A Strategy for Assessing Potential Future Changes

in Climate, Hydrology, and Vegetation

in the Western United States

U.S. Department of the Interior

U.S. Geological Survey

U.S. Geological Survey Circular 1153
Cover. The left-hand illustration depicts climate anomalies for July temperature calculated by subtracting simulated present-day climate conditions from simulated conditions

under a 2×CO2 climate—see figure 3J. The right-hand illustration depicts simulated

present-day lake surface temperatures—see figure 4A.
M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE INTERIOR

A Strategy for Assessing Potential Future Changes 

in Climate, Hydrology, and Vegetation 

in the Western United States

By Robert S. Thompson, Steven W. Hostetler, 

Patrick J. Bartlein, and Katherine H. Anderson

U.S. GEOLOGICAL SURVEY CIRCULAR 1153

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1998
Free on application to U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Thomas J. Casadevall

Acting Director

A strategy for assessing potential future changes in climate, hydrology, and vegetation in 

the Western United States / by Robert S. Thompson . . . [et al.].

p. cm.—(U.S. Geological Survey circular ; 1153)

Includes bibliographical references.

1. Climatic changes—West (U.S.) 2. Hydrology—West (U.S.)

3. Phytogeography—West (U.S.) I. Thompson, Robert Stephen. II. Series.

QC981.8.C5S78

551.6978—dc21 98–20975

CIP
III

CONTENTS

Abstract ................................................................................................................................ 1

Introduction ......................................................................................................................... 1

Strategy ................................................................................................................................ 1

Continental to Subcontinental Climate Modeling ............................................................... 4

Comparison of Model Simulations with Observed Present-Day Climate.................... 4

Simulation of a Potential 2×CO2 Climate..................................................................... 4

Simulated Changes in Surface Hydrology............................................................................ 6

Regional Lake Responses............................................................................................. 6

Lake-Specific Responses.............................................................................................. 8

Streamflow.................................................................................................................... 11

Simulated Changes in Plant Distributions............................................................................ 12

Present-Day Climate and Plant Distributions............................................................... 12

Comparison of Present-Day and Potential Future Ranges ........................................... 12

Discussion of Changes in Plant Distributions .............................................................. 14

Conclusions .......................................................................................................................... 17

Acknowledgments ................................................................................................................ 18

References Cited................................................................................................................... 18

FIGURES

1. Maps illustrating spatial and topographic resolution of two climate models relative to actual topography

in the Western United States ...................................................................................................................................... 2

2. Maps showing comparison of present-day climate as depicted by GENESIS, RegCM, and observed

conditions................................................................................................................................................................... 5

3. RegCM maps of the present-day climate, the 2×CO2 climate, and the difference between them ............................. 7

4. Maps showing lake conditions under RegCM-simulated present-day climate, lake conditions under

RegCM-simulated 2×CO2 climate, and lake conditions under RegCM-simulated present-day climate

minus lake conditions under RegCM-simulated 2×CO2 climate for hypothetical lakes ........................................... 9

5–8. Graphs of:

5. Present-day and 2×CO2 climate simulations for physical characteristics of Yellowstone Lake,

Wyoming, during four winters and three summers............................................................................................. 10

6. Present-day and 2×CO2 climate simulations for physical characteristics of Pyramid Lake, Nevada,

during four winters and three summers .............................................................................................................. 10

7. Present-day and 2×CO2 summer stream-temperature simulations of Steamboat Creek, Oregon ...................... 11

8. Present-day and 2×CO2 air temperature, precipitation, and streamflow simulations of

Steamboat Creek, Oregon................................................................................................................................... 11

9. Maps of observed present-day and simulated 2×CO2 climates illustrated on 15-km grid......................................... 13

10. Bivariate scatter plots of January vs. July temperature and January vs. July log precipitation showing

presence/absence data for four plant species in western North America................................................................... 15

11. Maps of plant-species distributions showing observed present-day distributions, simulated present-day

distributions, simulated distributions for 2×CO2 climate, and changes in species distribution between

simulated present-day distribution and simulated distribution under 2×CO2 climate ............................................... 16

12. Maps showing potential changes in distributions between simulated present-day distributions and

 simulated distributions under 2×CO2 climate that could occur for 16 tree species in western North America........ 18
IV CONTENTS

TABLES

1. Overview of scientific strategy used in this report..................................................................................................... 3

2. Common and scientific names of plant species with simulated present-day ranges and potential ranges

under a simulated 2×CO2 future climate.................................................................................................................... 14

ACRONYMS USED IN THIS REPORT

Acronym Meaning

AGCM ...........................atmospheric general circulation model

BATS .............................biosphere-atmosphere transfer scheme (Dickinson and others, 1993)

GENESIS.......................global environmental and ecological simulation of interactive systems

GHG ..............................greenhouse gases

INSTAAR ......................Institute for Arctic and Alpine Research (Boulder, Colo.)

NCAR............................National Center for Atmospheric Research (Boulder, Colo.)

RegCM ..........................regional climate model
1

A Strategy for Assessing Potential Future Changes

in Climate, Hydrology, and Vegetation 

in the Western United States

By Robert S. Thompson,1 Steven W. Hostetler,2

Patrick J. Bartlein,3 and Katherine H. Anderson4

ABSTRACT

Historical and geological data indicate that significant

changes can occur in the Earth’s climate on time scales

ranging from years to millennia. In addition to natural climatic change, climatic changes may occur in the near future

due to increased concentrations of carbon dioxide and other

trace gases in the atmosphere that are the result of human

activities. International research efforts using atmospheric

general circulation models (AGCM’s) to assess potential

climatic conditions under atmospheric carbon dioxide concentrations of twice the pre-industrial level (a “2×CO2”

atmosphere) conclude that climate would warm on a global

basis. However, it is difficult to assess how the projected

warmer climatic conditions would be distributed on a

regional scale and what the effects of such warming would

be on the landscape, especially for temperate mountainous

regions such as the Western United States. In this report, we

present a strategy to assess the regional sensitivity to global

climatic change. The strategy makes use of a hierarchy of

models ranging from an AGCM, to a regional climate

model, to landscape-scale process models of hydrology and

vegetation. A 2×CO2 global climate simulation conducted

with the National Center for Atmospheric Research

(NCAR) GENESIS AGCM on a grid of approximately 4.5°

of latitude by 7.5° of longitude was used to drive the NCAR

regional climate model (RegCM) over the Western United

States on a grid of 60 km by 60 km. The output from the

RegCM is used directly (for hydrologic models) or interpolated onto a 15-km grid (for vegetation models) to quantify

possible future environmental conditions on a spatial scale

relevant to policy makers and land managers.

1Global Change and Climate History Team, U.S. Geological Survey,

Denver, CO. 2 U.S. Geological Survey, Corvallis, OR.

3 Department of Geography, University of Oregon, Eugene, OR.

4 INSTAAR, University of Colorado, Boulder, CO.

INTRODUCTION

Although there remains considerable debate on the

issue of global warming, computer models suggest that the

increasing atmospheric concentrations of carbon dioxide

and other “greenhouse” gases (GHG) released by human

activities will cause global warming during the next century.

