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Pliocene Environments

R Z Poore, Center for Coastal and Watershed

Studies, FL, USA

Published by Elsevier B.V.

Introduction

The Pliocene spans the interval of Earth history fromca. 5.3 to 1.8 million years ago (Ma; including theGelasian Stage of the Upper Pliocene, which is nowconsidered the base of the Quaternary subera, begin-ning at 2.6 Ma). During the Pliocene, the Earth com-pleted a transition from relatively warm climaticconditions with little or no continental ice in theNorthern Hemisphere to more variable climatic con-ditions with significant continental ice sheets in theNorthern Hemisphere and alternating glacialinterglacial conditions. The early Pliocene, fromabout 5.3 to 3.0 Ma, represents the last time in

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Earths history when climates were generally warmerand more equable than modern climate. At the startof the late Pliocene (after 3.0 Ma) significantNorthern Hemisphere ice sheets began to build-up,mid- and high-latitude areas cooled, and climatevariability, as measured by 18O in deep-sea carbonate

microfossils, increased. Distinct glacialinterglacialcycles developed between 2.9 and 2.7 Ma.The high-latitude cooling and onset of Northern

Hemisphere glaciation (NHG) that occurred in thelate Pliocene represents the culmination of a trendthat started 50 Ma in the early Eocene. Figure 1shows a composite 65 Ma 18O record measured incalcareous benthic foraminifers in deep-sea sedi-ments. The 18O is a measure of the ratio of16O:18O in the shells of the foraminifera which iscontrolled primarily by the temperature of seawater and the 18O of sea water when the shell isformed (see Oxygen Isotope Stratigraphy of the

Oceans). Lower temperatures and/or more globalcontinental ice volume at the time of shell formationreduces the 16O in the shell and results in morepositive 18O. The global deep-sea 18O record forthe last 50 Ma shows an overall increase toward thepresent day. From about 40 Ma to the present much

of the change in the18

O record reflects the build-upof polar ice volume and related cooling of high lati-tudes (Zachos et al., 2001). Prior to 3 Ma most ofthe ice build-up and variability was in Antarctica.The increase in 18O after 3 Ma is primarily relatedto the development and variability of NorthernHemisphere ice sheets. Most workers concur thatthe long-term trend seen in Figure 1 is related to theoverall decrease in atmosphere CO2 concentrations(Fig. 2) that led to an overall cooling of Earth.

Changes in the Earths orbit alter the distributionof incoming solar radiation. The change in solarradiation causes long-term 104106 oscillations in

the Earths climate (see Milankovitch Theory andPaleoclimate). Orbital cycles include 1923 thousandyear (ky) cycles related to precession of the Earthsrotational axis, a 41-ky cycle related to the tilt of theEarths rotational axis (obliquity), and a 100-ky cyclerelated to changes in the shape of the Earths orbitaround the Sun (eccentricity). The orbital forcing andthe Earths climate response to the forcing varythrough time.

Figure 3 shows a typical composite deep-seabottom-water 18O record for the last 5 millionyears (My). Most 18O values from 5 to 3 Ma vary

within a narrow range between 3.5 and 3.0 per mil.Isotopic values are generally similar or smaller thanLate Pleistocene interglacial values (shown by dashedvertical line onFig. 3), which indicates that mid- andhigh-latitude conditions during the early Pliocenewere usually as warm or warmer than latePleistocene interglacial intervals. The amplitude ofvariability in the isotopic record is relatively smallcompared to the late Pliocene and Pleistocene, whichindicates that variability in early Pliocene climate was

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Figure 1 Composite global oxygen isotope record complied

from measurements of 18O in shells of deep-sea benthic forami-

nifers. The measurements were adjusted for vital effects. The red

line represents an adjusted five-point running mean. Highly gen-

eralized occurrence of polar ice-sheets is indicated by black bars.

