quantitative paleoclimatic … for a changing world quantitative paleoclimatic reconstructions from...
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science for a changing world
QUANTITATIVE PALEOCLIMATIC RECONSTRUCTIONS FROM
LATE PLEISTOCENE PLANT MACROFOSSILS OF
THE YUCCA MOUNTAIN REGION
by Robert S. Thompson1 , Katherine H. Anderson2, and Patrick J. Bardein3
Open-File Report 99-338
1999
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards (or with the North American Stratigraphic Code). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY
^Denver, Coloradory^Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 'Department of Geography, University of Oregon, Eugene, Oregon
INTRODUCTION
Plant macro fossil 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.
Spaulding's approach provided "state-of-the-art" 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.
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 macro fossil
assemblages preserved in ancient packrat middens from southern Nevada. Our approach
involves: 1) compiling a comprehensive list of plant macro fossil 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 Macro fossil 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,
Figu
re 1
. Lo
catio
ns o
f pac
krat
mid
dens
(circ
les)
use
d in
this
stu
dy b
y tim
e pe
riod.
The
loca
tion
of Y
ucca
Mou
ntai
n is
sho
wn
as a
fille
d tri
angl
e on
eac
h m
ap.
Cal
ifor
nia
14-1
1.S
kaA
rizo
na
Cal
ifor
nia
20.5
-18
kaA
rizo
na
N, 27-2
3 ka
Cal
ifor
nia
Ari
zona
N
Cal
ifor
nia
35-3
0 ka
Ari
zona
rather than placing more (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
divaricatd) 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.
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.
Pinus flexilis Juniperus osteosperma
Abies concolor Atriplex confertifolia
Table 1. Plant species for which maps were compiled by the USGS and/or DRI and digitized by the 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 phimePurshia tridentata antelope brushSymphoricarpos longiflorus snowberrySymphoricarpos oreophilus snowberry
Table 2. Present-day and Late Glacial Maximum (LGM) climatic estimates forapproximately 5000 ft (1524 m) elevation in the Yucca Mountain area (based on plant macrofossils from packrat middens).
Annual Annual Temperature Precipitation
PRESENT DAYThis study 13.4° C 125mmSpaulding, 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
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 "analogues" for the
Pleistocene packrat middens). The modem 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 modem 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 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 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
Figure 3. Comparison of observed (x-axis) and estimated (y-axis) present-day climatenormals for January and My 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).
20
0
-20
-40 -4
4
1000
100
10
11
r2 = O r 91 (0.86)i
;/-*f
'
0 -20 0 20 January Temperature (°C)
r2 = 0.71 (0.64) '.
^ x^** **"<|f»H !
30
20
10
0 C
1000
100
10
1
r2 = 0.84 (0.80) Jv«_^_«
< ^
Jar yt . ->:'
) 10 20 30 July Temperature (°C)
i r2 = 0.79 (0.77)
i i
i& &-
..: :.? i
1 10 100 1000 1 10 100 1000 January Precipitation (mm) July Precipitation (mm)
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 Finns flexilis (Figure 5), indicating that relatively
moist forest environments did not extend below this elevation during the last ~40,000
years. However, climate did vary through the late Pleistocene, and to quantify these
changes we analyzed suites of packrat midden assemblages grouped by the
10
Figu
re 4
. E
xam
ple
of th
e pr
esen
t-day
ana
logu
es (
from
the
mod
ern
clim
ate
grid
) fo
r a la
te P
leis
toce
ne p
ackr
at m
idde
n pl
ant m
acro
foss
ilas
sem
blag
e fr
om n
ear Y
ucca
Mou
ntai
n.
The
map
on
the
left
illu
stra
tes
the
geog
raph
ic p
lace
men
t of t
he a
nalo
gues
(ch
arac
teriz
ed
by th
eir d
egre
e of
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
). Th
e ce
ntra
l and
righ
t-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
l ana
logu
es (
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
resp
ect t
o Ja
nuar
y an
d Ju
ly T
empe
ratu
re (c
entra
l pan
el)
and
Janu
ary
and
July
Pre
cipi
tatio
n (o
n lo
gari
thm
ic s
cale
in ri
ght-
hand
pan
el).
