Emissions pathways, climate change, and impacts
, Daniel Cayan
, Christopher B. Field
, Peter C. Frumhoff
, Edwin P. Maurer
, Norman L. Miller
Susanne C. Moser
, Stephen H. Schneider
, Kimberly Nicholas Cahill
, Elsa E. Cleland
, Larry Dale
, Ray Drapek
R. Michael Hanemann
, Laurence S. Kalkstein
, James Lenihan
, Claire K. Lunch
, Ronald P. Neilson
Scott C. Sheridan
, and Julia H. Verville
ATMOS Research and Consulting, 809 West Colfax Avenue, South Bend, IN 46601;
Climate Research Division, The Scripps Institution of Oceanography, and
Water Resources Division, U.S. Geological Survey, 9500 Gilman Drive, La Jolla, CA 92093-0224;
Department of Global Ecology, Carnegie Institution of
Washington, 260 Panama Street, Stanford, CA 94305;
Union of Concerned Scientists, Two Brattle Square, Cambridge, MA 02238;
Department, Santa Clara University, Santa Clara, CA 95053;
Atmosphere and Ocean Sciences Group, Earth Sciences Division, Lawrence Berkeley National
Laboratory, 1 Cyclotron Road, Berkeley, CA 94720;
Environmental and Societal Impacts Group, National Center for Atmospheric Research, P.O. Box 3000,
Boulder, CO 80307;
Department of Biological Sciences and Institute for International Studies, Stanford University, Stanford, CA 94305;
Sciences Laboratory, U.S. Department of Agriculture Forest Service, 3200 SW Jefferson Way, Corvallis, OR 97331;
Department of Agricultural and Resource
Economics, University of California, Berkeley, CA 94720;
Center for Climatic Research, Department of Geography, University of Delaware, Newark, DE
Department of Geography, Kent State University, Kent, OH 44242
Contributed by Christopher B. Field, June 23, 2004
The magnitude of future climate change depends substantially on
the greenhouse gas emission pathways we choose. Here we
explore the implications of the highest and lowest Intergovern-
mental Panel on Climate Change emissions pathways for climate
change and associated impacts in California. Based on climate
projections from two state-of-the-art climate models with low and
medium sensitivity (Parallel Climate Model and Hadley Centre
Climate Model, version 3, respectively), we ﬁnd that annual tem-
perature increases nearly double from the lower B1 to the higher
A1ﬁ emissions scenario before 2100. Three of four simulations also
show greater increases in summer temperatures as compared with
winter. Extreme heat and the associated impacts on a range of
temperature-sensitive sectors are substantially greater under the
higher emissions scenario, with some interscenario differences
apparent before midcentury. By the end of the century under the
B1 scenario, heatwaves and extreme heat in Los Angeles quadruple
in frequency while heat-related mortality increases two to three
times; alpine兾subalpine forests are reduced by 50 –75%; and Sierra
snowpack is reduced 30–70%. Under A1ﬁ, heatwaves in Los
Angeles are six to eight times more frequent, with heat-related
excess mortality increasing ﬁve to seven times; alpine兾subalpine
forests are reduced by 75–90%; and snowpack declines 73–90%,
with cascading impacts on runoff and streamﬂow that, combined
with projected modest declines in winter precipitation, could
fundamentally disrupt California’s water rights system. Although
interscenario differences in climate impacts and costs of adaptation
emerge mainly in the second half of the century, they are strongly
dependent on emissions from preceding decades.
alifornia, with its diverse range of climate zones, limited
water supply, and economic dependence on climate-
sensitive industries such as agriculture, provides a challenging
test case to evaluate impacts of regional-scale climate change
under alternative emissions pathways. As characterized by the
Intergovernmental Panel on Climate Change, demographic,
socioeconomic, and technological assumptions underlying long-
term emissions scenarios vary widely (1). Previous studies have
not systematically examined the difference between projected
regional-scale changes in climate and associated impacts across
scenarios. Nevertheless, such information is essential to evaluate
the potential for and costs of adaptation associated with alter-
native emissions futures and to inform mitigation policies (2).
Here, we examine a range of potential climate futures that
represent uncertainties in both the physical sensitivity of current
climate models and divergent greenhouse gas emissions path-
ways. Two global climate models, the low-sensitivity National
Center for Atmospheric Research兾Department of Energy Par-
allel Climate Model (PCM) (3) and the medium-sensitivity U.K.