In addition, instrumental measurements suggest that the

Earth’s climate has warmed during the past few decades,

perhaps reflecting the initial phase of “greenhouse” warming (Houghton and others, 1996). Geologic studies of past

periods of global warmth and simulations of these past climates by numerical models suggest that the degree of

warming can vary greatly across the globe and that precipitation regimes are affected differently in different regions

(Lauritzen and Anderson, 1995; Dowsett and others, 1994;

Thompson and Fleming, 1996; Poore and Sloan, 1996).

Given the complex nature of regional responses to global

warming and the fact that natural climate variability is a

complicating factor, better tools are needed to assess the

impacts of a range of likely future climate variations on the

Western United States and elsewhere. Climate change will

directly affect water availability and quality, agriculture,

forestry, power production from dammed rivers, and the

storage of toxic materials. Entire ecosystems will be

affected, as will a wide variety of public lands, including

Native Lands, National Parks, National Forests, and range

lands managed by the Bureau of Land Management. Thus, it

is important that policy makers, land managers, and other

interested parties understand the sensitivity of these ecosystems to potential climate change.

In this report, we present examples of the sensitivity of

water and vegetation resources to a possible scenario of

future climate in the Western United States.

STRATEGY

The Western United States has a diverse array of

regional climates due to physiography and the interplay of

air masses originating from different source areas (Mock,
2 ASSESSING FUTURE CHANGES IN CLIMATE, HYDROLOGY, AND VEGETATION, WESTERN UNITED STATES

0 100 200 500 1000 1500 2000 3000

GENESIS LAND GRIDBOXES

AND ELEVATIONS (m)

A

0 100 200 500 1000 1500 2000 3000

RegCM LAND GRIDBOXES 

AND ELEVATIONS (m)

B

0 100 200 500 1000 1500 2000 3000

15-km LAND GRID

POINT ELEVATIONS (m)

C

1996; Thompson and others, 1993). To deal with this

complex situation, we used the sequential application of a

hierarchy of climate and landscape models to simulate the

regional-scale responses to climate change of streams, lakes,

and natural vegetation (table 1). At the top of the hierarchy

of models, an atmospheric general circulation model

(AGCM) is used to simulate global climate in response to

specified large-scale boundary conditions (e.g., sea-surface

temperatures and atmospheric CO2 concentration). AGCM’s

incorporate the physics of atmospheric circulation and the

interactions of the atmosphere with the Earth’s surface.

AGCM’s are generally run at a rather coarse resolution (on

the order of several degrees of latitude and longitude) to balance the level of detail with computing requirements. Many

simulations of trace-gas-induced change have been conducted with AGCM’s (Houghton and others, 1996). Most

model simulations generally support the conclusion that global mean temperature will rise over the next century; however, the models disagree on the magnitude and spatial

distribution of large temperature and precipitation changes.

These problems are, in part, due to imperfect representations

of atmospheric physics (e.g., cloud processes) and the coarse

resolution of the models.

For our study, we use a 2×CO2 climate simulation

(Giorgi, Shields-Brodeur, and Bates, 1994) conducted with

the GENESIS AGCM, developed at the National Center for

Atmospheric Research (NCAR), to provide estimates of

environmental sensitivity to global warming. The output

from these simulations is on a rectangular grid of 4.4° of latitude by 7.5° of longitude—a coarse resolution for regionalscale analyses. To achieve finer resolution, we used results

from a companion simulation conducted with the NCAR

regional climate model (RegCM) with a 60-km by 60-km

grid (Giorgi, Shields-Brodeur, and Bates, 1994). The

RegCM simulation was initialized and driven with boundary conditions derived from the GENESIS simulation.

The landscape of the Western United States is dominated by a great number of mountain ranges, with the highest

peaks reaching more than 4,000 m in elevation. The coarse

resolution of the GENESIS AGCM portrays the topography

of the Western United States as dome-like, with a very flat

slope with a maximum elevation of less than 3,000 m centered over Utah and Colorado (fig. 1A, table 1). At the 60-km

resolution of the RegCM (fig. 1B), the basin-and-range

topography begins to emerge and the Sierra Nevada, Rocky

Mountains, and intermountain plateaus are discernible.

Figure 1. Maps illustrating spatial and topographic resolution of the two climate models—A, the GENESIS global atmospheric general circulation model (AGCM) and B, the regional climate model (RegCM)—relative to actual topography in the Western United States. The latter, shown in C, is displayed by elevations on a 15-km grid, sampled from the

5-minute ETOPO5 data set (Edwards, 1992). The GENESIS map (A) shows how the complex topography of the Western United States is greatly smoothed and reduced in elevation in this model. The RegCM map (B), in comparison with

the map of observed topography (C), shows how many physiographic features in the real landscape that have an important mesoscale climatic influence, such as the Sierra Nevada, Colombia Basin, and Snake River Plain, are explicitly represented in the model. Some features, such as the Cascade Range and the individual basins and ranges, are still not well

represented by RegCM.
Table 1. Overview of scientific strategy used in this report.[∆, delta or “change in;” GHG, greenhouse gases; AGCM, atmospheric general circulation model; BATS, biosphere-atmosphere transfer scheme (Dickinson and others, 1993)]

Components of a projection strategy Components utilized in this report Components of potential future 

Climate-system boundary condition changes (i.e., 2×CO2. ∆CO2, ∆ other GHG; ∆ troposp

projected changes in the large-scale controls surface cover.of the climate system).

Global-scale climate simulation (e.g., AGCM, global GENESIS, version 1 (4.4° of latitude × 7.5° of longi- Higher resolution ACGM’s (ap

scope, coarse spatial resolution (hundreds of tude). kilometers); fully coupled kilometers)). vegetation, and oceans.

Procedures for downscaling to regional and local scales RegCM (60-km resolution, BATS surface-physics Higher resolution regional clim

(i.e., to tens of kilometers). package, interactive lakes). surface hydrology and veg

Environmental system process models (e.g., vegetation, Plant taxon probability surfaces (empirical, equilibrium Dynamic vegetation models; c

hydrology). vegetation model); evaporation climatonomy simulation models.

watershed model; lake thermal model.

Projected responses. Projected plant taxon distribution changes; projected Projected plant taxon distribut

monthly streamflow changes; projected changes in in carbon balances, ecosyslake temperature and ice-cover duration. projected changes in variab

yields, ground-water yield
4 ASSESSING FUTURE CHANGES IN CLIMATE, HYDROLOGY, AND VEGETATION, WESTERN UNITED STATES

However, compared with the actual topography at 15-km

resolution (fig. 1C), much of the detail is still missing.

We use output from the RegCM simulation to drive

hydrological and vegetation models to quantify fine-scale

environmental responses to global climate change. We are

not predicting the exact nature of future climatic changes in

the Western United States; rather, our objective is to demonstrate how the effects of large-scale simulations of global

climatic change can be portrayed on a spatial scale that is

meaningful to society.

CONTINENTAL TO 

SUBCONTINENTAL

CLIMATE MODELING

COMPARISON OF

MODEL SIMULATIONS WITH 

OBSERVED PRESENT-DAY CLIMATE

Figure 2 illustrates present-day mean January and July

temperature and precipitation in the Western United States as

simulated by GENESIS (figs. 2A–2D) and RegCM (figs.

