchange. Climatic Change, 42, 243–283.Gumbel, E. J., 1958: Statistics of Extremes. Columbia UniversityPress, 375 pp.Hahn, G. L., and T. L. Mader, 1997: Heat waves in relation tothermoregulation, feeding behavior, and mortality of feedlotcattle. Proceedings of the Fifth International Livestock Envi-ronment Symposium, R. W. Bottcher and S. J. Hoff, Eds., Vol.1, American Society of Agricultural Engineers, 563–571.Kalkstein, L. S., and K. E. Smoyer, 1993: The impact of climatechange on human health: Some international perspectives.Experientia, 49, 969–979.Katz, R. W., and B. G. Brown, 1992: Extreme events in a chang-ing climate: Variability is more important than averages. Cli-matic Change, 21, 289–302.Kunkel, K. E., R. A. Pielke Jr., and S. A. Changnon, 1999: Tem-poral fluctuations in weather and climate extremes that causeeconomic and human health impacts: A review. Bull. Amer.Meteor. Soc., 80, 1077–1098.Leadbetter, M. R., G. Lindgren, and H. Rootzen, 1983: Extremesand Related Properties of Random Sequences and Process.Springer-Verlag, 336 pp.Mearns, L. O., R. W. Katz, and S. H. Schneider, 1984: Extremehigh temperature events: Changes in their probabilities withchanges in mean temperature. J. Climate Appl. Meteor., 23,1601–1613.Pielke, R. A., Jr., and R. A. Pielke Sr., 1997: Hurricanes: TheirNature and Impacts on Society. John Wiley and Sons, 298 pp.Smith, R. L., J. A. Tawn, and S. G. Coles, 1997: Markov chainmodels for threshold exceedances. Biometrika, 84, 249–268.

An introduction to Trends in Extreme Weather and Climate Events: Observations, Socioeconomic Impacts, Terrestrial Ecological Impacts, and Model Projections

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An introduction to Trends in Extreme Weather and Climate Events: Observations, Socioeconomic Impacts, Terrestrial Ecological Impacts, and Model Projections

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