Black vertical bars show the occurrence of east and west

Antarctic ice sheets and Northern Hemisphere ice sheets. Small

and ephemeral ice sheets may have developed in the Northern

Hemisphere as early as the late Miocene but permanent ice

sheets did not develop until the middle of the Pliocene. Modified

from Zachos J,et al. (2001) Trends, rhythms, and aberrations in

global climate 65 Ma to present. Science 292: 686693, repro-

duced with permission.

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Figure 2 Estimates of atmospheric pCO2 for the last 60Ma.

From Zachos J, et al. (2001) Trends, rhythms, and aberrations in

global climate 65 Ma to present. Science 292: 686693, repro-

duced with permission. 2001 AAAS.

PALEOCLIMATE RECONSTRUCTION/Pliocene Environments 1949


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timing of inferred Pliocene conditions can be ques-tioned. However, the major differences indicatedbetween the present day and early Pliocene are sup-ported by multiple studies(Dowsettet al., 1999).SSTin high and mid-latitudes were several degreesCelsius above modern. Evergreen vegetation wasmuch more extensive in the Northern Hemispherehigh latitudes during the early Pliocene, largely atthe expense of more arid and cooler tundra andpolar desert environments. Note the reduction incontinental ice sheets (glaciers) compared to the pre-sent day and the absence of summer sea ice in theArctic Ocean. The inferred reduction in continentalice sheets, as well as the elevation of geomorphicfeatures such as Pliocene shorelines on continentalmargins, indicates that early Pliocene sea level was2030 m above modern sea level.

Mechanisms for Pliocene Warmth

Although climate records from a variety of areasindicate that climate during the early Pliocene wasoften warmer than modern climate, the explanationfor the warmer climate is still a matter of debate.Mechanisms commonly mentioned as the forcing

for warm early Pliocene climate include higher atmo-spheric concentrations of carbon dioxide (CO2),increased oceanic heat transport, and a permanentEl Nino SST patterns in the equatorial Pacific Ocean.

Estimates based on carbon isotope records in mar-ine deposits (Raymo et al., 1996) indicate that max-imum global atmospheric CO2concentrations in theearly Pliocene were around 425 parts per million(ppm) which is above the preindustrial (1850) atmo-spheric CO2level of 280 ppm but only slightly higherthan the 2,000 level of ,370 ppm. Estimates ofPliocene atmospheric CO2 concentrations based on

leaf stomatal frequency and size variation (Kurschneret al., 1996) suggest values varied between 280 and370 ppm (see CO2 Reconstruction from FossilLeaves). The difference in atmospheric CO2 levelsbetween the Pliocene and today seems too small toaccount for the climate differences inferred from thegeologic record (Crowley, 1996). In addition, warm-ing due solely to increased atmospheric CO2 shouldresult in warming at all locations. In contrast, theavailable Pliocene observations indicate temperaturesin most tropical areas were very similar to modern.

Increased oceanic heat transport is another

mechanism that has been proposed to explainPliocene warmth. Mapping of Pliocene SST estimatesin the North Atlantic Ocean (Dowsett et al., 1992)and estimates of deep-ocean circulation based oncarbon isotope gradients in deep-ocean basins indi-cate that thermohaline circulation (THC) was inten-sified during much of the early Pliocene. IncreasedTHC would result in increased transport of warmlow-latitude waters via the Gulf Stream into theNorth Atlantic Ocean and explain the increasedSST inferred from microfossil assemblages preservedin deep-sea sediments (Fig. 5). Increased transport ofheat into the North Atlantic should result in cooler

equatorial SST. However, available SST estimatesfrom equatorial areas of the Atlantic indicate SSTwas similar to modern. It is possible that earlyPliocene warming was due to some combination ofslightly higher atmospheric CO2and increased THC.

