quantitative paleoclimatic reconstructions from …
TRANSCRIPT
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.
14-1
1.5
ka20
.5-1
8 ka
27-2
3 ka
35-3
0 ka
Nev
ada
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iforn
ia
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ona
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Fig
ure
1. L
ocat
ions
of p
ackr
at m
idde
ns (
circ
les)
use
d in
this
stu
dy b
y tim
e pe
riod.
The
loca
tion
of Y
ucca
Mou
ntai
n
is
show
n as
a fi
lled
tria
ngle
on
each
map
.
N
NN
N
4
+ +
+++
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++++++++++++ +++++
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+
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+++++++++++++
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+
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+
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+
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+
++++++++ ++++++++
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++++++++++++
Abies concolor Atriplex confertifolia
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++
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+++ + ++ ++ + +++ + +++ + + + ++ + + +++ + ++++ + +++ +++ + +++ +++ + ++ + ++ + + + ++ +++++
+++++
+
Pinus flexilis
++ ++++ +++ +++++ ++++ + ++ ++ ++ +++ + +++ + + ++ +++ + ++ + + + +++ ++ + ++++ ++ + ++++++++ +
+ ++++++ + +++ +++ + +++
+++ ++ + +++
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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
logu
es fo
r a
13,7
40 y
r B
.P. P
ackr
at M
idde
n A
ssem
blag
e.
0
20
-40
-20
20
<0.
4.4
-.6
.6-.
8.8
-1.0
Mid
den
Loca
tion
and
Mod
ern
Clim
ate
Cub
ic W
eigh
ted
Ave
rage
of F
ossi
l Ass
embl
age
Jacc
ard
Dis
tanc
e M
easu
re (
1.0
= p
erfe
ct m
atch
)
July
Tem
pera
ture
(ºC
)
January Temperature (ºC)
1
1
0
100
100
0
1
1
0
10
0
100
0
July
Pre
cipi
tatio
n (m
m)
January Precipitation (mm)
0
Fig
ure
4. E
xam
ple
of th
e pr
esen
t-da
y an
alog
ues
(fro
m th
e m
oder
n cl
imat
e gr
id)
for
a la
te P
leis
toce
ne p
ackr
at m
idde
n pl
ant
mac
rofo
ssil
asse
mbl
age
from
nea
r Y
ucca
Mou
ntai
n. T
he m
ap o
n th
e le
ft ill
ustr
ates
the
geog
raph
ic p
lace
men
t of t
hean
alog
ues
(cha
ract
eriz
ed b
y th
eir
degr
ee o
f sim
ilarit
y to
the
mid
den
asse
mbl
age
as d
eter
min
ed b
y th
e Ja
ccar
d co
effic
ient
).T
he c
entr
al a
nd r
ight
-han
d pa
nels
illu
stra
te th
e pl
acem
ent o
f the
est
imat
ed p
rese
nt-d
ay c
limat
e (f
illed
squ
are)
, pot
entia
lan
alog
ues
(circ
les,
cha
ract
eriz
ed b
y de
gree
of s
imila
rity)
, and
est
imat
ed p
ast c
limat
e (f
illed
tria
ngle
) in
res
pect
to J
anua
ryan
d Ju
ly T
empe
ratu
re (
cent
ral p
anel
) an
d Ja
nuar
y an
d Ju
ly P
reci
pita
tion
(on
loga
rithm
ic s
cale
in r
ight
-han
d pa
nel).
11
1000
015
000
2000
025
000
3000
035
000
4000
0
Rad
ioca
rbon
Age
(yr
B.P
.)
500
1000
1500
2000
Elevation (m)
Pin
us fl
exili
s pr
esen
t
Pin
us fl
exili
s ab
sent
2500
Fig
ure
5. T
he o
ccur
renc
e of
lim
ber
pine
(P
inus
flex
ilis)
in p
ackr
at m
idde
ns fr
om th
e Y
ucca
Mou
ntai
n re
gion
dur
ing
the
late
P
leis
toce
ne (
plot
ted
by a
ge o
n th
e ho
rizon
tal a
xis
and
elev
atio
n on
the
vert
ical
axi
s).
Fill
ed c
ircle
s in
dica
te th
at
lim
ber
pine
was
pre
sent
, ope
n ci
rcle
s in
dica
te th
at it
was
abs
ent.
As
illus
trat
ed h
ere,
the
site
of t
he p
ropo
sed
r
epos
itory
at a
ppro
xim
atel
y 15
24 m
ele
vatio
n at
Yuc
ca M
ount
ain
was
at (
or b
elow
) th
e lo
wer
lim
it of
lim
ber
pin
e
thr
ough
the
late
Ple
isto
cene
.
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
Juniperus osteosperma
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
Juniperus californica
Pinus longaeva
Pinus ponderosa
Juniperus scopulorum
Picea pungens
Picea engelmannii
Pinus albicaulis
Tsuga mertensiana
Artemisia tridentata
Pinus flexilis
Larrea divaricata
Pinus monophylla
Pinus edulis
Abies magnifica
Pseudotsuga menziesii
Juniperus communis
Tsuga heterophylla
Pinus contorta
Abies lasiocarpa
Juniperus osteosperma
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
Juniperus californica
Pinus ponderosa
Pinus monophylla
Pinus edulis
Pinus longaeva
Pseudotsuga menziesii
Tsuga heterophylla
Juniperus scopulorum
Abies magnifica
Tsuga mertensiana
Picea pungens
Pinus contorta
Picea engelmannii
Pinus albicaulis
Abies lasiocarpa
Juniperus communis
Juniperus osteosperma
Abies concolor
Artemisia tridentata
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
Juniperus scopulorum
Juniperus californica
Abies lasiocarpa
Picea engelmannii
Pinus ponderosa
Pinus albicaulis
Pinus contorta
Abies magnifica
Pseudotsuga menziesii
Tsuga mertensiana
Tsuga heterophylla
Juniperus osteosperma
Artemisia tridentata
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
Pseudotsuga menziesii
Juniperus communis
Juniperus californica
Abies magnifica
Pinus monophylla
Pinus longaeva
Juniperus scopulorum
Picea engelmannii
Pinus edulis
Pinus albicaulis
Abies lasiocarpa
Larrea divaricata
Pinus contorta
Tsuga heterophylla
Pinus ponderosa
Tsuga mertensiana
Artemisia tridentata
Juniperus osteosperma
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
Juniperus californica
Picea pungens
Juniperus communis
Picea engelmannii
Pinus albicaulis
Pseudotsuga menziesii
Tsuga heterophylla
Artemisia tridentata
Abies concolor
Larrea divaricata
Pinus edulis
Pinus monophylla
Pinus longaeva
Juniperus scopulorum
Pinus ponderosa
Abies lasiocarpa
Abies magnifica
Pinus contorta
Tsuga mertensiana
Juniperus osteosperma
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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