Ana
logu
es fo
r a
13,7
40 y
r B
.P.
Pac
krat
Mid
den
Ass
embl
age.
Jacc
ard
Dis
tanc
e M
easu
re (
1.0
= pe
rfec
t m
atch
)
o20
£ 3 2
0 o> Q
.
o>-2
0
flj 3 (0
-40
<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
E 1
000
4t«
f>
o10
0
0 20
Ju
ly T
empe
ratu
re (
°C)
'g.
'o flj i
10 1
,,-
* ^
% " 0
1 10
10
0 10
00
July
Pre
cipi
tatio
n (m
m)
Figu
re 5
. T
he o
ccur
renc
e of
lim
ber p
ine
(Pin
us f
lexi
lis)
in p
ackr
at m
idde
ns f
rom
the
Yuc
ca M
ount
ain
regi
on d
urin
g th
e la
te
Plei
stoc
ene
(plo
tted
by a
ge o
n th
e ho
rizon
tal a
xis
and
elev
atio
n on
the
vert
ical
axi
s).
Fille
d ci
rcle
s in
dica
te th
at
limbe
r pin
e w
as p
rese
nt, o
pen
circ
les
indi
cate
that
it w
as a
bsen
t. A
s ill
ustr
ated
her
e, th
e si
te o
f the
pro
pose
d re
posi
tory
at a
ppro
xim
atel
y 15
24 m
ele
vatio
n at
Yuc
ca M
ount
ain
was
at (
or b
elow
) the
low
er li
mit
of li
mbe
r pin
e th
roug
h th
e la
te P
leis
toce
ne.
C .o ^4 » CO JD
LJJ
2000-
1500
-
1000
-
500
99m
<f>
o _
_ ---
-1
oo
oo
^^^
l~
O"O
" { '
o
f
or-
- - ~
A~
'T~
---
--
vj
u
On
n
0 ^ «
K °
___ _
__
__
_1
.__
__
__
__
__
J-
8 d
o o
. _
__
_ _
1o o>
q
r- ip r -----
L_
__
__
__
__
_
> r~ ~
~
~
P I "
L r i-
P
inus
flex
ilis
pres
ent
O P
inus
flex
ilis
abse
nt
O---------i
_ _ _
_.j
O
i> ----
----
---
_ _
__ _
_.
1000
015000
2000
025000
3000
035000
40000
Rad
ioca
rbon
Age
(yr
B.P
.)
Figu
re 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 f
our
inte
rval
s of
late
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 o
f ass
embl
ages
. A
s di
scus
sed
in th
e te
xt, e
leva
tion
appa
rent
ly in
fluen
ces t
he d
egre
e of
te
mpe
ratu
re a
nd p
reci
pita
tion
chan
ge th
roug
h tim
e, a
nd th
is e
ffec
t is
not t
aken
into
acc
ount
in th
is fi
gure
.
u>
T3 O I
I Q.
<D
T3
-3
O
-4 -5I
*-« 2 <D
Q
.
<D 13 j?
-7 -84
35 to
30
ka
(n =
4)
27 to
23
ka
14 to
11.
5 ka
(n
=
20.5
to 1
8 ka
1.2
1.4
1.6
1.8
2 2.
2 2.
4
Ann
ual
Pre
cipi
tatio
n (m
ultip
lied
by m
oder
n va
lues
)
2.6
2.8
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.
Compilations of paleoclimatic proxy data and numerical model simulations of past
14
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 monophylld), 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?
The present-day climatic and plant distributional data in Thompson and others
15
(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 ofLarrea divaricata (in accordance with Spaulding's [1985] interpretation) or
Juniperus californica. Conversely, winter temperatures were apparently too warm for
Piceapungens, 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
'absent' 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 Spaulding's 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 Spaulding's, the absolute differences between the two approaches are not
16
Figu
re 7
. Pr
esen
t-da
y co
rres
pond
ence
s be
twee
n m
ean
Janu
ary
tem
pera
ture
and
the
dist
ribu
tion
of se
lect
ed tr
ee a
nd s
hrub
spe
cies
fro
m w
este
rn N
orth
A
mer
ica
(dat
a ar
e th
e 10
% to
90%
dis
trib
utio
nal l
imits
fro
m T
hom
pson
and
oth
ers,
199
9).