Another proposed explanation for Pliocenewarmth is that a permanent El Nino-like state existedin the early Pliocene. In the modern ocean, easterlywinds in the equatorial Pacific essentially pile upwarm water in the western Pacific and cause upwel-ling of deeper cool waters to the surface in the eastern

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Figure 4 Comparison of generalized reconstruction of global

environmental conditions for modern (top panel) and early

Pliocene (bottom panel) warm intervals. Reconstruction is

designed for use in modeling experiments and is done on a

2 latitude by 2 longitude grid. Top panel shows modern summer

conditions. Bottom panel shows early Pliocene warm interval

summer conditions. Land cover including glaciers and sea ice

extent are color coded. Sea-surface temperatures are shown by

isotherms in degrees centigrade. Open ocean areas are white.

See text for discussion. Adapted from Dowsett HJ et al. (1999)

Middle Pliocene paleoenvironmental reconstruction; PRISM2.

US. Geological Survey Open-File Report 99535, reproduced

with permission.



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equatorial Pacific. The result is a west-to-east ther-mal gradient of about 37 C. During El Nino events,the easterly winds relax and the warm water in thewestern equatorial Pacific flows back to the eastresulting in higher SST in the east and a reductionin the west-to-east thermal gradient. The change in

equatorial surface water conditions alters atmo-spheric circulation patterns and affects climate con-ditions in a number of areas of the world throughatmospheric teleconnections. The permanent ElNino theory for Pliocene warming argues that themodern temperature gradient across the equatorialPacific Ocean was absent due to warmer SST in theeastern equatorial Pacific, possibly because theIndonesian Seaway was more open, which wouldprevent pooling of water in the western equatorialPacific. The more uniform temperature conditionsfrom west to east would then foster increased mer-idional atmospheric heat transport (enhanced

Hadley cell circulation) which provided themechanism for increased warming in high andmid-latitudes (Molnar and Cane, 2002).Comparison of early Pliocene reconstructed climateestimates with climate anomalies experienced dur-ing El Nino events (Fig. 6) show some similaritiesand provide support for the interpretation thatmodern El Nino teleconnections could explainearly Pliocene conditions. The comparison must beevaluated with caution because the estimates ofPliocene conditions are derived from different

times in the Pliocene and integrate different inter-vals of time. A recent SST reconstruction for theeastern equatorial Pacific (Wara et al., 2005) indi-cate that the temperature gradient across the equa-torial Pacific was substantially reduced during thePliocene compared to today, which is consistent

with a permanent El Nino-like state (Fig. 7).However, the reduced temperature gradient per-sisted until , 1.8 Ma, well after high-latitude cool-ing and build-up of continental ice sheets began inthe late Pliocene. Thus, there is conflicting evidencefor the correspondence of a reduced west-to-eastthermal gradient in the equatorial Pacific and mid-to-high-latitude warming.

A number of modeling studies using PRISMboundary conditions have attempted to test the dif-ferent mechanisms for explaining the warm Plioceneclimates. A recent experiment using a fully coupledoceanatmosphere model with PRISM boundaryconditions and an atmospheric CO2 level of400 ppm indicated that lower albedo related toreduction of continental and sea ice may be the pri-mary reason for increased warmth in mid- and highlatitudes. The model results also suggest that pole-ward heat transport was not an important factor inPliocene warmth (Fig. 8; Haywood and Valdes,2004).

Intensification of Northern Hemisphere

Glaciation

The late Pliocene is marked by the development ofsignificant Northern Hemisphere continental icesheets, associated cooling of high latitudes, and thedevelopment of distinct glacialinterglacial cyclesthat are characteristic of the Pleistocene. The historyof NHG is difficult to decipher from continentalrecords because recurring glaciations often overrideand obscure the evidence for prior glacial advances.Examining ice-rafted grains in deep-sea sedimentsadjacent to continents is often used to trace the evo-lution of continental ice sheets (see GlacimarineSediments and Ice-Rafted Debris). When continental

ice sheets reach coastlines, icebergs are calved andfloat out into the ocean. The subsequent melting ofthe icebergs releases rock fragments and other debristrapped in the ice and the debris drops to the seafloor. The amount, composition, and areal distribu-tion of the ice-rafted debris in marine sediments canthen be used to infer the history of ice sheets andglaciers on the continents. Examination of sedimentcores from the Iceland and Voring Plateaus revealssporadic occurrences of ice-rafted debris as far backas the Miocene. However, the ice-rafted material is

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Figure 5 Map of North Atlantic Ocean showing difference in

summer sea-surface temperature (SST) between early Pliocene

warm interval and modern. Circled dotes represent localities at

which early Pliocene SST estimates were made. Contours repre-

sent estimated differences in SST in degrees Celsius. Early

Pliocene SST in northeastern Atlantic were 56C higher in

early Pliocene compared to today suggesting increased heattransport by the Gulf Stream. Adapted from Dowsett HJ et al.