2E–2H), compared with observed values that have been

interpolated onto a 15-km grid (figs. 2I–2L). In the observed

climate, January temperatures are above freezing only in

southern portions of California, Arizona, New Mexico,

Texas, and along the Pacific Coast. Extremely cold winter

temperatures occur largely on the northern Great Plains. Due

in large part to its coarse depiction of topography, GENESIS

for January simulates relatively warm winter temperatures

across much of the interior Western United States; extremely

cold temperatures are restricted to the far-northern Great

Plains in Canada. In contrast, the RegCM simulation provides a spatial temperature pattern that better matches the

observed present-day pattern. The location of the freezing

line in the RegCM simulation is very close to that of the

observed data, and cold temperatures on the northern Great

Plains are more accurately depicted. For July temperatures,

the results are similar: GENESIS simulates the general,

broad pattern of July temperatures, but the Western Cordillera is not evident. The RegCM simulation provides an

acceptable pattern of warm temperatures in the Southwest

and cooler temperatures along the Western Cordillera

mountain chain from northern Mexico to British Columbia.

Precipitation in the Western United States is presently

dominated by two distinctly seasonal features of

atmospheric circulation. Winter precipitation, associated

with low-pressure cells and westerly wind patterns (jet

stream) off the Pacific Ocean, is the dominant feature along

the Pacific Coast and into the northern Rocky Mountains

(fig. 2K). In contrast, summer rainfall, associated with convective storms from monsoonal flow originating from the

subtropical Pacific Ocean and Gulf of Mexico, occurs in the

southern and eastern portions of the West (fig. 2L). The wetwest/dry-east pattern of January precipitation is present in

the GENESIS simulation (fig. 2C), although the spatial distribution does not closely match observations (fig. 2K). The

RegCM simulation of January precipitation is a better match

with the observed pattern, although, as with GENESIS, the

simulated precipitation is too high in the Great Basin and

the Southwest. The results for July show that both models

simulate too much precipitation throughout the Rocky

Mountains and that the RegCM captures the gross features

of observed precipitation.

SIMULATION OF A

POTENTIAL 2×CO2 CLIMATE

The GENESIS 2×CO2 simulation used a 50-m-thick

“slab ocean” and atmospheric CO2 concentrations prescribed at 680 ppm—twice the level used in the present-day

simulation (Giorgi, Shields-Brodeur, and Bates, 1994). The

model simulated 20 years before achieving an equilibrium

climate. A 31/2-year RegCM simulation was conducted

Figure 2 (facing page). Maps showing comparison of presentday climate (average or mean January and July temperature and

precipitation) as depicted by A–D, GENESIS; E–H, RegCM; and

I–L, observed conditions. Observed present-day climate data (I–L)

is interpolated onto a 15-km grid from climate stations using a locally weighted trend-surface regression approach (e.g., Lipsitz,

1988). The GENESIS maps (A–D) show, in part, how the crude depiction of topography in the model (see fig. 1A) is translated into the

simulation of temperature and precipitation. Because of the reduction in overall elevation in the smoothed representation of topography in the model, lower values of January temperatures are not

simulated correctly and the overall representation of topography as

a broad dome centered over Wyoming, Colorado, and Utah spreads

out areas of higher precipitation relative to those of the observed

present-day climate. The RegCM maps (E–H), on the other hand,

show that this higher resolution model, even though forced by the

coarse-resolution GENESIS, produces simulations that are closer in

overall appearance to observed climatic patterns (I–L).
CONTINENTAL TO SUBCONTINENTAL CLIMATE MODELING 5

GENESIS SIMULATION OF PRESENT-DAY CLIMATE

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

January Temperature ( ̊C)

A -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

July Temperature ( ̊C)

B 10 25 50 75 100 150 200 300 400 600

January Precipitation (mm)

C 10 25 50 75 100 150 200 300 400 600

July Precipitation (mm)

D

RegCM SIMULATION OF PRESENT-DAY CLIMATE

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

January Temperature ( ̊C)

E -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

July Temperature ( ̊C)

F 10 25 50 75 100 150 200 300 400 600

January Precipitation (mm)

G 10 25 50 75 100 150 200 300 400 600

July Precipitation (mm)

H

OBSERVED PRESENT-DAY CLIMATE ON 15-km GRID

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

January Temperature ( ̊C)

I -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

July Temperature ( ̊C)

J 10 25 50 75 100 150 200 300 400 600

January Precipitation (mm)

K 10 25 50 75 100 150 200 300 400 600

July Precipitation (mm)

L
6 ASSESSING FUTURE CHANGES IN CLIMATE, HYDROLOGY, AND VEGETATION, WESTERN UNITED STATES

using the last 31/2 years of the GENESIS run as boundary

conditions. The coarse-scale fields of GENESIS are interpolated to the finer scale boundary of the RegCM to provide

forcing or inputs for the regional simulations. Giorgi,

Shields-Brodeur, and Bates (1994) provide complete details

and analyses of these simulations. Figure 3 shows a comparison of the simulated January and July temperatures and

precipitation for the present-day climate (control simulation) (figs. 3A–3D) and 2×CO2 climate (figs. 3E–3H). Figures 3I–3L show the climatic differences (anomalies)

between these two simulations. The anomaly maps for January and July temperatures (figs. 3I and 3J) indicate general

and substantial warming of the region for the 2×CO2 simulation. January temperatures are as much as 5°C warmer on

the northern Great Plains and 3°C or more warmer in parts

of Oregon and Idaho, whereas the southwestern deserts

warm by only 1°C to 2°C. The spatial pattern for July is

quite different—British Columbia and the Sonoran Desert

warm by 4°–5°C, whereas the northern Great Plains and the

Oregon/Idaho region warm by 3°C or less.

The anomaly maps for precipitation (figs. 3K and 3L)

indicate that, for the 2×CO2 simulation, winter precipitation

is greater than in the modeled present-day climate (control

simulation) over the Pacific Northwest and along the Pacific

Coast to southern California. Scattered areas of increased

winter precipitation also are located over portions of the

Great Basin, northern Mexico, and parts of the Great

Plains. January precipitation values that are less than those

modeled for the present-day climate are located over the

Sonoran Desert, the Four Corners region, eastern Oregon,

and most of Utah, Wyoming, Montana, Alberta, and the

northern Great Plains in the United States. July precipitation for the 2×CO2 climate is generally greater than that

modeled for the present-day climate in the northern half of

the region and is less than that simulated for the presentday climate in the southern half of the region. In the 2×CO2

simulation, arid summer conditions are simulated for the

Southwestern United States, California, and most of the

Great Basin, as well as for portions of the northern Great

Plains of the United States.

These simulations demonstrate the potential complexity of climate change in the Western United States. The

amount of warming and the changes in precipitation in the

2×CO2 simulation (figs. 3I–3L) vary greatly with geography and with season, with most regions of the Western

United States receiving a mixture of winter and summer

precipitation that is quite different from that of the presentday simulation.

SIMULATED CHANGES IN

SURFACE HYDROLOGY

The quantity and quality of water in lakes, reservoirs,

streams, and rivers are directly affected by climate and climatic change. Water quantity is determined by net moisture,

which, on average, is the difference between what is supplied by precipitation and what is lost to evaporation. Water

quality, which is closely associated to water quantity, and

climatic fluctuations can affect the temperature and biological and chemical properties of both lakes and rivers.

REGIONAL LAKE RESPONSES

A physically based model of lake temperature and

evaporation (Hostetler and Bartlein, 1990) is used to assess

the regional and local sensitivity of lakes to climate change.

