QUANTITATIVE PALEOCLIMATIC RECONSTRUCTIONS FROM …

QUANTITATIVE PALEOCLIMATIC RECONSTRUCTIONS FROM

LATE PLEISTOCENE PLANT MACROFOSSILS OF

THE YUCCA MOUNTAIN REGION

by Robert S. Thompson1, Katherine H. Anderson2, and Patrick J. Bartlein3

Open-File Report 99-338

1999

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorialstandards (or with the North American Stratigraphic Code). Any use of trade, product, or firm names is fordescriptive purposes only and does not imply endorsement by the U.S. Government.

U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

1Denver, Colorado2Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado3Department of Geography, University of Oregon, Eugene, Oregon

1

INTRODUCTION

Plant macrofossil assemblages recovered from packrat (Neotoma) middens of late

Pleistocene age from the present-day Mojave Desert of southern Nevada contain plant species

that today live at higher elevations and/or farther north than the midden collection sites. Previous

reconstructions of late Pleistocene climates from packrat midden assemblages in this region

(Spaulding, 1985) assessed the minimum climatic differences from today by estimating the

present-day climatic differences between the fossil midden sites and the nearest current

occurrences of key plant species recovered from the Pleistocene middens. From this approach

Spaulding (1985) concluded that although late Pleistocene temperatures were considerably below

those of today, only modest increases in precipitation (relative to today) were necessary for

these plant species to survive in the current Mojave Desert during the late Pleistocene.

SpauldingÕs approach provided Òstate-of-the-artÓ results from an intensive careful

examination of the best data available at the time. However, data and techniques developed since

the mid-1980s suggest that there are two possible short-comings to this approach: 1) the use of

lowest elevational and (frequently) most southerly occurrences of key plant species results in

minimal estimates of the differences between Pleistocene and present-day climates, and 2) the

instrumental climate data set available to Spaulding was limited in duration, non-standard in its

method of collection, and indicated a modern climate wetter than the long-term historic mean,

which resulted in relatively small apparent differences between late Pleistocene and present-day

mean annual precipitation levels. In this report we use a more standard (close to the long-term

mean) modern calibration period and a modern plant distribution data set that permits us to

identify modern analogues for the Pleistocene vegetation. This reexamination permits a more

robust reconstruction of the past climate, and results in estimates of mean annual temperature for

the glacial maximum at Yucca Mountain that are 1.0¡ to 1.4¡ C warmer than those of Spaulding,

and estimates of mean annual precipitation that are 60 mm or more higher than his.

2

METHODS

In this report we use present-day climatic and vegetational data from across North

America to estimate past temperature and precipitation values from plant macrofossil

assemblages preserved in ancient packrat middens from southern Nevada. Our approach

involves: 1) compiling a comprehensive list of plant macrofossil assemblages from late

Pleistocene packrat middens of the region, 2) employing a ~25 km grid of present-day climate and

plant distributions in North America (Thompson and others, 1999), and 3) applying numerical

analysis that compared the North American data with the packrat middens to produce estimates

of past climates. The following text describes each of these aspects in greater detail.

Plant Macrofossil Assemblages from Packrat Middens.

Previous studies (e.g. Spaulding, 1985; Wigand and others, 1995) recovered numerous

packrat middens from southern Nevada dating to the most recent period of continental glaciation

during the late Pleistocene (~40,000 to 12,000 yr B.P. [~40 to 12 ka]). This report focuses on

quantitative paleoclimatic interpretations of the plant macrofossil assemblages reported by

Spaulding and Wigand, with particular emphasis on reconstructions of the climate from the last

glacial maximum (LGM, ~18 ka). In addition to this interval, we also estimated the past climates

for each of four intervals of the late Pleistocene (35 -30 ka, 27 -23 ka, 20.5 -18 ka, and 14-11.5

ka) that a panel of scientists from the Desert Research Institute (DRI), Dames and Moore

(D&M), DOE, University of Colorado (CU) and the USGS selected as key times in the

paleoclimatic history of the Yucca Mountain region (Figure 1). These time periods were selected

based on previous paleoclimatic studies involving packrat middens, ostracodes, and other data

sets that suggested that the climatic characteristics of each period were different. Initially, the

period from 35-30 ka was thought to be the wettest interval of the late Pleistocene, and the 20.5

to 18 ka the coldest.

Presence-absence information on all of the plant species identified from 39 packrat

middens in this region (Figure 1) were compiled by DRI and USGS researchers. The presence-

absence approach places equal weight on the occurrences of all species, rather than placing more

3

(or less) emphasis based on the apparent abundance of a given species. This approach seems

warranted, as there is no strong evidence that the abundance of a given plant species in a packrat

midden assemblage reflects the actual abundance on the landscape surrounding the midden.

The late Pleistocene packrat middens from the Yucca Mountain region were collected

from sites where today the plant cover is dominated by creosote bush (Larrea divaricata) and

other plants adapted to the hot and dry current Mojave Desert. In contrast, packrat midden data

indicate that during the late Pleistocene these sites hosted plants that today grow farther north

and/or at higher elevations (including limber pine [Pinus flexilis], white fir [Abies concolor], Utah

juniper [Juniperus osteosperma], big sagebrush [Artemisia tridentata], and shadscale [Atriplex

confertifolia]; Figure 2). Collectively the modern distributions of these plant species imply that

the late Pleistocene climate was cooler and wetter than that of today.

Present-Day Climate and Plant Distributions.

As part of other research (Thompson and others, 1999), we estimated the present-day

climate of North America for each of 31,363 points on a 25-km equal-area grid covering the

continent (Bartlein et al, 1994). We then digitized maps of the current distributions (from

published sources, including: Benson and Darrow, 1981; Critchfield and Little, 1966; Little,

1971, 1976; and Yang, 1970) of more than 300 major trees and shrubs and aligned these

distributions with the 25-km grid. This procedure provides the basis for direct comparisons

between the distributions of plant species and climatic parameters and is the foundation for the

paleoclimatic reconstructions in this report. In addition to the plant taxa presented in Thompson

and others (1999), the panel of DRI, D&M, DOE, and USGS scientists determined that

additional maps (not available from previous publications) were required for the modern ranges of

twelve additional plant taxa that were common in late Pleistocene packrat middens from the

Yucca Mountain region (Table 1). USGS and DRI personnel worked together to compile this

information, and USGS contractors digitized the maps and entered this information into the

dataset for paleoclimatic analysis.