Met Office Hadley Centre Climate Model, version 3 (HadCM3),
model (4, 5) are used to calculate climate change resulting from
the SRES (Special Report on Emission Scenarios) B1 (lower)
and A1fi (higher) emissions scenarios (1). These scenarios
bracket a large part of the range of Intergovernmental Panel on
Climate Change nonintervention emissions futures with atmo-
spheric concentrations of CO
reaching ⬇550 ppm (B1) and
⬇970 ppm (A1fi) by 2100 (see Emissions Scenarios in Supporting
Text, which is published as supporting information on the PNAS
web site). Although the SRES scenarios do not explicitly assume
any specific climate mitigation policies, they do serve as useful
proxies for assessing the outcome of emissions pathways that
could result from different emissions reduction policies. The
scenarios at the lower end of the SRES family are comparable
to emissions pathways that could be achieved by relatively
aggressive emissions reduction policies, whereas those at the
higher end are comparable to emissions pathways that would be
more likely to occur in the absence of such policies.
Downscaling Methods. For hydrological and agricultural analyses,
HadCM3 and PCM output was statistically downscaled to a 1兾8°
grid (⬇150 km
) (6) and to individual weather stations (7) for
analyses of temperature and precipitation extremes and health
impacts. Downscaling to the 1兾8° grid used an empirical statis-
tical technique that maps the probability density functions for
modelled monthly precipitation and temperature for the clima-
tological period (1961–1990) onto those of gridded historical
observed data, so the mean and variability of observations are
reproduced by the climate model data. The bias correction and
spatial disaggregation technique is one originally developed for
adjusting General Circulation Model output for long-range
streamflow forecasting (6), later adapted for use in studies
examining the hydrologic impacts of climate change (8), and
compares favorably to different statistical and dynamic down-
scaling techniques (9) in the context of hydrologic impact studies.
Station-level downscaling for analyses of temperature and
precipitation extremes and health impacts used a deterministic
method in which grid-cell values of temperatures and precipi-
Freely available online through the PNAS open access option.
Abbreviations: DJF, December, January, February; HadCM3, Hadley Centre Climate Model,
version 3; JJA, June, July, August; PCM, Parallel Climate Model; SRES, Special Report on
Emission Scenarios; SWE, snow water equivalent.
To whom correspondence should be addressed. E-mail: firstname.lastname@example.org.
© 2004 by The National Academy of Sciences of the USA
August 24, 2004
no. 34 www.pnas.org兾cgi兾doi兾10.1073兾pnas.0404500101
tation from the reference period were rescaled by simple
monthly regression relations to ensure that the overall proba-
bility distributions of the simulated daily values closely approx-
imated the observed probability distributions at selected long-
term weather stations (7). The same regression relations were
then applied to future simulations, such that rescaled values
share the weather statistics observed at the selected stations. At
the daily scales addressed by this method, the need to extrapolate
beyond the range of the historically observed parts of the
probability distributions was rare even in the future simulations
(typically ⬍1% of the future days) because most of the climate
changes involve more frequent warm days than actual truly
warmer-than-ever-observed days (7).
Except where otherwise noted, we present projected climate
anomalies and impacts averaged over 2020–2049 (with a mid-
point of 2035) and 2070–2099 (here designated as end-of-
century, with a midpoint of 2085), relative to a 1961–1990
Temperature. All simulations show increases in annual average
temperature before midcentury that are slightly greater under
the higher A1fi emissions scenario (see Fig. 4, which is published
as supporting information on the PNAS web site). By end-of-
century, projected temperature increases under A1fi are nearly
twice those under B1, with the more sensitive HadCM3 model
producing larger absolute changes (Table 1). Downscaled sea-
sonal mean temperature projections (10) show consistent spatial
patterns across California, with lesser warming along the south-
west coast and increasing warming to the north and northeast
(Fig. 1). Statewide, the range in projected average temperature
increases is higher than previously reported (11–14), particularly
for summer temperature increases that are equal to or greater
than increases in winter temperatures.