In this experiment, daily averaged meteorological parameters (air temperature, humidity, wind speed, solar radiation,

and atmospheric radiation) derived from the RegCM 3-year

present-day (control) and 2×CO2 simulations were used as

inputs to the lake model. In response to these initial conditions, the model computes daily average temperature profiles (i.e., the temperature at each meter of depth within the

lake), mixing and stratification, evaporation, and, if applicable, ice and snow thickness.

Figure 3 (facing page). RegCM simulation of January and July

temperature and precipitation for the present-day climate (A–D),

for the 2×CO2 climate (E–H), and the difference (anomalies) between the 2×CO2 climate and the present-day climate (I–L). The

RegCM present-day climate simulation (A–D) used the GENESIS

simulation of present-day climate (see figs. 2A–2D) as lateral

boundary conditions or inputs, whereas the RegCM 2×CO2 climate simulation (E–H) used the GENESIS 2×CO2 simulation for

lateral boundary conditions. The climate anomalies (RegCM of

2×CO2 climate minus RegCM of present-day climate) are shown

in I–L. The temperature anomalies show greater warming in winter

than in summer, with the maximum warming in winter in the interior of the continent (I). Precipitation anomalies reveal increases of

precipitation in the 2×CO2 simulation relative to the present-day

climate along the West Coast in January (K), and in a broad region

extending from the Pacific Northwest to the Great Plains in July

(L), with generally drier conditions simulated elsewhere in the interior. A hint of a stronger summer monsoon in the 2×CO2 climate

is also apparent.
SIMULATED CHANGES IN SURFACE HYDROLOGY 7

RegCM SIMULATION OF PRESENT-DAY CLIMATE

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

January Temperature ( ̊C)

A -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

July Temperature ( ̊C)

B 10 25 50 75 100 150 200 300 400 600

January Precipitation (mm)

C 10 25 50 75 100 150 200 300 400 600

July Precipitation (mm)

D

RegCM SIMULATION OF 2×CO2 CLIMATE

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

January Temperature ( ̊C)

E -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

July Temperature ( ̊C)

F 10 25 50 75 100 150 200 300 400 600

January Precipitation (mm)

G 10 25 50 75 100 150 200 300 400 600

July Precipitation (mm)

H

CLIMATE ANOMALIES—RegCM OF 2×CO2 CLIMATE MINUS RegCM OF PRESENT-DAY CLIMATE

-30 -10 0 +10 +30

January Precipitation (mm)

K -30 -10 0 +10 +30

July Precipitation (mm)

L +1 +2 +3 +4 +5

January Temperature ( ̊C)

I +1 +2 +3 +4 +5

July Temperature ( ̊C)

J
8 ASSESSING FUTURE CHANGES IN CLIMATE, HYDROLOGY, AND VEGETATION, WESTERN UNITED STATES

Regional lake simulations were conducted by sequentially running a lake model at each grid point for the 3-year

period, thereby producing a gridded set of simulated lake

parameters for the domain. The results presented here are

for a hypothetical lake with a surface area of 0.5 km2 (about

125 acres) and a maximum depth of 5 m (about 16 ft), a size

that is typical of many of the smaller lakes throughout the

West. The distributions of average annual lake-surface temperatures (figs. 4A, 4E, 4I) reflect the topographic and climatological variations simulated across the Western United

States by the RegCM. Simulated water temperatures were

generally coldest over mountain ranges and the eastern High

Plains and warmest in the Southwest and along the Pacific

Coast. Temperatures are warmer over the entire domain for

the 2×CO2 simulation (fig. 4I), with the greatest warming

occurring over the western High Plains northward into the

Canadian prairies. Even though lake evaporation increases

for the 2×CO2 climate, the net moisture (precipitation minus

evaporation) computed over lakes is generally higher (fig.

4J) than that calculated for the present-day climate. This

increase in net moisture on an annual basis is simulated

despite decreases in precipitation and increased temperatures in January and July for the 2×CO2 climate, indicating

that precipitation is increased in months not illustrated in

this report. The increased warmth for the 2×CO2 climate

results in a reduction in the number of days that lakes are ice

covered in winter (fig. 4K) and the number of days per year

that the lakes fully mix (turn over) (fig. 4L). Interpreting

these regional patterns at a specific lake is complicated by

lake-specific factors such as the actual size of the lake, the

shape of the lake basin, and water chemistry.

Where present-day lake temperatures are relatively

warm, deep lakes usually mix or turn over during the winter

when water temperatures cool to near 4°C and the lake

becomes more susceptible to convective and wind-driven

mixing. Shallow lakes usually mix more than deep lakes. In

warm lakes, it has been demonstrated that increased warming results in more stability and, hence, less opportunity for

turnover. Because water chemistry, nutrient flux, and biological activity depend on turnover for mixing and redistribution of important nutrients, warmer waters could reduce

or eliminate turnover and thus degrade water quality and

biological productivity.

The short growing seasons in colder areas today limit

lake productivity—in these areas, warmer water temperatures, reduced seasonal ice cover, and little change or slight

increase in the number of mixing days associated with the

2×CO2 climate would lead to enhanced lake productivity.

Water quality and quantity would not be adversely affected

by higher water losses to evaporation because net moisture

values are generally more positive for the 2×CO2 climate

than they are for the simulated present-day climate.

LAKE-SPECIFIC RESPONSES

The lake model described above was applied to Yellowstone Lake, Wyoming, and Pyramid Lake, Nevada—two

large lakes of contrasting climatic setting (Hostetler and

Giorgi, 1994). For both lakes, 3-year meteorological inputs

for the lake model were derived by averaging the output for

the four RegCM grid points closest to the lake. Actual depth

and areal coverage data for each of the lakes were used in

the model runs, which are summarized below.

Yellowstone Lake lies within Yellowstone National

Park in northwest Wyoming and is the largest (surface area

360 km2, maximum depth 98 m), high-altitude (2,537 m)

freshwater lake in North America. More than 100 tributaries

provide water to the lake, which is drained by the Yellowstone River. The lake is relatively cold, covered by ice during winter, and usually turns over before ice forms and after

ice melts. The lake is the native habitat of the Yellowstone

cutthroat trout (Oncorhynchus clarki bouvieri), which is

important both as a source of food for wildlife and as a

game fish. The 3-year average surface temperature for icefree periods is 11.6°C for the simulated present-day

Figure 4 (facing page). Maps showing lake conditions under

RegCM-simulated present-day climate (A–D), lake conditions under RegCM-simulated 2×CO2 climate (E–H), and lake responses

(lake conditions under RegCM-simulated 2×CO2 climate minus

lake conditions under RegCM-simulated present-day climate) (I–L)

for hypothetical lakes (here, a lake with surface area of 0.5 km2 and

depth of 5 m). Lake conditions calculated by physically based model of lake temperature and evaporation by Hostetler and Bartlein

(1990). Annual surface temperatures for lakes are substantially

greater across the region under the 2×CO2 climate than under the

present-day climate, with lake temperatures as much as 5°C warmer

on the northern Great Plains (I). Annual precipitation minus evaporation is enhanced across much of the region under the 2×CO2 climate, but drier-than-present-day conditions are simulated for parts

of the southwestern deserts and the Great Plains (J). Ice days are, in

general, reduced under the 2×CO2 climate (K). The total number of

days of full-depth mixing shows a complex pattern of response—with increases over most of the Great Plains and in parts

of Mexico, the Southwest, and along the Pacific Coast (L).
SIMULATED CHANGES IN SURFACE HYDROLOGY 9