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Abies concolor Atriplex confertifolia

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Juniperus osteosperma

Figure 2. The present-day distributions of four key plant species (Pinus flexilis [limber pine], Juniperus osteosperma [Utah juniper], Abies concolor [white fir], and Atriplex confertifolia [shadscale]) recovered from late Pleistocene packrat middens in the Yucca Mountain region.

5

6

Table 1. Plant species for which maps were compiled by the USGS and/or DRI and digitized bythe USGS.

Scientific Name Common NameAmbrosia dumosa white bur sageAtriplex canescens four-wing saltbushCercocarpus intricatus little-leaf mountain mahoganyChamaebatiaria millifollium fernbushChrysothamnus nauseosus rabbit brushColegyne ramosissima blackbrushEphedra nevadensis boundary ephedraEphedra viridis green ephedraFallugia paradoxa Apache plumePurshia tridentata antelope brushSymphoricarpos longiflorus snowberrySymphoricarpos oreophilus snowberry

Table 2. Present-day and Late Glacial Maximum (LGM) climatic estimates for approximately5000 ft (1524 m) elevation in the Yucca Mountain area (based on plant macrofossilsfrom packrat middens).

Annual AnnualTemperature Precipitation

PRESENT DAYThis study 13.4¡ C 125 mmSpaulding, 1985 13.5¡ C 189 mm

LAST GLACIAL MAXIMUMThis study 7.9 to 8.5¡ C 266 to 321 mmSpaulding, 1985 6.5 to 7.5¡ C 246 to 265 mm

7

Estimation of Climatic Parameters from Vegetation Data.

The inventory of fossil plant remains recovered from each packrat midden was compared

with the plant list from each North American grid point to identify those grid points whose

present-day vegetation is similar to the late Pleistocene vegetation found in the packrat midden

(these modern sites are hereafter referred to as ÒanaloguesÓ for the Pleistocene packrat middens).

The modern temperature and precipitation values at the analogue sites provide estimates for past

climates in the region surrounding Yucca Mountain. We used the latter information to identify

climatic patterns for selected past time periods by mapping the paleoclimatic estimates from

multiple packrat midden fossil localities, and by plotting them against the geographic locations

and elevations of the packrat midden sample sites. Climatic values for the late Pleistocene of

Yucca Mountain were estimated by using the climate-elevation relations derived from this

approach.

Modern analogue techniques, methods that identify sites where plants found in fossil

assemblages are living today, are widely used to reconstruct past climates in eastern North

America and Europe (for examples, see Overpeck and others, 1985 and Guiot, 1990). The fossil

and the gridded modern vegetation data in this study are both presence-absence data. In order to

obtain a measure of similarity between modern and fossil presence-absence data, we used the

binary Jaccard matching coefficient (Jaccard, 1908; Schweitzer, 1994) to compare each packrat

midden plant assemblage with each gridpoint within the modern data set. The Jaccard coefficient

provides a measure of how similar each modern gridpoint assemblage is to the fossil assemblage,

with a value of 0.0 indicating no shared species between the modern and fossil assemblages and a

value 1.0 indicating that the two assemblages have exactly the same list of species.

The Jaccard matching coefficient identifies modern sites that have vegetation that resembles

(to varying degrees as measured by this coefficient) the Pleistocene vegetation recorded in the

packrat midden samples. Some of these potential analogues may include some of the taxa in the

fossil assemblage, whereas other potential analogues may lack these but include other taxa

present in the fossil assemblage. To provide paleoclimatic estimates from fossil plant

assemblages under these circumstances, we used weighted averages of climatic parameters from a

8

large number of possible analogues for each fossil sample. To accomplish this, we had to make

three decisions: 1) what is the threshold value for the Jaccard coefficient below which we do not

consider the modern vegetation analogue to be similar enough to the fossil vegetation to be

included in the analysis? 2) how many possible analogues above a threshold value should be

included in each paleoclimatic estimate?, and 3) how should we weight the climatic information

from each analogue so that the information from the least similar modern samples (in comparison

with the fossil assemblage under consideration) does not overshadow the information from the

most similar modern samples? There are no firm guidelines for any of these questions, so we

experimented with different values for each of these three parameters and then objectively judged

the results by comparing climates at a suite of modern gridpoints with those estimated by our

method (see below).

Based on our experiments, we discarded potential modern analogues with Jaccard

coefficients less than 0.3, and then selected the 200 analogues with the highest Jaccard

coefficients above that value (if there were fewer than 200 analogues above that value, we used all

of them). As higher Jaccard coefficients imply greater similarity between the modern and fossil

vegetation than do low coefficients, we weighted the climatic data from the analogues by the ratio

of the cubes of their Jaccard coefficients. Hence the climatic data from a modern sample with a

Jaccard coefficient of 0.9 would be weighted approximately six times the data from a sample with

a coefficient of 0.5 (0.93 = 0.73; 0.53 = 0.13; 0.73/0.13 = 5.62). This ensures that the modern

sites with the greatest similarity contribute the most information to a given paleoclimatic

estimate, while also allowing a broad examination of the potential climates that could account for

the fossil vegetation.

We used the method described above with the present-day vegetation to estimate the modern

climate at each of 80 grid points from the western interior of the United States where weather

stations occur near the grid points (Figure 3). Regression analysis of the observed versus

estimated January, July, and annual temperature at the grid points yielded r2 values of 0.84 to

0.91 for temperature, indicating that, at least in the present-day situation, this method provides

very reliable estimates of temperature. The comparisons of observed and estimated modern

-40 -20 0 20January Temperature (ºC)

0

-40

-20

20r2 = 0.91 (0.86)

0 10 20 30July Temperature (ºC)

0

10

20

30 r2 = 0.84 (0.80)

1 10 100 1000January Precipitation (mm)

1

10

100

1000r2 = 0.71 (0.64)

1 10 100 1000July Precipitation (mm)

1

10

100

1000r2 = 0.79 (0.77)

Figure 3. Comparison of observed (x-axis) and estimated (y-axis) present-day climate normals for January and July temperature and precipitation based on the gridded

vegetation at 80 grid points in the interior of the western United States (r2 values in parentheses indicate the comparison between observed climate at weather stations near the grid points and the predicted value for the grid points).