Table 1. Summary of midcentury (2020–2049) and end-of-century (2070–2099) climate and impact projections for the HadCM3 and
PCM B1 and A1ﬁ scenarios
PCM HadCM3 PCM HadCM3
B1 A1ﬁ B1 A1ﬁ B1 A1ﬁ B1 A1ﬁ
Change in statewide avg temperatures
Annual °C 15.0 1.35 1.5 1.6 2.0 2.3 3.8 3.3 5.8
Summer (JJA) °C 22.8 1.2 1.4 2.2 3.1 2.15 4.1 4.6 8.3
Winter (DJF) °C 7.6 1.3 1.2 1.4 1.45 2.15 3.0 2.3 4.0
Change in statewide avg precipitation
Annual mm 544 ⫺37 ⫺51 ⫹6 ⫺70 ⫹38 ⫺91 ⫺117 ⫺157
Summer (JJA) mm 20 ⫺3 ⫹2 ⫺1 ⫺7 ⫹4 ⫺46 ⫺5 ⫺1
Winter (DJF) mm 269 ⫺45 ⫺55 ⫹4 ⫺44 ⫹13 ⫺13 ⫺79 ⫺92
Sea level rise cm — 8.7 9.5 11.6 12.7 19.2 28.8 26.8 40.9
Los Angeles Days 12 28 35 24 36 44 76 47 95
Sacramento Days 58 91 101 93 104 109 134 115 138
Fresno Days 92 113 120 111 116 126 147 126 149
El Centro Days 162 185 185 176 180 191 213 197 218
Length of heatwave season* Days 115 135 142 132 141 149 178 162 204
Excess mortality for Los Angeles
Without acclimatization avg no. of
— — — — — 394 948 667 1,429
With acclimatization avg no. of
165 — — — — 319 790 551 1,182
Change in April 1 snowpack SWE
1,000–2,000 m elevation % 3.6 km
⫺60 ⫺56 ⫺58 ⫺66 ⫺65 ⫺95 ⫺87 ⫺97
2,000–3,000 m elevation % 6.5 km
⫺34 ⫺34 ⫺24 ⫺36 ⫺22 ⫺73 ⫺75 ⫺93
3,000–4,000 m elevation % 2.3 km
⫺11 ⫺15 4 ⫺16 15 ⫺33 ⫺48 ⫺68
All elevations % 12.4 km
⫺38 ⫺37 ⫺26 ⫺40 ⫺29 ⫺73 ⫺72 ⫺89
Change in annual reservoir inﬂow
Total % 21.7 km
⫺18 ⫺22 5 ⫺10 12 ⫺29 ⫺24 ⫺30
Northern Sierra % 15.2 km
⫺19 ⫺22 3 ⫺99⫺29 ⫺20 ⫺24
Southern Sierra % 6.5 km
⫺16 ⫺23 10 ⫺14 17 ⫺30 ⫺33 ⫺43
Change in April–June reservoir inﬂow
Total % 9.1 km
⫺20 ⫺24 ⫺11 ⫺19 ⫺1 ⫺46 ⫺41 ⫺54
Northern Sierra % 5.5 km
⫺21 ⫺24 ⫺16 ⫺19 ⫺6 ⫺45 ⫺34 ⫺47
Southern Sierra % 3.6 km
⫺18 ⫺24 ⫺2 ⫺19 5 ⫺47 ⫺52 ⫺65
Change water year ﬂow centroid
Total Days 03兾26 0 2 ⫺15 ⫺7 ⫺7 ⫺14 ⫺23 ⫺32
Northern Sierra Days 03兾13 0 3 ⫺16 ⫺5 ⫺3 ⫺11 ⫺18 ⫺24
Southern Sierra Days 05兾01 ⫺10 ⫺7 ⫺19 ⫺12 ⫺22 ⫺34 ⫺34 ⫺43
avg, average; JJA, June, July, August; DJF, December, January, February; SWE, snow water equivalent.
*The number of days between the beginning of the year’s ﬁrst and end of the year’s last heatwave.
Reference period is 1990–1999, and projections are for the period 2090 –2099.
Results are for inﬂows to seven major dams and reservoirs in the Sacramento兾San Joaquin water system, including three in the Northern Sierra (Shasta, Oroville,
and Folsom) and four in the Southern Sierra (New Melones, New Don Pedro, Lake McClure, and Pine Flat).
Hayhoe et al. PNAS
August 24, 2004
Precipitation. Precipitation shows a tendency toward slight de-
creases in the second half of the century with no obvious
interscenario differences in magnitude or frequency (see Figs.
5–10, which are published as supporting information on the
PNAS web site). Three of four simulations project winter
decreases of ⫺15% to ⫺30%, with reductions concentrated in
the Central Valley and along the north Pacific Coast. Only PCM
B1 projects slight increases (⬇7%) by the end of the century
(Table 1). These results differ from previous projections showing
precipitation increases of 75–200% by 2100 (11–13), but they are
consistent with recent PCM-based midrange projections (14, 15).