(1992) Micropaleontological evidence for increased meridional

heat transport in the North Atlantic Ocean during the Pliocene.

Science 258: 11331135, reproduced with permission. 2001

AAAS.



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sheets in the Northern Hemisphere is shown inFigure 10. The formation of small ice sheets beganduring the early Pliocene in southern Greenland andthen spread to northern Greenland by about 3.0 Ma.Between 2.8 and 2.5 Ma ice sheets developed in

Eurasia, Alaska, and North America. As noted inFigure 10, the development of Northern Hemispherecontinental ice sheets was a phased process. As muchas 200 ka may have elapsed between the establishmentof major ice sheets in Alaska and Eurasia and estab-lishment of ice sheets in North America. Glacialinter-glacial oscillations become distinct in the deep-seaoxygen isotope records between 2.9 and 2.7 Ma coin-cident with the continued build-up of the NorthernHemisphere continental ice sheets. At the same timethat continental ice sheets were building up in the

Northern Hemisphere, the Antarctic Ice Sheet wasalso expanding. Details of the late Pliocene history ofthe Antarctic Ice Sheet are not well-known.

Cause of Northern Hemisphere Glaciation

Several tectonic events that altered oceanic or per-haps atmospheric circulation have been proposed to

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zonalSSTdifference

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Figure 7 Estimated temperature gradient between western and

eastern equatorial Pacific Ocean for last 5 Ma. Note that the

gradient was greatly reduced from 1.8 Ma to 5.0Ma compared

to gradient of 4 C 7 C after 1.8 Ma. See text for discussion.

Adapted from Wara M, et al.(2005) Permanent El Nino-like con-

ditions during the Pliocene warm period. Science309: 758761,

reproduced with permission. 2005 AAAS.

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Figure 8 Model predicted difference in poleward heat transport

(PW) between Pliocene coupled and present-day coupled model

experiments using the HadCM3 GCM. The plot indicates pole-

ward heat transport in the Pliocene and modern simulations are

very similar. Reprinted from Earth and Planetary Science Letters

218, Haywood AM and Valdes PJ, Modelling Pliocene warmth:

contributions of atmosphere, oceans and cryosphere, 363377.

2004, reproduced with permission from Elsevier.

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Figure 9 Abundance of ice rafted debris (IRD) in sediments

from ODP Site 907 just east of southern Greenland. Note con-

sistent occurrence of significant amounts of ice-rafted debris after

3.0 Ma. Adapted from Jansen E, et al., 2000, Pliocene

Pleistocene ice rafting history and cyclicity in the Nordic Seas

during the last 3.5 Myr. Paleoceanography 15: 709721, repro-duced with permission. 2000 American Geophysical Union.



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explain the intensification of NHG in the latePliocene. Some of the possible events include theclosing of the Isthmus of Panama, the closure of the

Indonesian seaway, the uplift of the Tibetan Plateau,the deepening of the Bering Straits, and the deepeningof the GreenlandScotland Ridge (Maslin et al.,1998; Cane and Molnar, 2001). When examinedclosely, none of these events appears to provide acomplete explanation for the intensification ofNHG. For example, the closure of the Isthmus ofPanama caused an increase in salinity of theCaribbean and presumably increased the strength ofthe Gulf Stream. It has been suggested that increasedtransport of warm, salty water to the North Atlantic

by an enhanced Gulf Stream would have provided asource of moisture to grow ice sheets on the adjacentcontinents and also increase formation of North

Atlantic deep water (NADW). However, the closureof the Isthmus of Panama was not a discrete event; itoccurred gradually and over a long period, fromabout 4.5 to 2.0 Ma. In addition, carbon isotoperecords from North Atlantic deep-sea cores indicateNADW formation decreased from 3.5 to 2.0 Ma.