LAKE CONDITIONS—RegCM-SIMULATED PRESENT-DAY CLIMATE

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Annual Surface Temperature ( ̊C)

A -3 -2 -1 1 2 3

Precipitation–Evaporation (mm/day)

B 0 20 30 40 50 60 70 80

Ice Days

C 0 10 20 30 40 50 60 70 80

Mixing Days

D

LAKE CONDITIONS—RegCM-SIMULATED 2×CO2 CLIMATE

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Annual Surface Temperature ( ̊C)

E -3 -2 -1 1 2 3

Precipitation–Evaporation (mm/day)

F 0 20 30 40 50 60 70 80

Ice Days

G 0 10 20 30 40 50 60 70 80

Mixing Days

H

LAKE RESPONSES—RegCM-SIMULATED 2×CO2 CLIMATE MINUS RegCM-SIMULATED PRESENT-DAY CLIMATE

-4 -3 -2 -1 0 1 2 3 4 5

Annual Surface Temperature ( ̊C)

I -3 -2 -1 1 2 3

Precipitation–Evaporation (mm/day)

J -50 -40 -30 -20 -10 0 10 20 30

Ice Days

K -35 -30 -25 -20 -15 -10 -5 0 5 10 15

Mixing Days

L
10 ASSESSING FUTURE CHANGES IN CLIMATE, HYDROLOGY, AND VEGETATION, WESTERN UNITED STATES

0

5

10

15

20

25

0 200 400 600 800 1000 1200 1400

SURFACE TEMPERATURE,

IN DEGREES CELSIUS

100

75

50

25

0

0 200 400 600 800 1000 1200 1400

MIXED LAYER DEPTH,

IN METERS

0.25

0.5

0.75

SNOW DEPTH,

IN METERS

1

0

1

0.75

0.5

0.25

0 200 400 600 800 1000 1200 1400

ICE THICKNESS, 

IN METERS

TIME SINCE BEGINNING OF SIMULATION, IN DAYS

YELLOWSTONE LAKE, WYOMNG

A

B

C

Simulated present-day climate conditions

Simulated 2×CO2 climate conditions

EXPLANATION

(control) climate and 13.2°C for the 2×CO2 climate (fig.

5A). The lake changes from being predominantly ice covered (196 days of ice cover) under the simulated present-day

climate to being predominantly open (152 days of ice cover)

under 2×CO2 climate conditions. The resulting longer

growing season, together with increased water supply to the

lake (Hostetler and Giorgi, 1994), would lead to increased

productivity of the Yellowstone cutthroat trout under the

2×CO2 climate.

In contrast to Yellowstone Lake, Pyramid Lake is a

warm, monomictic (i.e., ice free, with one period of turnover annually), closed-basin (no river outflow) lake located

at an altitude of 1,065 m in the high desert that lies within

the Paiute Indian Reservation in the lee of the Sierra

Nevada. The lake is the native habitat of the Lahontan cutthroat trout (Oncorhynchus clarki henshawi) and the cui-ui

sucker (Chasmistes cujus), which are important economic

and cultural resources for the Paiute Tribe. The average surface temperature of Pyramid Lake over the 3-year presentday (control) simulation is 12.7°C, compared to 15.5°C

under the 2×CO2 climate simulation. The depth of the

mixed layer of Pyramid Lake varies seasonally, with turnover occurring in January. Under modeled present-day (control) conditions, turnover of the lake occurred in January for

each of the 3 years of simulation (fig. 6); however, under the

2×CO2 climate simulation, turnover did not occur in any of

the 3 years as a result of warming in the upper 30 m of the

lake’s waters. The average annual evaporation rate

Figure 5. Present-day climate (blue) and 2×CO2 climate (red)

simulations for physical characteristics of Yellowstone Lake, Wyoming, during four winters and three summers. Physical characteristics modeled are A, surface temperature; B, mixed layer depth; C,

snow depth and ice thickness. When compared to simulated presentday climate conditions, the simulated 2×CO2 climate results in

warmer summer water temperatures (+1.6°C), longer ice-free periods (as illustrated by the times of mixing in this dimictic lake), less

snow depth in the winter, and generally thinner winter ice. (A dimictic lake has two turnovers or periods of mixing per year.)

0

5

10

15

20

25

0 200 400 600 800 1000 1200 1400

100

75

50

25

0

0 200 400 600 800 1000 1200 1400

TIME SINCE BEGINNING OF SIMULATION, IN DAYS

PYRAMID LAKE, NEVADA

MIXED LAYER DEPTH,

IN METERS

SURFACE TEMPERATURE,

IN DEGREES CELSIUS

A

B

Simulated present-day climate conditions

Simulated 2×CO2 climate conditions

EXPLANATION

Figure 6. Present-day climate (blue) and 2×CO2 (red) simulations for physical characteristics of Pyramid Lake, Nevada, during

four winters and three summers. Physical characteristics modeled

are A, surface temperature; B, mixed layer depth. The simulated

2×CO2 climate results in warmer summer water temperatures

than under simulated present-day climate conditions (+2.8°C).

Part B of the figure shows that, for this lake, warmer water-surface temperatures (predicted by simulated 2×CO2 climate conditions) have led to a cessation of the overturning process, which

would affect the chemistry and biology of the lake. (This has

changed the lake from a monomictic lake—one with one yearly

turnover or period of mixing—into a lake with no turnovers or pe-
SIMULATED CHANGES IN SURFACE HYDROLOGY 11

6

8

10

12

14

16

18

20

22

24

26

28

WATER TEMPERATURE, IN DEGREES CELSIUS

JUNE JULY

MONTH

SUMMER STREAM TEMPERATURES

STEAMBOAT CREEK, OREGON

AUGUST SEPTEMBER

Simulated present-day water temperature

Simulated water temperature under 2×CO2 climate

EXPLANATION

Tolerance level for

juvenile steelhead trout

50

100

150

200

250

10

5

0

0

0

15

20

25

50

100

150

200

250

AIR TEMPERATURE,

IN DEGREES CELSIUS

PRECIPITATION,

IN MILLIMETERS PER MONTH

STREAMFLOW,

IN MILLIMETERS PER MONTH

A

B

C JAN FEB MAR APR MAY

MONTH

JUN JUL AUG SEP OCT NOV DEC

Observed present-day air temperature (avg = 12 ̊C)

Air temperature calculated by statistical model of

Hostetler and Bartlein (1990) under RegCM-calculated

2×CO2 climate conditions (avg = 13 ̊C)

Observed present-day precipitation

 amount (total = 1,345 mm)

Precipitation amount calculated by

statistical model of Hostetler and Bartlein (1990)

under RegCM-calculated 2×CO2 climate conditions

(total = 1,686 mm)

Observed present-day streamflow (total = 861 mm)

Streamflow calculated by statistical model of 

Hostetler and Bartlein (1990) under RegCM-calculated

2×CO2 climate conditions (total = 1,066 mm)

computed for the 2×CO2 climate (1,595 mm) is 14 percent

higher than that calculated for the modeled present-day climate. However, as in the case of Yellowstone Lake,

increased precipitation in the 2×CO2 climate simulation

resulted in a net increase in moisture over the lake. The lack

of turnover under the 2×CO2 simulation would result in

reduced productivity, which could have large negative

effects on the fish populations in the lake.