The
17 s
peci
es o
n th
e le
ft ar
e sp
ecie
s th
at d
o no
t oc
cur i
n la
te P
leis
toce
ne p
ackr
at m
idde
n as
sem
blag
es f
rom
the
Yuc
ca M
ount
ain
regi
on (
alth
ough
Jun
iper
us s
copu
loru
m, P
inus
mon
ophy
lla,
Pin
us lo
ngae
va, a
nd P
seud
otsu
ga m
enzi
esii
and
have
bee
n re
cove
red
in th
e br
oade
r so
uthe
aste
rn C
alif
orni
a/so
uthe
rn N
evad
a /s
outh
wes
tern
U
tah
area
). Th
e 4
spec
ies
on th
e ri
ght h
ave
been
reco
vere
d fr
om m
ultip
le m
idde
n as
sem
blag
es in
the
Yuc
ca M
ount
ain
regi
on (a
lthou
gh
Pin
us fl
exili
s w
as a
ppar
ently
at o
r nea
r its
low
est l
imits
at t
he e
leva
tion
of th
e pr
opos
ed re
posi
tory
). Th
e gr
ay b
and
repr
esen
ts th
e ra
nge
of
pale
oclim
atic
est
imat
es f
or th
e La
st G
laci
al M
axim
um f
rom
the
mod
ern
anal
ogue
ana
lysi
s.
20
Janu
ary
Tem
pera
ture
15 10 5 -I
O
0o ¥ 3
-5
£ 0)
Q
. I -10 -1
5
-20
-25
-30
-35
Figu
re 8
. Pre
sent
-day
cor
resp
onde
nces
bet
wee
n m
ean
July
tem
pera
ture
and
the
dist
ribu
tion
of se
lect
ed tr
ee a
nd s
hrub
spe
cies
fro
m
wes
tern
Nor
th A
mer
ica
(see
cap
tion
for F
igur
e 7
for a
mor
e co
mpl
ete
expl
anat
ion)
.
oo
CD o> a.
July
Tem
pera
ture
35
30
25 20 15 10 0
Figu
re 9
. Pre
sent
-day
cor
resp
onde
nces
bet
wee
n m
ean
annu
al te
mpe
ratu
re a
nd th
e di
stri
butio
n of
sele
cted
tree
and
shr
ub
spec
ies
from
wes
tern
Nor
th A
mer
ica
(see
cap
tion
for F
igur
e 7
for a
mor
e co
mpl
ete
expl
anat
ion)
.
Ann
ual T
empe
ratu
re
CO
*§ .c
£
*1
CO .8 -a
<*
.*» § jj I
.c
-10
Figu
re 1
0. 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
stri
butio
n of
sele
cted
tree
and
shr
ub s
peci
es
from
wes
tern
Nor
th A
mer
ica
(see
cap
tion
for F
igur
e 7
for a
mor
e co
mpl
ete
expl
anat
ion)
.
Janu
ary
Pre
cipi
tatio
n350
300
250
E £ c .0 s '5. 'o
200
150
100 50
Figu
re 1
1. P
rese
nt-d
ay c
orre
spon
denc
es b
etw
een
mea
n Ju
ly p
reci
pita
tion
and
the
dist
ribu
tion
of se
lect
ed tr
ee a
nd s
hrub
spe
cies
fro
m
wes
tern
Nor
th A
mer
ica
(see
cap
tion
for F
igur
e 7
for a
mor
e co
mpl
ete
expl
anat
ion)
.
July
Pre
cipi
tatio
n14
0
120
100 80
g. "o £
60Q
.
40 20
I
1 §
Figu
re 1
2. 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
stri
butio
n of
sele
cted
tree
and
shr
ub s
peci
es f
rom
wes
tern
Nor
th A
mer
ica
(see
cap
tion
for F
igur
e 7
for
a m
ore
com
plet
e ex
plan
atio
n).
3000
Ann
ual
Pre
cipi
tatio
n
2500
to
to2000
I15
00
1000
500
g .«2
S 1
CO
*"^
fc
co
E
co
§>
8
o
S
^
iS
§
"*
wg
.co
^^lo
-g
g c'S
&o&
coC
SCO
^ 1
®
in
. ^
Q>
®
Q-
^
"0 1
c
g
.0
o
.&
'5
1 I
I" |
1 "
"^
Q.