9

10

January, July, and annual precipitation provided r2 values of 0.70 to 0.79 for precipitation and

0.71 to 0.80 for the log of precipitation. This indicates that our method produces somewhat less

robust, but still acceptable, estimates of precipitation.

ANALOGUE-BASED PALEOCLIMATIC RECONSTRUCTIONS

Modern Analogues for Pleistocene Vegetation and Climatic Differences from Today.

Using the method described above, the fossil data from packrat middens were compared to

our North American grid to locate the best modern analogues for the late Pleistocene vegetation.

Figure 4 provides an example of the geographic and climatic spread of the modern analogues

(before averaging) for a 13,740 yr B.P. packrat midden from southern Nevada. As seen in this

figure, most of the modern analogues for this sample (and for other late Pleistocene middens from

the Yucca Mountain region) lie in the steppe and woodland regions of the central Great Basin,

and to a lesser extent, the Colorado Plateau. The climates of these analogue sites are cooler and

wetter than the modern climates at this fossil-midden site (and at other midden sites).

We estimated the modern temperature and precipitation at each packrat midden site using the

same techniques we used to assign climatic values to our North American grid points (Thompson

and others, 1999). "Anomalies" for each climatic parameter at each site were then calculated by

taking the difference between the estimated modern climatic parameters at the packrat collection

site and the late Pleistocene estimates.

RECONSTRUCTION OF LATE PLEISTOCENE CLIMATES

Reconstructed Climates Through Time. The proposed site of the nuclear waste repository at

approximately 5000 ft (1524 m) elevation at Yucca Mountain was apparently at or near the

lower elevational limits of Pinus flexilis (Figure 5), indicating that relatively moist forest

environments did not extend below this elevation during the last ~40,000 years. However,

Ana

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ure

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Fig

ure

5. T

he o

ccur

renc

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lim

ber

pine

(P

inus

flex

ilis)

in p

ackr

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idde

ns fr

om th

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12

13

climate did vary through the late Pleistocene, and to quantify these changes we analyzed suites of

packrat midden assemblages grouped by the time periods selected by the advisory panel

(discussed above). Comparisons between time periods may be misleading, as the number of

samples and the elevational range vary greatly between periods. Setting aside these potential

problems, the analogue-based reconstructions suggest that the paleoclimate from 35 to 30 ka was

approximately 4¡ C colder than today in the Yucca Mountain region, and mean annual

precipitation was one and one-half times greater than today (Figure 6). The climate apparently

cooled and became wetter through 27 to 23 ka, culminating in the coldest period during the late

Wisconsin during the Last Glacial Maximum (LGM; 20.5 to 18 ka). Climatic conditions warmed

somewhat by 14 to 11.5 ka, which was the period of the highest level of reconstructed

precipitation during the late Pleistocene. In the following text we concentrate on reconstructions

of the LGM (Table 2) when the estimated climate was the coldest and mean annual precipitation

was significantly greater than that of the present-day.

Reconstructed Climates Versus Elevation. Maps of the reconstructed climate reveal

differences in the size of the anomalies (difference between modern and reconstructed past

climate parameters) within short distances, apparently due to the influence of the high

physiographic relief of southern Nevada. For the LGM, there are very strong relationships

between elevation and the amplitude of the temperature difference from today (r2 = 0.97 for

January, 0.96 for July, and 0.98 for annual temperature). There are weaker, but still strong

relations for precipitation anomalies versus elevation (r2 = 0.51 for January, 0.82 for July, and

0.88 for annual precipitation) for the late glacial period (14 to 11.5 ka). These paleoclimatic

reconstructions imply that during the late Pleistocene the gradients of decreasing temperature and

increasing precipitation with rising elevation were more gradual than those of today in southern

Nevada. Subsequent analyses of packrat midden assemblages from farther south in the Mojave

Desert to as far north as the Bonneville basin indicate that these apparent shallower-than-modern

relations between increasing elevation with decreasing temperature and increasing precipitation

occurred during the LGM throughout this region.

-8-7-6-5-4-3-2-10

11

.21

.41

.61

.82

2.2

2.4

2.6

2.8

Ann

ual P

reci

pita

tion

(mul

tiplie

d by

mod

ern

valu

es)

35 t

o 30

ka

(

n =

4)

27 t

o 23

ka

(

n =

5)

20

.5 t

o 1

8 k

a

(n

= 1

1)

14

to

11

.5 k

a

(

n =

19

)

Tod

ay

Annual Temperature (°C departure from Today)

Fig

ure

6. A

vera

ge p

aleo

clim

atic

cha

nges

infe

rred

from

pac

krat

mid

dens

in th

e Y

ucca

Mou

ntai

n re

gion

for

four

i

nter

vals

of l

ate

Ple

isto

cene

tim

e. A

rrow

s in

dica

te th

e ap

pare

nt d

irect

ions

of c

hang

e th

roug

h tim

e;

n =

num

ber

of a

ssem

blag

es.

As

disc

usse

d in

the

text

, ele

vatio

n ap

pare

ntly

influ

ence

s th

e de

gree

of

t

empe

ratu

re a

nd p

reci

pita

tion

chan

ge th

roug

h tim

e, a

nd th

is e

ffect

is n

ot ta

ken

into

acc

ount

in th

is fi

gure

.

14

15

Compilations of paleoclimatic proxy data and numerical model simulations of past

climates (Thompson and others, 1993) both provide support for the hypothesis that the

westerlies were displaced southward from their modern position into the southwestern United

States during the late Pleistocene. The persistence through the year of a strong pole-to-equator

temperature gradient at that time also caused the westerlies to be much stronger during the

summer season than they are today. These factors would have led to more frequent cyclonic

storms in the Yucca Mountain region throughout the year, enhanced cloudiness, and a

replacement of convective precipitation regime by frontal storms that provide precipitation

across the elevational range. This change in storm patterns would have reduced the rate of change

(relative to today) of decreasing temperature with rising elevation and the generally wetter climate

would also lower the temperature lapse rate.