A reduction in atmospheric CO2 concentrationhas been proposed as the cause for NHG.However, there is no clear evidence for a significantlowering of atmospheric CO2 levels coincident withthe intensification of NHG. In addition, a cooling

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Figure 10 Map and graph showing location and timing of build-up of Northern Hemisphere continental ice sheets large enough to

release icebergs into the ocean. Repeated glacial build-up occurred in Iceland during the Pliocene but the timing of the first sustained ice

sheet is not well established. Open circles with three digit numbers represent Ocean Drilling Program cores used to establish ice sheet

history. Arrows and fronts outline possible atmospheric circulation in preglacial Pliocene. Adapted from Quaternary Science Reviews17,

Maslin et al. The contribution of orbital forcing to the progressive intensification of Northern Hemisphere glaciation, 411426. 1998,

reproduced with permission from Elsevier.



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related to lowered atmospheric CO2 should berecorded in low latitudes but there is no evidencethat tropical climates cooled at the beginning of thelate Pliocene.

Variations in solar insolation reaching Earths sur-face due to changes in Earths orbit (orbital forcing)

have also been proposed as an important cause forthe intensification of NHG. In theory, ice sheetsshould form when summer temperatures are lowenough to allow winter frozen precipitation (snowand ice) to survive the summer and thus begin apositive cooling feedback through increased surfacealbedo. The initial build-up of ice sheets in Alaska,Eurasia, and northeastern North America between2.75 and 2.55 Ma (Fig. 10) occurred when calcula-tions show Northern Hemisphere summer insolationat 65 N reached minimum values (Maslin et al.,1998;Fig. 6). However similar or even lower summerinsolation values occur periodically earlier in the

Pliocene so summer insolation variability is not thesole cause of NHG.

Stratification and the resulting late summerwarming of North Pacific surface waters at about2.7 Ma have also been linked with the intensifica-tion of NHG. In addition to cooling, a source ofmoisture must be available to permit sufficientsnow to grow ice sheets. In the modern oceanthere is a large seasonal variation in SST in thesub-Arctic Northwest Pacific due to the formationof a seasonal thermocline. During winter, when nothermocline is present, surface waters are well

mixed with subsurface water. SST is about 1

C(February) and surface waters are rich in nutrientsmixed from below. In summer, when a thermoclineis present, surface waters are isolated from coolsubsurface waters and late summer SST(September) is 12 C. Estimates of North PacificSST derived from analyses of alkenones, whichlikely reflect late summer SST (Haug et al., 2005)indicated SST increased by about 7 C near 2.7 Ma.The increase in temperature is considered to reflectthe development of a seasonal thermocline in theNorth Pacific, which would allow the large seaso-nal range in SST observed in the modern ocean.The presence of warm surface waters in the NorthPacific in fall would enhance transport of moistureonto the adjacent cooler continents and increasesnowfall and thus promote build-up of continentalice sheets.Haug et al. (2005)proposed that prior tothe development of a seasonal thermocline at2.7 Ma, surface waters would have remained coolduring the seasonal cycle due to mixing with coolersubsurface waters. One problem with this theory isthat alkenone SST estimates indicate that subarcticNorth Pacific SST were nearly as warm at 2.9 and

3.0 Ma as they were at 2.7 Ma. In addition, theassumption that seasonal signals can be extractedfrom Pliocene proxy records needs additional test-ing.

Comparison of detailed climate proxy recordsfrom tropical to subpolar areas in the Northern

Hemisphere indicates no clear pattern of coolingin the late Pliocene across latitudes or regions.Ravelo et al. (2004) concluded that regionally spe-cific processes caused climatic cooling at differenttimes and that the termination of the early to mid-Pliocene warm interval was not forced by a singleevent whose effects were propagated globally.Thus, as of this writing, no clear single cause hasbeen identified as the triggering event leading tothe NHG.