STREAMFLOW

Streamflow models are still under development, generally stream-specific, and not yet capable of being applied on

a regional scale. In addition, in some areas a substantial portion of river water is derived from ground water, and

ground-water flow models are difficult to implement in the

Western United States where complex topography and geology determine recharge areas and subsurface flow patterns.

Nonetheless, it is still possible to demonstrate the potential

sensitivity of specific streams to climate change, such as at

Steamboat Creek, Oregon, which lies in a forested watershed on the western slope of the Cascade Mountains.

Steamboat Creek and its tributaries provide important

spawning and rearing habitats for native steelhead trout

Figure 7. Simulated summer stream temperatures under presentday climate (control, in blue) and simulated temperatures under

2×CO2 climate (red) of Steamboat Creek, Oregon. The dashed horizontal line at 24°C on the “water temperature” axis indicates the

summer temperature tolerance limit of juvenile steelhead trout (Oncorhynchus mykiss). Under the 2×CO2 climate simulation, the statistical model of Hostetler (1991) suggests that the length of time

within the year when the tolerance limit is exceeded is more than

twice as long as under simulated present-day climate conditions

(control). Shaded area surrounding 2×CO2 temperature curve indicates one standard deviation (σ) from the mean.

Figure 8. Comparisons of present-day (control, in blue) and

2×CO2 (red) air temperature (A), precipitation (B), and streamflow

(C) simulations of Steamboat Creek, Oregon. In this case, the simulated annual air temperature of the drainage rises only 1°C. However, the simulated air temperature during the summer period of

minimum flow is 3°C higher in the 2×CO2 simulation than in the

control simulation. The 2×CO2 simulation has higher precipitation

levels than in the control simulation—hence, streamflow increases

under the 2×CO2 simulation and shifts slightly seasonally. The

change in streamflow lags the temperature and precipitation changes by a month or more, probably due to the time required for soil

moisture to respond to changes in climate.
12 ASSESSING FUTURE CHANGES IN CLIMATE, HYDROLOGY, AND VEGETATION, WESTERN UNITED STATES

(Oncorhynchus mykiss). During part of July and August,

present-day average maximum stream temperatures are

often near the upper limit of tolerance (24°C) for juvenile

steelhead trout (fig. 7).

A statistical model of stream temperature (Hostetler,

1991) provides quantitative estimates of the change in summer water temperatures in this stream in response to climate simulated by RegCM. Although the annual average

air temperature of the Steamboat Creek drainage was only

about 1°C warmer for the simulated 2×CO2 climate than

for the control case (RegCM-simulated present-day conditions) (fig. 8), it was as much as 3°C warmer during the

minimum flows of the summer. Precipitation increased 25

percent annually for 2×CO2 climate relative to simulated

present-day conditions. As a result of this increase in precipitation and change in air temperature, streamflow

increased and is shifted slightly seasonally (fig. 8C). During

summer, the effect of higher air temperatures on water temperatures was not offset by increased flow and maximum

stream temperatures—stream water temperatures calculated

for the 2×CO2 climate exceeded the tolerance level of steelhead trout for a period more than twice as long as the simulated present-day average (fig. 7). Relatively small changes

in climate could thus lead to significant degradation of

spawning habitat. The potential for natural warming

implies that, if the steelhead trout’s habitat is to be maintained, land-use decisions in the Steamboat Creek basin

will need to be based on extensive analysis of the effects of

both natural climate variability and human modifications.

SIMULATED CHANGES IN

PLANT DISTRIBUTIONS

The geographic ranges of plant species are controlled

by climate change—in this section, we explore potential

changes in the distributions of selected western plant species due to future climate change. Following the methods

described in Lipsitz (1988) and Bartlein and others (1994),

we interpolated present-day climatic data onto a 15-km

equal-area grid of the Western United States to provide a

fine-scale basis for the exploration of potential changes in

plant distributions (fig. 9). The differences between the

RegCM-simulated present-day climate and the RegCMsimulated 2×CO2 climate were applied to this target grid to

provide inputs into vegetation models simulating potential

changes in plant distributions that would occur when comparing present-day and potential future climates.

PRESENT-DAY CLIMATE

AND PLANT DISTRIBUTIONS

Bartlein and others (1994) prepared an equal-area 25-

km grid of 30-year climate “normals” for North America.

January and July temperature and precipitation data from

this grid were used in the analyses presented below.

GSMAP software (Selner and Taylor, 1992) was used to

digitize maps of tree distributions (Little, 1971, 1976) and to

determine which of the 32,211 points in the North American

climate grid were within the geographic range of the each

taxon and which were outside of this range. The presence/

absence data were matched with temperature and precipitation data to determine the present-day relationships between

plant distributions and climate (Thompson and others, in

press). The distributions of 16 common western trees and

shrubs (table 2) are analyzed for the present study. Scatter

diagrams (fig. 10) illustrate the climatic tolerances of four of

these plants in terms of seasonal temperature and precipitation. For example, Sitka spruce lives where January mean

temperatures are above –10°C and mean July temperatures

are below 20°C. In contrast, Douglas fir lives under a wide

variety of temperature regimes, and the frost-sensitive species, California white oak and Joshua tree, live where mean

January temperatures are above freezing. Sitka spruce is

largely limited to sites with very high levels of January precipitation and relatively low levels of July precipitation.

Douglas fir lives in a range of seasonal precipitation

regimes, whereas California white oak lives in areas with

high winter precipitation and pronounced summer drought.

Joshua tree lives in very dry climates where winter precipitation tends to exceed that of summer.

COMPARISON OF PRESENT-DAY AND

POTENTIAL FUTURE RANGES

The relationships illustrated in figure 10 can be analyzed statistically to produce derived presence/absence

response surfaces (e.g., Bartlein and others, 1986) that

determine the probability of occurrence for each plant for a

given combination of seasonal temperature and precipitation characteristics. These statistical relationships can then

be used with observed climate data to estimate the presentday range of the species. The relationships can also be used

with climate-model output to simulate the potential future

range of the plant. Figure 11 shows the simulated presentday and potential future ranges of the four plants discussed

above. In the left-hand illustration of each row (figs. 11A,

11E, 11I, 11M) is shown the observed present-day range of

the plant on the 15-km target grid. For comparison, the next

illustration to the right (figs. 11B, 11F, 11J, 11N) shows the

probability of occurrence of the species produced by

response-surface analysis of the observed present-day climatic and plant-distribution data. In most cases, the presentday range is aligned with approximately the 0.4 probability

of occurrence. The third illustration from the left (figs. 11C,

11G, 11K, 11O) illustrates the simulated probability of

occurrence for the same taxa under the 2×CO2 climate simulation interpolated onto the 15-km target grid. The righthand illustrations (figs. 11D, 11H, 11L, 11P) illustrate

changes in the distribution of each species at the 0.4
SIMULATED CHANGES IN PLANT DISTRIBUTIONS 13

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

January Temperature ( ̊C)

A -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

July Temperature ( ̊C)

B 10 25 50 75 100 150 200 300 400 600

January Precipitation (mm)

C 10 25 50 75 100 150 200 300 400 600

July Precipitation (mm)

D

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

January Temperature ( ̊C)

E -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

July Temperature ( ̊C)

F 10 25 50 75 100 150 200 300 400 600

January Precipitation (mm)

G 10 25 50 75 100 150 200 300 400 600

July Precipitation (mm)

H

OBSERVED PRESENT-DAY CLIMATE ON 15-km GRID

RegCM SIMULATION OF 2×CO2 CLIMATE ON 15-km GRID

probability level between the present-day simulated distribution and the distribution calculated for the 2×CO2 climate. In this case (i.e., the right-hand illustrations of the

figure), green represents areas occupied by the species

under both the present-day and 2×CO2 climate simulations,

red represents range lost by the species between the presentday and 2×CO2 simulations, and blue represents new areas

that could potentially be colonized by the species under the

2×CO2 climate simulation. In the examples presented here,

all of the four species could potentially increase their total

coverage, although not necessarily within or adjacent to

their current range.