Paleoclimatic Inferences from Missing Plant Species During the Late Pleistocene. As

discussed above, the late Pleistocene occurrences of limber pine (Pinus flexilis), Utah juniper

(Juniperus osteosperma), and other montane and steppe plants in modern desert environments

suggest cooler than modern temperatures, and greater moisture availability than today (Spaulding,

1985). Spruce (Picea engelmannii, P. pungens), lodgepole pine (Pinus contorta), subalpine fir

(Abies lasiocarpa), prostrate juniper (Juniperus communis) and other boreal plants have not been

found in packrat middens in southern Nevada, suggesting that the late Pleistocene climate was not

cold or wet enough for these boreal plants. On the other hand, Pleistocene-age middens from the

Yucca Mountain area lack evidence of ponderosa pine (Pinus ponderosa), Douglas-fir

(Pseudotsuga menziesii), and other montane trees that live today in the mountains of southern

Nevada. Their absence, and the rarity of single-needle pinyon pine (Pinus monophylla), may

suggest that Pleistocene climates were too cold for those plants. Similarly, the absence of the

now widespread creosote bush (Larrea divaricata) suggests that late Pleistocene temperatures

were much colder than those of today (Spaulding, 1985). In this section we address the question:

do our paleoclimatic estimates (if correct) provide an adequate explanation for the absence of

these forest and desert plants?

16

The present-day climatic and plant distributional data in Thompson and others (1999)

provide the basis for examining the apparent climatic meaning of the late Pleistocene absences of

key plant species in the Yucca Mountain Region. As illustrated in Figure 7, our reconstructed

LGM January temperatures were apparently too cold for the survival of Larrea divaricata (in

accordance with SpauldingÕs [1985] interpretation) or Juniperus californica. Conversely, winter

temperatures were apparently too warm for Picea pungens, Picea engelmannii, Abies lasiocarpa,

Juniperus communis, and Pinus flexilis (which, as illustrated in Figure 5, was apparently near its

lower elevational limits here during the LGM). The subsequent figures provide similar

illustrations of the reconstructed climates compared with the apparent tolerances of these species

in respect to July and annual temperature (Figures 8 and 9), and to January, July, and annual

precipitation (Figures 10 to 12). These figures collectively indicate that many of the ÔabsentÕ

taxa presently live under climatic conditions that differ in at least one aspect of seasonal or annual

temperature or precipitation from the analogue-based reconstruction of the LGM climate.

Table 3 summarizes the overall patterns observed in these figures, and as seen here, both

temperature and precipitation were important in excluding many of these taxa. However, the

reasons for the exclusions of Pinus edulis, Pinus longaeva, and Pinus monophylla are unclear. On

the other hand, all three of these species lived in or near southern Nevada during parts of the late

Pleistocene, so it is not surprising that the reconstructed climates would apparently permit their

growth near Yucca Mountain.

EVALUATION OF CLIMATIC ANOMALIES FOR THE LGM

As discussed above, Spaulding (1985) estimated climatic anomalies for the late

Pleistocene based on the present-day southern-most and/or lowest elevational occurrences of key

plant species that were recovered from packrat middens in the Yucca Mountain region. Tables 2

and 4 illustrate the differences between SpauldingÕs and our paleoclimatic reconstructions for the

LGM (interpolated to the 5000 ft [1524 m] elevation of the proposed repository). Although our

results indicate somewhat warmer and wetter conditions than SpauldingÕs, the absolute

Pinus monophylla

Tsuga heterophylla

Tsuga mertensiana

Abies magnifica

Pinus contorta

Picea engelmannii

Abies lasiocarpa

Juniperus communis

Larrea divaricata

Juniperus californica

Pinus ponderosa

Pseudotsuga menziesii

Pinus edulis

Pinus longaeva

Juniperus scopulorum

Picea pungens

Pinus albicaulis

Abies concolor


Artemisia tridentata

Pinus flexilis

-35

-30

-25

-20

-15

-10-505

101520Ja

nuar

y T

empe

ratu

re

Temperature (°C)

Fig

ure

7. P

rese

nt-d

ay c

orre

spon

denc

es b

etw

een

mea

n Ja

nuar

y te

mpe

ratu

re a

nd th

e di

strib

utio

n of

sel

ecte

d tr

ee a

nd s

hrub

spe

cies

from

wes

tern

Nor

th A

mer

ica

(dat

a ar

e th

e 10

% to

90%

dis

trib

utio

nal l

imits

from

Tho

mps

on a

nd o

ther

s, 1

999)

. T

he17

spe

cies

on

the

left

are

spec

ies

that

do

not

occu

r in

late

Ple

isto

cene

pac

krat

mid

den

asse

mbl

ages

from

the

Yuc

caM

ount

ain

regi

on (

alth

ough

Ju

nip

eru

s sc

op

ulo

rum, P

inu

s m

on

op

hyl

la, P

inu

s lo

ng

ae

va, a

nd P

seu

do

tsu

ga

me

nzi

esi

i a

ndha

ve b

een

reco

vere

d in

the

broa

der

sout

heas

tern

Cal

iforn

ia/s

outh

ern

Nev

ada

/sou

thw

este

rn U

tah

area

). T

he 4

spe

cies

on th

e rig

ht h

ave

been

rec

over

ed fr

om m

ultip

le m

idde

n as

sem

blag

es in

the

Yuc

ca M

ount

ain

regi

on (

alth

ough

P

inu

sfle

xilis

was

app

aren

tly a

t or

near

its

low

est l

imits

at t

he e

leva

tion

of th

e pr

opos

ed r

epos

itory

). T

he g

ray

band

repr

esen

ts th

e ra

nge

of p

aleo

clim

atic

est

imat

es fo

r th

e La

st G

laci

al M

axim

um fr

om th

e m

oder

n an

alog

ue a

naly

sis.