^
Pinus
ponderosa
Picea
engelmannii
Abies lasiocarpa
CO | S +£ 2 C
s 1
nnus aioicauiis Abies
magn
ifi
Pseudotsuga menx
CD (
P/nus
contort,
CO 1
.to 55 1 <D C CO OJ
£
CD
5t
1 .
* 1
till
fi *
3 £
°
S
.£
-Q
S2
co
Q-
^
C
'53
P»
Q)
Table 3. Climatic factors that may have excluded selected taxa from the Yucca Mountain region during the late Pleistocene.
" 'Abies concolor
Abies lasiocarpa
Abies magnified
Juniperus californicaJuniperus communis^Juniperus scopulorum
Larrea divaricata
°icea engelmannii
Picea pungens
Pinus albicaulis
3inus contortaDinus edulis
*Pinus flexilis
^Pinus longaeva
*Pinus monophylla
Pinus ponderosa*Pseudotsuga menziesii
Tsuga heterophyllarsuga mertensiana
January
H
CH
CHHH
H
KEY:H = too hot for species, C = too cold, D
TEMPERATUREJuly
H
MH
CHHHH
H
MHH
= marginal, W =
Annual
H
CH
CHHHH
H
M
PRECIPITATIONJanuary July
DDM W
DM
D MD
D MD
DD D
too wet, M = marginal
AnnualDDD
MM
DDDD
D
DDDD
* = present near Yucca Mountain during part of the late PleistoceneA = present regionally during the late Pleistocene
Januaryleason for exclusion:
too warm or dry 6too cold or wet 2
TEMPERATUREJuly
Plants for whom reason of exclusion unclear: Pinus edulis,
AnnualPRECIPITATION
January July
10 8 6 8121 1
Pinus longaeva, Pinus monophylla
Annual
140
23
Tab
le 4
. R
econ
stru
cted
LG
M c
limat
es a
nd c
limat
ic an
omal
ies b
ased
on
the
pres
ent-
day
clim
ate
estim
ates
use
d in
this
repo
rt c
ontr
aste
d w
ith th
e pr
esen
t-da
y va
lues
use
d by
Spa
uldi
ng(1
985)
.
Mea
n A
nnua
l Tem
pera
ture
Spau
ldin
g T
his
Rep
ort
Mea
n A
nnua
l Pre
cipi
tatio
nSp
auld
ing
Thi
s Rep
ort
Est
imat
ed
Val
ues
MO
DE
RN
°C 13.5
13
.4
mm
18
9 12
5
Rec
onst
ruct
ed
Val
ues
LG
M
LG
M
Min
umum
M
axim
um
°C
°C6.
5 7.
5 7.
9 8.
5
mm
m
m
246
265
266
321
Mul
tipl
ier
for A
nnua
l Pre
cipi
tati
onSp
auld
ing
Thi
s Rep
ort
Ano
mal
ies b
ased
on
Spau
ldin
g M
oder
n V
alue
s
LG
M
LG
M
Min
umum
M
axim
um
°C
°C7
6 5.
6 5
mm
m
m57
76
77
13
2
Mod
. X
Mod
. X
1.3
1.4
2.1
2.6
Ano
mal
ies b
ased
on
This
Rep
ort
Mod
em V
alue
s
LG
M
LG
M
Min
umum
M
axim
um
°C
°C6.
9 5.
9 5.
5 4.
9
mm
m
m
121
140
141
196
Mod
. X
Mod
. X
2.0
2.1
1.4
1.7
Mod
. X =
Mod
ern
mea
n an
nual
pre
cipi
tatio
n m
ultip
liedb
y th
is n
umbe
r
great for either mean annual temperature (6.5° C versus 7.5 ° C) nor for mean annual
precipitation (246 - 265 mm versus 266 - 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, Spaulding's anomalies for LGM mean
annual precipitation (based on Beatly's 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 "normals" for the period 1951 to
1980, with the great majority of our data obtained from U.S. Weather Service calculations
of these "normal" 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)
Spaulding's estimates of modem 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
25
(unfortunately) Yucca Mountain is 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 fells near the long-term historic
mean, whereas that employed by Spaulding (1963 to 1972) covers a much wetter period
(and, in addition, Beatly's 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 Nino and La Nina 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
26
Figu
re 1
3. H
isto
ric v
aria
tions
in a
nnua
l tem
pera
ture
dep
icte
d as
10-
year
runn
ing
mea
ns.