Figure 12 illustrates potential range changes that could

occur under the 2×CO2 climate simulation. Here the habitats

of plants along the Pacific Coast from northern California to

British Columbia (Sitka spruce, western red cedar, western

hemlock) are reduced slightly, but new habitat develops in

the northern interior (figs. 12A, 12B, 12C). Incense cedar

nearly dies out in its current range in California and Oregon,

but potential habitat opens up in the present-day steppe environment of the Four Corners region (fig. 12D). Under this

scenario, major forest trees and range shrubs (Engelmann

spruce, Douglas fir, lodgepole pine, and big sagebrush) die

off across much of their present-day range without much

replacement habitat becoming available (figs. 12E, 12G,

12M). Ponderosa pine would lose much of its western range

and find new potential habitat east of its present-day limits

(fig. 12H). California white oak and Oregon white oak could

potentially grow under a 2×CO2 winter-wet summer-dry

climate in the Southwest (figs. 12I, 12J). Gambell oak and

pinyon pine would die off in the Southwest, but could potentially find new range in the northern interior (figs. 12K, 12L).

Joshua tree and creosote bush, two shrubs adapted to arid

conditions, would considerably expand their spatial

Figure 9. Observed present-day (A–D) and simulated 2×CO2 (E–H) climates illustrated on 15-km grid. These latter values were formed by interpolating the RegCM climate anomalies (see bottom row of fig. 3) onto the 15-km grid and adding

interpolated values to present-day values.
14 ASSESSING FUTURE CHANGES IN CLIMATE, HYDROLOGY, AND VEGETATION, WESTERN UNITED STATES

coverage relative to today under the 2×CO2 simulation (figs.

12N, 12O). Saguaro, a frost-limited, dry-adapted plant of the

Sonoran Desert, would largely die off in its present-day

range, but could potentially find new range farther east and

at higher elevations (fig. 12P).

If dispersal rates and (or) human intervention allowed

these plants to reach their new potential habitats, how large

would the changes be in the coverage of each species?

Joshua tree and California white oak have potential future

ranges under the 2×CO2 simulation that would cover more

than seven times their current total area (table 2). Incense

cedar, Oregon white oak, and Sitka spruce would expand to

more than twice their current areal extents; and creosote

bush, ponderosa pine, and saguaro would have somewhat

greater-than-present-day coverage, although generally not

within their current range limits. Western hemlock and

western red cedar could occupy modest amounts of new

range near their present-day limits, whereas Gambell oak

and pinyon pine would have slightly larger and slightly

smaller coverages, respectively—but these species would

have to disperse hundreds of kilometers northward to reach

their new habitats. Douglas fir, big sagebrush, and lodgepole

pine would have half or less of their present-day extents,

whereas Engelmann spruce would have less than one-fifth

of its present-day areal coverage. Could some of the forest

trees have potential habitat north of the 15-km target grid

under the 2×CO2 climate simulation? This is probably not

the case—if potential habitat existed north of the target grid,

it would be logical to assume that the species would be surviving in cool habitats in higher elevations on the northern

edge of the grid. Inspection of the output (fig. 12) indicates

that this is not the case. The surviving trees are instead

located in the lower elevations in this region.

DISCUSSION OF CHANGES IN

PLANT DISTRIBUTIONS

The range changes simulated to occur between today

and the 2×CO2 climate would dramatically affect ecosystems across the Western United States. Similarly large shifts

in plant distributions have been simulated for a potential

2×CO2 climate in Europe (Dahl, 1990; Huntley and others,

1995; Sykes and others, 1996). The plant species modeled

Table 2. Common and scientific names of plant species with simulated present-day ranges and

potential ranges under a simulated 2×CO2 future climate.

[The 15-km grid used in this study has 23,961 cells in western North America. “Present number” and “2×CO2 number” are

the number of those cells occupied by the species at the 0.40 level of probability for the present-day and potential future climate simulations, respectively. “Percent of present coverage” represents the percentage increase or decrease of the taxon under the future climate scenario compared to the present-day simulation (present-day range = 100 percent)]

Common Scientific Present 2× CO2 Percent of

name name number number present coverage

Sitka spruce .................Picea sitchensis.................... 493 .................1,033 .................210

Western red cedar........Thuja plicata......................1,010 .................1,143 .................113

Western hemlock.........Tsuga heterophylla............... 945 .................1,185 .................125

Incense cedar...............Libocedrus decurrens........... 517 .................1,103 .................213

Engelmann spruce.......Picea engelmannii .............3,387 ................... 566 .................. 17

Douglas fir...................Pseudotsuga menziesii ......4,898 .................2,474 .................. 51

Lodgepole pine............Pinus contorta....................4,350 .................1,508 .................. 35

Ponderosa pine ............Pinus ponderosa.................2,319 .................3,147 .................136

Oregon white oak ........Quercus garryana ................ 626 .................1,330 .................212

California white oak....Quercus lobata..................... 447 .................3,322 .................743

Gambel oak .................Quercus gambelli .................753 ....................959 .................127

Pinyon pine .................Pinus edulis...........................974 ....................935 ...................96

Big sagebrush..............Artemisia tridentata...........5,733 .................2,356 ...................41

Joshua tree...................Yucca brevifolia.....................187 .................1,482 .................793

Creosote bush..............Larrea divaricata...............3,891 .................6,235 .................160

Saguaro........................Cereus giganteus...................626 ....................806 .................129
SIMULATED CHANGES IN PLANT DISTRIBUTIONS 15

 -40

 -30

 -20

 -10

 0

 10

 20

 30 Sitka spruce

A

 1

 10

 100

1000

B

 -40

 -30

 -20

 -10

 0

 10

 20

 30

Douglas fir

C

 1

 10

 100

1000

D

 -40

 -30

 -20

 -10

 0

 10

 20

 30

California white oak

JANUARY TEMPERATURE, IN DEGREES CELSIUS

E

 1

 10

 100

1000

JANUARY PRECIPITATION, IN MILLIMETERS

F

 0 10 20 30 -40

 -30

 -20

 -10

 0

 10

 20

 30

Joshua tree

JULY TEMPERATURE,

IN DEGREES CELSIUS

G 1 10 100 1000 1

 10

 100

1000

JULY PRECIPITATION,

IN MILLIMETERS

H

Figure 10. Bivariate scatter plots of January

vs. July temperature (left column) and January

vs. July log precipitation (right column) showing

presence/absence data (green and gray, respectively) for four plant species in western North