17


Pinus longaeva

Pinus ponderosa


Picea pungens

Picea engelmannii

Pinus albicaulis

Tsuga mertensiana


Pinus flexilis

Larrea divaricata

Pinus monophylla

Pinus edulis

Abies magnifica


Juniperus communis

Tsuga heterophylla

Pinus contorta

Abies lasiocarpa


Abies concolor

05

101520253035

July

Tem

pera

ture

Temperature (°C)

Fig

ure

8. P

rese

nt-d

ay c

orre

spon

denc

es b

etw

een

mea

n Ju

ly te

mpe

ratu

re a

nd th

e di

strib

utio

n of

sel

ecte

d tr

ee a

ndsh

rub

spec

ies

from

wes

tern

Nor

th A

mer

ica

(see

cap

tion

for

Fig

ure

7 fo

r a

mor

e co

mpl

ete

expl

anat

ion)

.

18

Ann

ual T

empe

ratu

re

-10

-5

0510152025

Temperature ( C)

Larrea divaricata


Pinus ponderosa

Pinus monophylla

Pinus edulis

Pinus longaeva


Tsuga heterophylla


Abies magnifica

Tsuga mertensiana

Picea pungens

Pinus contorta

Picea engelmannii

Pinus albicaulis

Abies lasiocarpa

Juniperus communis


Abies concolor


Pinus flexilis

Fig

ure

9. P

rese

nt-d

ay c

orre

spon

denc

es b

etw

een

mea

n an

nual

tem

pera

ture

and

the

dist

ribut

ion

of s

elec

ted

tree

and

shr

ubsp

ecie

s fr

om w

este

rn N

orth

Am

eric

a (s

ee c

aptio

n fo

r F

igur

e 7

for

a m

ore

com

plet

e ex

plan

atio

n).

19

Larrea divaricata

Pinus edulis

Pinus longaeva

Pinus monophylla

Juniperus communis

Picea pungens



Abies lasiocarpa

Picea engelmannii

Pinus ponderosa

Pinus albicaulis

Pinus contorta

Abies magnifica


Tsuga mertensiana

Tsuga heterophylla



Pinus flexilis

Abies concolor

050

100

150

200

250

300

350

Janu

ary

Pre

cipi

tatio

n

Precipitation (mm)

Fig

ure

10. P

rese

nt-d

ay c

orre

spon

denc

es b

etw

een

mea

n Ja

nuar

y pr

ecip

itatio

n an

d th

e di

strib

utio

n of

sel

ecte

d tr

ee a

ndsh

rub

spec

ies

from

wes

tern

Nor

th A

mer

ica

(see

cap

tion

for

Fig

ure

7 fo

r a

mor

e co

mpl

ete

expl

anat

ion)

.

20

Picea pungens


Juniperus communis


Abies magnifica

Pinus monophylla

Pinus longaeva


Picea engelmannii

Pinus edulis

Pinus albicaulis

Abies lasiocarpa

Larrea divaricata

Pinus contorta

Tsuga heterophylla

Pinus ponderosa

Tsuga mertensiana



Pinus flexilis

Abies concolor

020

40

60

80

100

120

140

July

Pre

cipi

tatio

n

Precipitation (mm)

Fig

ure

11. P

rese

nt-d

ay c

orre

spon

denc

es b

etw

een

mea

n Ju

ly p

reci

pita

tion

and

the

dist

ribut

ion

of s

elec

ted

tree

and

shru

b sp

ecie

s fr

om w

este

rn N

orth

Am

eric

a (s

ee c

aptio

n fo

r F

igur

e 7

for

a m

ore

com

plet

e ex

plan

atio

n).

21

Ann

ual P

reci

pita

tion


Picea pungens

Juniperus communis

Picea engelmannii

Pinus albicaulis


Tsuga heterophylla


Abies concolor

Larrea divaricata

Pinus edulis

Pinus monophylla

Pinus longaeva


Pinus ponderosa

Abies lasiocarpa

Abies magnifica

Pinus contorta

Tsuga mertensiana


Pinus flexilis

0

500

1000

1500

2000

2500

3000

Annual Precipitation (mm)

Fig

ure

12. P

rese

nt-d

ay c

orre

spon

denc

es b

etw

een

mea

n an

nual

pre

cipi

tatio

n an

d th

e di

strib

utio

n of

sel

ecte

d tr

ee a

ndsh

rub

spec

ies

from

wes

tern

Nor

th A

mer

ica

(see

cap

tion

for

Fig

ure

7 fo

r a

mor

e co

mpl

ete

expl

anat

ion)

.

22

23

Table 3. Climatic factors that may have excluded selected taxa from the Yucca Mountain region duringthe late Pleistocene.

Ê Ê TEMPERATURE Ê Ê PRECIPITATION ÊÊ January July Annual January July Annual

*Abies concolor Ê Ê DAbies lasiocarpa H H H Ê D DAbies magnifica Ê D DJuniperus californica C M C M W ÊJuniperus communis H H H Ê D M^Juniperus scopulorum Ê Ê M MLarrea divaricata C C C Ê ÊPicea engelmannii H H H D M DPicea pungens H H H Ê D DPinus albicaulis H H H D M DPinus contorta Ê H H Ê D DPinus edulis Ê Ê Ê*Pinus flexilis H H H Ê D^Pinus longaeva Ê Ê Ê*Pinus monophylla Ê Ê ÊPinus ponderosa Ê Ê D*Pseudotsuga menziesii Ê M Ê DTsuga heterophylla Ê H D DTsuga mertensiana Ê H M D D DÊ Ê Ê ÊÊ Ê Ê Ê Ê Ê Ê

Ê Ê KEY: Ê H = too hot for species, C = too cold, D = too dry, W = too wet, M = marginal Ê * = present near Yucca Mountain during part of the late Pleistocene Ê ^ = present regionally during the late Pleistocene Ê

Ê Ê

Ê Ê TEMPERATURE Ê Ê PRECIPITATION ÊÊ January July Annual January July Annual

Reason for exclusion: Êtoo warm or dry 6 10 8 6 8 14

too cold or wet 2 1 2 1 1 0

Plants for whom reason of exclusion unclear: Pinus edulis, Pinus longaeva, Pinus monophyllaÊ Ê Ê Ê Ê Ê Ê

Tab

le 4

. R

econ

stru

cted

LG

M c

limat

es a

nd c

limat

ic a

nom

alie

s ba

sed

on th

e pr

esen

t-da

y cl

imat

e es

timat

es u

sed

in th

is r

epor

t con

tras

ted

w

ith th

e pr

esen

t-da

y va

lues

use

d by

Spa

uldi

ng (

1985

).