Links Between Northern Hemisphere

Glaciation and Human Evolution

Records of African climate and climate variabilityderived from studies of deep-sea cores and conti-nental records generally show a change from wetterand warmer conditions in the early Pliocene tocooler and drier conditions with higher variabilityin the late Pliocene. Pollen records from a variety ofsites indicate that woodland areas were replaced bymore open savannah grasslands (Thompson andFleming, 1996; deMenocal, 2004). After about3.0 Ma, variability in African wetdry cycles as

monitored by the abundance of African-sourcedust in deep-sea cores indicates that African climatecycles correspond closely with variation in high-latitude continental ice volume cycles. In general,glacial climates result in cooler and more arid con-ditions. Changes in the African mammal faunaincluding hominids broadly coincides with thechange in climate and landscape that marks thedevelopment and intensification of NHG (Fig. 11).Analyses of bovids (antelope group) and micro-mammal assemblages (e.g., rodents) indicate thatarid-adapted taxa were more prevalent after3.0 Ma. The diversification of hominids coincides

with the change in pace of climate variability andthe expansion of grasslands. It is not clear if thechange in timing and magnitude of climate varia-bility or the change to more arid conditions orsome combination of both was the primary causeof the change in mammal faunas (deMenocal,2004). Although the details are still being debated,the available evidence indicates that the evolutionof hominids was linked to the change in climateassociated with the onset and intensification ofNHG during the late Pliocene.



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Abbreviations

CO2 carbon dioxideIRD ice rafted debriska thousand years agoky thousand yearsMa million years agoNADW North Atlantic deep waterNHG Northern Hemisphere glaciationppm parts per millonPRISM Pliocene Research, Interpretation,

and Synoptic MappingSST sea-surface temperatureTHC thermohaline circulation

See also:Glacial Landforms, Sediments: Glacimarine

Sediments and Ice-Rafted Debris.Glaciation, Causes:

Milankovitch Theory and Paleoclimate.

Paleoceanography, Biological Proxies: Alkenone

Paleothermometry from Coccoliths. Paleoceanography,

Physical and Chemical Proxies: Oxygen Isotope

Stratigraphy of the Oceans. Paleoclimate

Reconstruction: Paleodroughts and Society.

References

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way as a precursor to east African aridification around 34

million years ago. Nature411,157162.Crowley, T. J. (1996). Pliocene climates: the nature of the problem.

Marine Micropaleontology27, 312.deMenocal, P. B. (2004). African climate change and faunal evolu-

tion during the PliocenePleistocene. Earth and PlanetaryScience Letters 220,324.

Dowsett, H. J., Barron, J. A., and Poore, R. Z. (1996). Middle

Pliocene sea surface temperatures: a global reconstruction.

Marine Micropaleontology27, 1325.Dowsett, H. J., Barron, J. A., Poore, R. Z., et al. (1999) Middle

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Geological Survey Open-File Report99535.Dowsett, H. J., Cronin, T. M., Poore, R. Z., Thompson, R. S.,

Whatley, R. C., and Wood, A. M. (1992). Micropaleontological

evidence for increased meridional heat transport in the North

Atlantic Ocean during the Pliocene.Science258,11331135.Haug, G. H., Ganopolski, A., Sigman, D. M.,et al. (2005). North

Pacific seasonality and the glaciation of North America 2.7

million years ago. Nature433,821825.Haywood, A. M., and Valdes, P. J. (2004). Modelling Pliocene

warmth: contributions of atmosphere, oceans and cryosphere.

Earth and Planetary Science Letters 218,363377.