America. These illustrations depict the unique

climatic adaptations of various tree species in

western North America. Sitka spruce (A, B), a

tree of the moist Pacific Northwest coast, lives in

some of the wettest winter climates on the continent under January temperatures between –10°C

and +10°C and July temperatures between 5°C

and 20°C. Douglas fir (C, D) lives under a broad

band of seasonal temperatures and precipitation

conditions. California white oak (E, F) is restricted to environments west of the Sierra Nevada in California and lives under a fairly wide

range of July temperatures but a very narrow

band of January temperatures. This tree receives

essentially all of its moisture in the winter and

lives under pronounced summer drought. Joshua

tree (G, H), an arborescent yucca of the Mojave

Desert, lives in places where summers are extremely hot and winters are rarely below freezing. It survives in very arid environments.
16 ASSESSING FUTURE CHANGES IN CLIMATE, HYDROLOGY, AND VEGETATION, WESTERN UNITED STATES

PRESENT-DAY OBSERVED

SPECIES DISTRIBUTION

PRESENT-DAY SIMULATED

SPECIES DISTRIBUTION

SPECIES DISTRIBUTION

SIMULATED FOR

2×CO2 CLIMATE

CHANGE IN SPECIES DISTRIBUTION 

BETWEEN PRESENT-DAY 

SIMULATED DISTRIBUTION AND

SIMULATED DISTRIBUTION

UNDER 2×CO2 CLIMATE

A B C D

E F G H

Present .0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0

Probability of Occurrence

No Change Contraction Extension

Sitka spruce

Douglas fir

here exhibit individualistic responses to climatic change, as

has been demonstrated in the response of western plants to

warming following the end of the last Ice Age (Thompson,

1988; Betancourt and others, 1990; Thompson and others,

1993). In the latter case, warming was relatively gradual and

plants had thousands of years to disperse from their old

ranges to new ones. In contrast, the anticipated onset of

“greenhouse” warming may occur in only a few decades,

and it is unknown whether or not displaced plants and animals could migrate to new habitats this rapidly. Many widespread taxa would lose much of their current range under

the 2×CO2 climate, whereas other presently uncommon taxa

could greatly expand their ranges (table 2). Similar post-IceAge patterns have been documented in packrat midden plant

Figure 11 (above and facing page). Maps of plant-species distributions showing, by column from left to right, observed present-day distributions (A, E, I, M) (Little, 1971, 1976), simulated present-day distributions at 0.4 probability of

occurrence (B, F, J, N), simulated distributions for 2×CO2 climate at 0.4 probability of occurrence (C, G, K, O), and

changes in species distribution between simulated present-day distribution and simulated distribution under 2×CO2 climate (D, H, L, P). Species shown are Sitka spruce (A–D), Douglas fir (E–H), California white oak (I–L), and Joshua tree

(M–P). Comparison of the simulated present-day distributions with observed present-day distributions demonstrates that

the presence/absence response surface methodology captures the overall distribution pattern of all of the species. Comparison of the simulated present-day distributions and the simulated 2×CO2 distributions illustrates the potential range

shifts that could occur if species were able to migrate rapidly enough to reach their new potential habitats during the transition from the present-day climate to a 2×CO2 climate. The right-hand columns of maps (D, H, L, P) show these shifts

more directly—green represents sites where species live today and where they could continue to live under the simulated

2×CO2 climate; red indicates sites where species live today but could not survive under the simulated 2×CO2 climate;

blue represents sites where species cannot live today but could potentially live under the simulated 2×CO2 future climate.
CONCLUSIONS 17

I J K L

M N O P

PRESENT-DAY OBSERVED

SPECIES DISTRIBUTION

PRESENT-DAY SIMULATED

SPECIES DISTRIBUTION

SPECIES DISTRIBUTION

SIMULATED FOR

2×CO2 CLIMATE

CHANGE IN SPECIES DISTRIBUTION 

BETWEEN PRESENT-DAY 

SIMULATED DISTRIBUTION AND

SIMULATED DISTRIBUTION

UNDER 2×CO2 CLIMATE

Present .0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0

Probability of Occurrence

No Change Contraction Extension

California white oak

Joshua tree

assemblages, illustrating the transition from Pleistocene to

Holocene vegetation in the American Southwest (Thompson, 1988). For example, pinyon pine (Pinus edulis) was relatively rare during the last Ice Age, whereas papershell

pinyon pine (Pinus remota) was common (Van Devender,

1990), a situation that reversed during the transition from

Pleistocene to Holocene conditions.

The economic impacts of projected changes in vegetation for the 2×CO2 climate could be substantial. Many forest trees, such as Engelmann spruce and lodgepole pine,

could be driven to the edge of extinction. The habitats of

National Parks, Bureau of Land Management (BLM) lands,

and Native American lands would be strongly affected by

these and other associated changes (e.g., Bartlein and others, 1997). Land managers could be faced with deciding if

they should be protecting the land or the ecosystems presently occupying the lands. The hierarchy of climate and process models used here provides detail and geographic scope

unavailable from the current generation of AGCM’s and

employs embedded physics and process models that

enhance the regional depiction of climate change. Analyses

such as this provide a means for land-resource managers to

identify and assess elements of ecosystems that are sensitive

to climatic variability and climate change.

CONCLUSIONS

The use of a hierarchy of atmospheric and process

models enables us to assess the effects of global climate

change at the scale of landscape-scale processes. In this

example, regional climate, hydrology, vegetation, and

wildlife resources are affected by simulated global climate

change. However, not all possible simulated or actual

future climates would necessarily have the same environmental effects. Our strategy does provide a methodology

for quantifying the response to a range of possible climatic

changes on national lands and resources and may serve to

identify regions and processes that are most vulnerable to

these changes.
18 ASSESSING FUTURE CHANGES IN CLIMATE, HYDROLOGY, AND VEGETATION, WESTERN UNITED STATES

Sitka spruce

A

Western red cedar

B

Western hemlock

C

Incense cedar

D

Engelmann spruce

E

Douglas fir

F

Lodgepole pine

G

Ponderosa pine

H

No Change Contraction Extension

CHANGES IN SPECIES DISTRIBUTION BETWEEN PRESENT-DAY SIMULATED DISTRIBUTION

AND SIMULATED DISTRIBUTION UNDER 2×CO2 CLIMATE

ACKNOWLEDGMENTS

Bev Lipsitz, Jenny Buchner, and Jeff Honke provided

technical help that made this project possible. Filippo

Giorgi and the staff at the National Center for Atmospheric

Research provided the GENESIS simulation output. Robert

S. Webb and Cathy Whitlock provided useful discussions

and ideas, and Dick Poore and Robert S. Webb provided

constructive criticisms of the manuscript. This effort was

funded by the U.S. Geological Survey Global Change and

Climate History Program. This report benefited greatly from

the editorial efforts of Rick Scott and the graphical experience of Carol Quesenberry.

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Published in the Central Region, Denver, Colorado

Manuscript approved for publication March 12, 1998

Edited by Richard W. Scott, Jr.

Graphics prepared by Carol A. Quesenberry; use made

of author-drafted material

Cover design and photocomposition by Carol A. Quesenberry

Online version by Joan G. Nadeau