A

nom

alie

s ba

sed

on

A

nom

alie

s ba

sed

on

Est

imat

ed

Rec

onst

ruct

ed

S

paul

ding

Thi

s R

epor

tV

alue

s

Val

ues

Mod

ern

Val

ues

Mod

ern

Val

ues

MO

DE

RN

LG

ML

GM

LG

ML

GM

LG

ML

GM

Min

umum

Max

imum

Min

umum

Max

imum

Min

umum

Max

imum

24

Mea

n A

nnua

l Tem

pera

ture

¡C¡C

¡C¡C

¡C¡C

¡CSp

auld

ing

13.5

6.5

7.5

76

6.9

5.9

Thi

s R

epor

t13

.47.

98.

55.

65

5.5

4.9

Mea

n A

nnua

l Pre

cipi

tati

onm

mm

mm

mm

mm

mm

mm

mSp

auld

ing

189

246

265

5776

121

140

Thi

s R

epor

t12

526

632

177

132

141

196

Mul

tipl

ier

for

Ann

ual P

reci

pita

tion

Mod

. XM

od. X

Mod

. XM

od. X

Spau

ldin

g1.

31.

42.

02.

1T

his

Rep

ort

2.1

2.6

1.4

1.7

Mod

. X =

Mod

ern

mea

n an

nual

pre

cipi

tatio

n m

ultip

lied

by th

is n

umbe

r

25

differences between the two approaches are not great for either mean annual temperature (6.5¡ C

versus 7.5 ¡ C) nor for mean annual precipitation (246 Ð 265 mm versus 266 Ð 321 mm). As

discussed in greater detail below, the differences between these estimates are substantially less

than the variability observed during the historic period.

Importance of Historical Climatic Baseline Data. Although the absolute differences between

our estimates and those of Spaulding (1985) are small, the calculated anomalies for precipitation

are large. As shown in Table 4, SpauldingÕs anomalies for LGM mean annual precipitation

(based on BeatlyÕs historic climate data) are 57 to 76 mm, which indicates precipitation at levels

1.3 to 1.4 times historic values. In contrast, our LGM mean annual precipitation anomalies

(based on our historic climate normals) are 141 to 196 mm, which implies LGM precipitation at

levels 2.1 to 2.6 times historic values. These comparisons point out the importance of the

modern climatic baseline data in the assessment of the differences between present-day and late

Pleistocene climates. As discussed above, our baseline is based on the climate ÒnormalsÓ for the

period 1951 to 1980, with the great majority of our data obtained from U.S. Weather Service

calculations of these ÒnormalÓ values. Thus these data provide a baseline that is continental in

scale, that is based on three decades of records, and that conforms to the U.S. Weather Service

standards. However, this data set does not provide detailed coverage in proximity to Yucca

Mountain. In contrast, Spaulding (1985) employed a modern baseline derived from data on the

Nevada Test Site collected by Beatly (1975, 1976) for the period 1963 to 1972. This dataset

provides more local data, but was collected with non-standard methods (which may over-

estimate precipitation) for only a decade.

For the analyses in this report we interpolated the modern climate estimates to the

elevation of the proposed repository at 5000 ft (1524 m). Although the two data sets have very

similar mean annual temperature estimates for this elevation (Tables 2 and 4) SpauldingÕs

estimates of modern mean annual precipitation are significantly higher than ours. The National

Oceanic and Atmospheric Administration (NOAA) provides historic climatic data by divisions

within each state (NCDC, 1994; Karl and others, 1986), and (unfortunately) Yucca Mountain is

26

located on the boundary between Nevada Divisions 3 and 4. These data indicate that annual

temperature varied relatively little between 1950 and 1972 (the combined period of the Spaulding

and our baseline data; Figure 13). However, precipitation increased almost monotonically over

this interval, with the result that our baseline (1951 to 1980) for annual precipitation falls near

the long-term historic mean, whereas that employed by Spaulding (1963 to 1972) covers a much

wetter period (and, in addition, BeatlyÕs measurements may overestimate precipitation, Figure

14). The differences in precipitation estimates in the modern baselines employed by Spaulding

(1985) and in this report greatly influence the calculated anomalies for the LGM (Table 4). In

fact, the differences in these anomalies is influenced more by the differences in the modern

baseline data than by differences in the estimation of late Pleistocene climates (Figure 15).

Comparison of Reconstructed LGM Climates With Historical Climatic Variations. As

discussed above, Yucca Mountain is located on the boundary between NOAA Nevada Climatic

Divisions 3 and 4. Over the past century, historic variations in precipitation in both of these

divisions have reached levels (on an annual basis) as high as those reconstructed for the LGM

(Figure 16). However, temperatures have remained above the LGM estimates throughout the

historic period, although rare years in Nevada Division 3 have approached the mean annual

temperature reconstruction for the LGM at Yucca Mountain (Figure 16). These individual years

are labeled on the scatterplot on Figure 17, where it can be seen that in many years between 1897

and 1917 in Nevada Division 3 the annual temperatures approached the mean annual

temperatures reconstructed for Yucca Mountain for the LGM. Several of those years

(particularly 1901, 1904, 1905, 1906, and 1913) also had precipitation levels comparable to those

reconstructed for the LGM. These years include both El Ni�o and La Ni�a years (Cayan and

Webb, 1992), although the amplitude of the difference between these patterns was apparently

smaller than in recent years. The first ~15 years of this century are estimated to have been

among the coldest and wettest of the last 400 years in the Intermountain basins and Southwest

Deserts (Fritts and Shao, 1992, p. 279-280), although precipitation levels of similar amplitudes

have been common in Nevada Division 3 through the latter half of the Holocene (Hughes and

68

101214161820

1890

1910

1930

1950

1970

1990

12

3

4

M

oder

n M

AT

(Spa

uldi

ng, 1

985)

M

oder

n M

AT

(T

his

Rep

ort)

Nev

ada

Div

isio

n 3

Nev

ada

Div

isio

n 4

LGM

Tem

pera

ture

(th

is r

epor

t)

LGM

Tem

pera

ture

(S

paul

ding

, 198

5)

10-y

ear

runn

ing

mea

n

Annual Temperature (°C)

YE

AR

Fig

ure

13.