Dat
a ar
e fr
om N
OA
A c
ompi
latio
ns o
f ins
trum
enta
l wea
ther
da
ta fo
r Nev
ada
regi
ons
3 an
d 4
(see
inse
t map
). Th
e m
odem
mea
n an
nual
tem
pera
ture
est
imat
es fo
r Yuc
ca M
ount
ain
(500
0 ft,
152
4 m
el
evat
ion)
from
Spa
uldi
ng (1
985)
and
this
repo
rt ar
e sh
own
on th
e rig
ht.
The
mea
n an
nual
tem
pera
ture
reco
nstru
ctio
ns fo
r the
Las
t Gla
cial
M
axim
um (L
GM
) fro
m S
paul
ding
(198
5) a
nd th
is re
port
are
show
n as
gra
y ba
nds
on th
e lo
wer
par
t of t
he fi
gure
.
K)
-4
20 18 16O 2
140>
Q
.
0> (U 3
c1
2 10 8
10-y
ear r
unni
ng m
ean
Nev
ada
Div
isio
n 4
Nev
ada
Div
isio
n 3
Mod
em M
AT
(Spa
uldi
ng,
1985
) M
M
Mod
em M
AT
(Thi
s R
epor
t)
1890
1910
1930
1950
YE
AR
1970
1990
Figu
re 1
4. H
isto
ric v
aria
tions
in a
nnua
l pre
cipi
tatio
n de
pict
ed a
s 10
-yea
r run
ning
mea
ns.
Dat
a ar
e fr
om N
OA
A c
ompi
latio
ns o
fin
stru
men
tal w
eath
er d
ata
for N
evad
a re
gion
s 3
and
4 (s
ee in
set m
ap).
The
mod
ern
mea
n an
nual
pre
cipi
tatio
n es
timat
es fo
r Y
ucca
Mou
ntai
n (5
000
ft, 1
524
m e
leva
tion)
from
Spa
uldi
ng (1
985)
and
this
repo
rt ar
e sh
own
on th
e rig
ht.
The
mea
n an
nual
pr
ecip
itatio
n re
cons
truct
ions
for t
he L
ast G
laci
al M
axim
um (L
GM
) fro
m S
paul
ding
(198
5) a
nd th
is re
port
are
show
n as
gra
y ba
nds
on th
e up
per p
art o
f the
figu
re.
K> 00
350
300
10-y
ear r
unni
ng m
ean
1890
LGM
Pre
cipi
tatio
n (t
his
repo
rt)
1910
1930
1950
YE
AR
1970
1990
LOI«
Pr«c
iplta
tlon
(Spa
uldi
ng, 1
986)
Nev
ada
Div
isio
n 3
Nev
ada
Div
isio
n 4
. M
oder
n M
AP
(Spa
uldi
ng,
1985
)
Mod
ern
MAP
(T
his
Rep
ort)
Figu
re 1
5. C
ompa
rison
s of
estim
ates
of t
he m
oder
n an
d La
st G
laci
al M
axim
um (
LGM
) clim
ates
at Y
ucca
Mou
ntai
n.
Spau
ldin
g (1
985)
in
ferr
ed a
sm
alle
r diff
eren
ce in
pre
cipi
tatio
n be
twee
n to
day
and
the
LGM
than
in th
is re
port,
in p
art b
ecau
se h
is e
stim
ate
of
mod
ern
prec
ipita
tion
is h
ighe
r tha
n th
e on
e us
ed h
ere.