A.afarensis

African ClimateVariability

Hominid Evolution

0

1

Age(Ma)

241 kyr

Acheulean

A.garhi A

.africanus

A.afarensis

Paranthropusaethiopicus

P.robu

stus

0 4 8 12 InterglacialGlacial

P.

bois

ei

Grassland Woodland

100 kyr

A.

bahrelgha

zali

Ardipithecusramidus

A.anamensis

Kenyanthropus.platyops

Oldowan

?

??

??

100 kyr

H.sapiens

H.erectus

H.habilis

Homorudolfensis

3

4

East AfricanSoil carb. 13C

18O

41 kyr

23-19 kyr

Figure 11 Summary of Hominid Evolution in Africa compared with evolution of Pliocene and Pleistocene climate as measured by soil

carbon isotope values in East Africa and composite benthic foraminifer deep-sea isotope record. Note increase in Hominid speciation

coincides with expansion of grasslands, initiation of NHG, and change to 41 ky cycles near 3.0 Ma. Adapted fromEarth and Planetary

Science Letters, 220, deMenocal PB, African climate change and faunal evolution during the PliocenePleistocene, 324. 2004,

reproduced with permission from Elsevier.



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Paleodroughts and SocietyC J Mock, University of South Carolina, Columbia,

USA

2007 Elsevier B.V. All rights reserved.

Introduction

Severe drought is a recurring natural hazard that hasprominent economic and agricultural consequenceson society. Many societal planning schemes are basedon the modern record of interannual and decadaldrought variability (Stockton, 1990). The modernrecord of drought is restricted temporally and geo-graphically, depending on the availability of systema-tic instrumental weather records. For example, ourobserved record of drought for North America coversmostly the last 100 yr, whereas such modern recordsare substantially longer in Europe and East Asia.Some scholars have conducted research on theobserved record of severe drought, noting periodiccycles of aridity for some regions, such as the 22-yrdrought cycle in interior North America. These

drought cycles have been somewhat disputed becauseof the temporal limitation of coverage by the moderninstrumental record. Paleoclimatic reconstructions ofdrought have been conducted from a variety of dif-ferent proxy data, with some reconstructions trans-lating proxy terms into drought indices (e.g., Stahle

et al., 1998), and others providing more relative orpresence/absence information on drought conditions(e.g.,Masonet al., 1994).These paleoclimatic recon-structions provide a longer perspective of droughtvariability for a region, enabling a clearer analysisof the forcing mechanisms of severe drought, improv-ing probabilistic models of predicting future drought,and contributing to a more comprehensive analysis ofdrought impacts on past societies. This article pre-sents an overview of how some of the importanttypes of terrestrial paleoclimatic data are utilized ina drought context at different temporal scales withinthe Holocene, and how they are assessed in conjunc-

tion with potential forcing mechanisms of past cli-mate, emphasizing examples from North America.The article emphasizes temporal variability at annualto decadal timescales, as this variability is thecommon element that directly links past droughtto the modern instrumental record (Overpeckand Webb, 2000). Data for the paleoclimaticreconstructions used in this paper were taken fromthe NOAA Paleoclimatology website at http://www.ncdc.noaa.gov.

Paleoclimatic Proxies of Past DroughtThis section describes neither the extreme details forall different types of proxy data nor the numerouscomplexities on drought signals from different proxydata types. For further information, refer toBradley(1999)and specific references cited in it concerningparticular proxy data and dating techniques.

Historical Records

Historical (noninstrumental) evidence of drought con-sists of exactly dated documentary records such asdiaries, newspapers, and chronicles (Baron, 1989).Documentary records provide the highest resolutionof all the paleoclimatic proxy data types, enablingdetailed reconstructions of daily weather events thatmay lead up to severe drought. Some documentaryevidence, such as annals, letters, and almanacs, possessless temporal resolution. They are reflective ofmonthly, seasonal, or annual conditions. However,these lines of evidence still provide some general infor-mation on extreme drought years (e.g.,Therrellet al.,2004). The documentary materials, however, cannotbe taken at face value, as poor data quality from

1958 PALEOCLIMATE RECONSTRUCTION/Paleodroughts and Society

pliocene environments, 2007 poore

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