His

toric

var

iatio

ns in

ann

ual t

empe

ratu

re d

epic

ted

as 1

0-ye

ar r

unni

ng m

eans

. D

ata

are

from

NO

AA

com

pila

tions

of in

stru

men

tal w

eath

er d

ata

for

Nev

ada

regi

ons

3 an

d 4

(see

inse

t map

). T

he m

oder

n m

ean

annu

al te

mpe

ratu

rees

timat

es fo

r Y

ucca

Mou

ntai

n (5

000

ft, 1

524

m e

leva

tion)

from

Spa

uldi

ng (

1985

) an

d th

is r

epor

t are

sho

wn

on th

erig

ht.

The

mea

n an

nual

tem

pera

ture

rec

onst

ruct

ions

for

the

Last

Gla

cial

Max

imum

(LG

M)

from

Spa

uldi

ng (

1985

)an

d th

is r

epor

t are

sho

wn

as g

ray

band

s on

the

low

er p

art o

f the

figu

re.

27

0

50

100

150

200

250

300

350

M

oder

n M

AP

(Spa

uldi

ng, 1

985)

Mod

ern

MA

P(T

his

Rep

ort)

LGM

Pre

cipi

tatio

n (t

his

repo

rt)

LGM

Pre

cipi

tatio

n (S

paul

ding

, 198

5)

Nev

ada

Div

isio

n 4

Nev

ada

Div

isio

n 3

12

3

4

10-y

ear

runn

ing

mea

n

1890

1910

1930

1950

1970

1990

YE

AR

Annual Precipitation (mm)

Fig

ure

14. H

isto

ric v

aria

tions

in a

nnua

l pre

cipi

tatio

n de

pict

ed a

s 10

-yea

r ru

nnin

g m

eans

. D

ata

are

from

NO

AA

com

pila

tions

of i

nstr

umen

tal w

eath

er d

ata

for

Nev

ada

regi

ons

3 an

d 4

(see

inse

t map

). T

he m

oder

n m

ean

annu

al p

reci

pita

tion

estim

ates

for

Yuc

ca M

ount

ain

(500

0 ft,

152

4 m

ele

vatio

n) fr

om S

paul

ding

(19

85)

and

this

repo

rt a

re s

how

n on

the

right

. T

he m

ean

annu

al p

reci

pita

tion

reco

nstr

uctio

ns fo

r th

e La

st G

laci

al M

axim

um(L

GM

) fr

om S

paul

ding

(19

85)

and

this

rep

ort a

re s

how

n as

gra

y ba

nds

on th

e up

per

part

of t

he fi

gure

.

28

010

020

030

0

Mea

n A

nnua

l Pre

cipi

tatio

n (m

m)

68101214

Spa

uldi

ng (

1985

) T

his

Rep

ort

Mean Annual Temperature ( °C)M

oder

n C

limat

e E

stim

ates

f

or Y

ucca

Mou

ntai

n

LG

M C

limat

e E

stim

ates

for

Yuc

ca M

ount

ain

Mo

de

rn a

nd

LG

M C

lima

teE

stim

ate

s a

re fo

r 5

00

0 ft

(15

24

m)

ele

vatio

n a

tY

ucc

a M

ou

nta

in

Fig

ure

15.

Com

paris

ons

of e

stim

ates

of t

he m

oder

n an

d La

st G

laci

al M

axim

um (

LGM

) cl

imat

es a

t Yuc

ca M

ount

ain.

Spa

uldi

ng (

1985

) in

ferr

ed a

sm

alle

r di

ffere

nce

in p

reci

pita

tion

betw

een

toda

y an

d th

e LG

M th

an in

this

rep

ort,

in p

art b

ecau

se h

is e

stim

ate

of m

oder

n pr

ecip

itatio

n is

hig

her

than

the

one

used

her

e.

29

0

20

40

60

6 8 10 12 14 16 18 20Temperature (°C)

NV3NV4 LGM Yucca

Mountain Today

Num

ber

of Y

ears

0

10

20

0 100 200 300Precipitation (mm)

NV3

NV4

YuccaMountain Today

LGM

Num

ber

of Y

ears

Figure 16. Annual precipitation and annual temperature for the period from 1895 to 1998 forNevada regions 3 and 4 (see inset map on Figure 13 for the definition of theseNOAA regions) characterized by the number of years when precipitation ortemperature fell within given ranges. The analogue-based estimates for the LastGlacial Maximum (LGM) are shown as vertical bars. Over the past century therehave been years in both regions when precipitation was as great as thatreconstructed for the LGM. There have also been a few years when temperaturesin region 3 were cool enough to approach those estimated for the LGM.

30

68

101214161820

050

100

150

200

250

300

350

400

Ann

ual P

reci

pita

tion

(mm

)

Annual Temperature (°C)

NV

4N

V3

1913

1904

1901

190619

82

1941

1983

1998

1978

1911

1912

1903

1905

1897

1916

1898

1917

His

toric

Ann

ual P

reci

pita

tion

vs. A

nnua

l Tem

pera

ture

Mod

ern

(thi

s re

port

)

Mod

ern

(Spa

uldi

ng)

L

GM

(thi

s re

port

)

LG

M(S

paul

ding

)

Fig

ure

17.

Com

paris

on o

f his

toric

clim

ates

in N

OA

A N

evad

a re

gion

s 3

(fill

ed c

ircle

s) a

nd 4

(op

en c

ircle

s) c

ompa

red

with

mod

ern

and

LGM

clim

ate

estim

ates

for

Yuc

ca M

ount

ain.

Sel

ecte

d ye

ars

in r

egio

n 3

are

labe

led.

As

show

n he

re, t

here

wer

e se

vera

l yea

rs in

the

first

par

t of t

his

cent

ury

(not

ably

: 19

01, 1

904,

190

5, 1

906,

191

3)w

hen

the

clim

ate

of r

egio

n 3

was

as

wet

, and

nea

rly a

s co

ld, a

s th

e LG

M r

econ

stru

ctio

ns (

at le

ast a

s vi

ewed

on

an a

nnua

l bas

is -

the

seas

onal

clim

ates

cou

ld s

till b

e qu

ite d

iffer

ent t

han

thos

e of

the

LGM

).