to sO
14 -
c3
12 ~
*H I
^
8 6 - 0
Mod
ern
Clim
ate
Estim
ates
fo
r Yuc
ca M
ount
ain
O
This
R
epor
t
Mod
ern
and
LG
M C
limat
eE
stim
ates
are
for
5000
ft(1
524
m)
elev
atio
n at
Yucc
a M
ount
ain
Spa
uldi
ng
(198
5)
LGM
Clim
ate
Estim
ates
for
Yuc
ca M
ount
ain
100
200
300
Mea
n A
nnua
l Pre
cipi
tatio
n (m
m)
Figure 16. Annual precipitation and annual temperature for the period from 1895 to 1998 for Nevada regions 3 and 4 (see inset map on Figure 13 for the definition of these NOAA regions) characterized by the number of years when precipitation or temperature fell within given ranges. The analogue-based estimates for the Last Glacial Maximum (LGM) are shown as vertical bars. Over the past century there have been years in both regions when precipitation was as great as that reconstructed for the LGM. There have also been a few years when temperatures in region 3 were cool enough to approach those estimated for the LGM.
Mountain Today
200 Precipitation (mm)
300
YuccaMcxntainToday
I/i / i
8 10 12 14 16 Temperature (°C)
INV4 I I IIII\ \
18 20
30
Figu
re 1
7. C
ompa
rison
of h
isto
ric c
limat
es in
NO
AA
Nev
ada
regi
ons
3 (f
illed
circ
les)
and
4 (o
pen
circ
les)
com
pare
d w
ith
mod
ern
and
LGM
clim
ate
estim
ates
for
Yuc
ca M
ount
ain.
Se
lect
ed y
ears
in re
gion
3 a
re la
bele
d. A
s sh
own
here
, the
re w
ere
seve
ral y
ears
in th
e fir
st p
art o
f thi
s ce
ntur
y (n
otab
ly:
1901
,190
4,19
05,1
906,
1913
) whe
n th
e cl
imat
e of
re
gion
3 w
as a
s w
et, a
nd n
early
as
cold
, as
the
LGM
reco
nstru
ctio
ns (a
t lea
st a
s vie
wed
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
).
U)
20
n
18 16 14 12 10
8
His
toric
Ann
ual
Pre
cipi
tatio
n vs
. Ann
ual T
empe
ratu
reo
o
o °o
o
o 0
ftof
lO
° <b
°o%
0
oO
Q
O
o N
V4
N
V3
p 0
o o0
o
o.
Q0
0
O
O
«
O
°00°S
b
O f
t °
0
%
<*>
°
o Jt
Fo
° °
° °
o °
°
Mod
em
Mod
ern
(this
rep
ort)
(Spa
uldi
ng)
o o
**s.
*
*
>?
»
.
1898 r
,0V
1916
191?
-1982
.1898
.19
11
1912
0191
1
(Spa
uldi
ng)
1978
LGM
(th
is r
epor
t)
5010
015
0 20
0 25
0
Ann
ual P
reci
pita
tion
(mm
)30
035
040
0
Figu
re 1
8. T
his
figur
e ill
ustra
tes
the
sam
e da
ta a
s in
Fig
ure
17, b
ut p
ortra
yed
as d
ecad
al a
vera
ges
plot
ted
by th
eir m
id
poin
ts.
The
deca
des
repr
esen
ted
by th
e po
ints
at 1
900
(189
5-19
04) a
nd 1
910
(190
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14) a
re th
e co
ldes
t on
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rd in
regi
on 3
, whe
reas
198
0 (1
975-
1984
), w
hich
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the
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e-sc
ale
El N
ifio
of th
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rly 1
980s
, is
the
wet
test
on
reco
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As
show
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re, t
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in a
nnua
l tem
pera
ture
ove
r the
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t cen
tury
has
bee
n 1.
8° C
in
regi
on 3
and
2.1
° C
in re
gion
4, w
here
as th
e va
riabi
lity
in a
nnua
l pre
cipi
tatio
n ha
s be
en 9
9 m
m in
regi
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and
96 m
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regi
on 4
.
Dec
adal
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18
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this
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ort)
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100
125
150
175
200
225
Ann
ual
Pre
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250
275
300
325
amplitudes have been common in Nevada Division 3 through the latter half of the
Holocene (Hughes and 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, Bamola 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
33
dioxide. At this point we have no way to assess how well this concept applies to the
Yucca Mountain region, but the 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 "modern" 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, Spaulding's estimates of the difference
between late Pleistocene and "modern" 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.
34
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.
35
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