31

LG

M (

this

re

po

rt)

LG

M (

Sp

au

ldin

g)

19

60 1

95

0

19

30

19

00 19

20

19

7019

901

99

7

19

40

19

10

19

80

19

60

19

50

19

30

19

00

19

20

19

90

19

40

19

70

19

10

19

97

19

80

M

od

ern

(th

is r

ep

ort

)

Mo

de

rn(S

pa

uld

ing

)Ra

ng

e =

2.1°

C

Ra

ng

e =

96

mm

Ra

ng

e =

1.8°

C

Ra

ng

e =

99

mm

68

101214161820

7510

012

515

017

520

022

525

027

530

032

5

Ann

ual P

reci

pita

tion

Annual Temperature

NV

4

NV

3

Dec

adal

Ave

rage

s

Fig

ure

18.

Thi

s fig

ure

illus

trat

es th

e sa

me

data

as

in F

igur

e 17

, but

por

tray

ed a

s de

cada

l ave

rage

s pl

otte

d by

thei

rm

idpo

ints

. T

he d

ecad

es r

epre

sent

ed b

y th

e po

ints

at 1

900

(189

5-19

04)

and

1910

(19

05-1

914)

are

the

cold

est

on r

ecor

d in

reg

ion

3, w

here

as 1

980

(197

5-19

84),

whi

ch e

ncom

pass

es th

e la

rge-

scal

e E

l Niñ

o of

the

early

1980

s, is

the

wet

test

on

reco

rd.

As

show

n he

re, t

he r

ange

in a

nnua

l tem

pera

ture

ove

r th

e pa

st c

entu

ry h

as b

een

1.8°

C in

reg

ion

3 an

d 2.

1° C

in r

egio

n 4,

whe

reas

the

varia

bilit

y in

ann

ual p

reci

pita

tion

has

been

99

mm

inre

gion

3 a

nd 9

6 m

m in

reg

ion

4.

32

33

Graumlich, 1996). Examination of the synoptic climatology of the early part of this century in

the Southwest could provide insights into the nature of Pleistocene climates, but is outside the

scope of this report.

Although individual years in the historic period had conditions that approached those

reconstructed for the LGM, on a decadal basis the climate of Nevada Division 3 has remained

warmer and (generally) drier than that of the LGM (Figure 18). The range of inter-decadal

variability over the past century has been ~2¡ C and ~97 mm in Nevada Divisions 3 and 4,

exceeding the differences in LGM estimates between Spaulding (1985) and this report.

Potential influences of changes in atmospheric chemistry on plant communities.

Atmospheric carbon dioxide concentrations during the late Pleistocene were greatly reduced

relative to pre-industrial Holocene levels, and during the LGM they were as low as ~190 ppmv

(versus ~280 ppmv during the Holocene, Barnola and others, 1987). In addition to its effects on

global climates, this change in atmospheric chemistry may have had influences on the physiology

of plants, their elevational distributions, and the sensitivity of plant species and communities to

climatic variations (Ehleringer and others, 1997). Woodward (1987) demonstrated that stomatal

density decreased during the last 200 years in a variety of species due to the increased availability

of carbon dioxide in the atmosphere following the industrial revolution. Van de Water and others

(1994) demonstrated that fossil needles of Pinus flexilis in the Southwestern United States had

17% greater stomatal density during the LGM than at 12,000 years ago (by which time

atmospheric carbon dioxide had increased 30% over LGM levels). By their calculations, water-

use efficiency for this species would have been ~15% lower at the LGM than at 12 ka, assuming

constant temperature and humidity.

Ecophysiological modeling by Jolly and Haxeltine (1997) suggests that much of the

lowering of upper treelines in East Africa during the LGM could be directly caused by the lower

concentrations of atmospheric carbon dioxide. They also found that some vegetational

assemblages increased their climatic tolerances under lower levels of carbon dioxide. At this point

we have no way to assess how well this concept applies to the Yucca Mountain region, but the

34

stability through time of the limber pine/Utah juniper woodland (as other indicators of climate

changed through time) could conceivably be due to changes in climatic sensitivity of vegetation

related to changes in atmospheric chemistry. Using the African model, it is possible that the

apparent changes in lapse rates for temperature could be due (at least in part) to an expanded

range of climatic tolerance of woodland vegetation (relative to today). In any case, the increased

stomatal density should have decreased water-use efficiency for most plants during the LGM,

and hence they would require greater-than-modern levels of precipitation to sustain themselves.

Hence, our estimates of late Pleistocene precipitation may be too low, especially for the LGM.

PRIMARY CONCLUSIONS.

1) For 5000 ft (1524 m) at the Yucca Mountain repository we estimate that mean annual

temperature (MAT) and precipitation (MAP) differed from our modern baseline data as

follows:

35 to 30 ka: MAT = ~4¡ C colder than today, MAP = 1.5X modern levels;

27 to 23 ka: MAT = ~5¡ C colder than today, MAP = 2.2X modern levels;

20.5 to 18 ka: MAT = ~8¡ C colder than today, MAP = 2.4X modern levels;

14 to 11.5 ka: MAT = ~5.5¡ C colder than today, MAP = 2.6X modern levels

2) The selection of historic baseline data has strong influence on the perceived differences

between Pleistocene and ÒmodernÓ climates. The baseline data used by Spaulding (1985) was

significantly wetter than either the baseline employed in this report or the long-term historic

mean. Consequently, SpauldingÕs estimates of the difference between late Pleistocene and

ÒmodernÓ precipitation levels is much lower than ours.

3) It is difficult to assess the potential effects of decreased atmospheric carbon dioxide

concentrations during the late Pleistocene on the physiology of various plant species.

However, it is plausible that nearly all species required higher levels of moisture than they do

under higher concentrations of carbon dioxide during the 20th century. Consequently, our

estimates of late Pleistocene precipitation levels may be too low.

35

4) During the early part of this century there were several years when the climate of the region

adjoining Yucca Mountain on the north approached the annual temperature and moisture

conditions reconstructed for the LGM at Yucca Mountain. Inter-decadal variability in the

historic climate record is greater than the differences in the LGM reconstructions of Spaulding

(1985) and this report.

36

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