The larger-scale pattern of rainfall over North America is more
uniform across scenarios, showing an area of decreased (or lesser
increase in) precipitation over California that contrasts with
increases further up the coast (see Fig. 11, which is published as
supporting information on the PNAS web site). Because inter-
decadal variability often dominates precipitation over Califor-
nia, projected changes in climate and impacts associated with the
direct effects of temperature should be considered more robust
than those determined by interactions between temperature and
precipitation or precipitation alone.
Extreme Heat and Heat-Related Mortality
Temperature extremes increase in both frequency and magni-
tude under all simulations, with the most dramatic increases
occurring under the A1fi scenario. Changes in local temperature
extremes were evaluated based on exceedance probability anal-
yses, by using the distribution of daily maximum temperatures
downscaled to representative locations (16). Exceedance prob-
abilities define a given temperature for which the probability
exists that X% of days throughout the year will fall below that
temperature (i.e., if the 35°C exceedance probability averages
95% for the period 2070–2099, this means that an average of
95% or ⬇347 days per year are likely to lie below 35°C). For the
four locations examined for extreme heat occurrence (Los
Angeles, Sacramento, Fresno, and Shasta Dam), mean and
maximum temperatures occurring 50% and 5% of the year
increase by 1.5–5°C under B1 and 3.5–9°C under A1fi by the end
of the century. Extreme temperatures experienced an average of
5% of the year during the historical period are also projected to
increase in frequency, accounting for 12–19% (B1) and 20–30%
(A1fi) of days annually by 2070–2099 (see Fig. 12, which is
published as supporting information on the PNAS web site).
The annual number of days classified as heatwave conditions
(3 or more consecutive days with temperature above 32°C)
increases under all simulations, with more heatwave days under
A1fi before midcentury (see Fig. 13, which is published as
supporting information on the PNAS web site). Among the four
locations analyzed, increases and interscenario differences are
proportionally greatest for Los Angeles, a location that currently
experiences relatively few heatwaves. By the end of the century,
the number of heatwave days in Los Angeles increases four times
under B1, and six to eight times under A1fi. Statewide, the length
of the heatwave season increases by 5–7 weeks under B1 and by
9–13 weeks under A1fi by the end of this century, with inter-
scenario differences emerging by midcentury (Table 1; see also
Fig. 14, which is published information on the PNAS web site).
The connection between extreme heat and summer excess
mortality is well established (17). Heat-related mortality esti-
mates for the Los Angeles metropolitan area were determined
Fig. 1. Downscaled winter (DJF) and summer (JJA) temperature change (°C) for 2070–2099, relative to 1961–1990 for a 1兾8° grid. Statewide, SRES B1 to A1ﬁ
winter temperature projections for the end of the century are 2.2–3°C and 2.3– 4°C for PCM and HadCM3, respectively, compared with previous projections of
1.2–2.5°C and 3–3.5°C for PCM and HadCM2, respectively. End-of-century B1 to A1ﬁ summer temperature projections are 2.2–4°C and 4.6– 8.3°C for PCM and
HadCM3, respectively, compared with previous projections of 1.3–3°C and 3–4°C for PCM and HadCM2, respectively (11–14).
www.pnas.org兾cgi兾doi兾10.1073兾pnas.0404500101 Hayhoe et al.
by threshold meteorological conditions beyond which mortality
tends to increase. An algorithm was developed to determine the
primary environmental factors (including maximum apparent
temperature, number of consecutive days above the threshold
apparent temperature, and time of year) that explain variability
in excess mortality for all days with apparent maximum temper-
atures at or above the derived daily threshold apparent temper-
ature (18) value of 34°C (see Heat-Related Mortality in Supporting
Text). Estimates do not account for changes in population or
From a baseline of ⬇165 excess deaths during the 1990s,
heat-related mortality in Los Angeles is projected to increase by
about two to three times under B1 and five to seven times under
A1fi by the 2090s if acclimatization is taken into account (see
Heat-Related Mortality in Supporting Text). Without acclimati-
zation, these estimates are about 20–25% higher (Table 1).
Actual impacts may be greater or lesser depending in part on
demographic changes and societal decisions affecting prepared-
ness, health care, and urban design. Individuals likely to be most
affected include elderly, children, the economically disadvan-
taged, and those who are already ill (19, 20).
Impacts on Snowpack, Runoff, and Water Supply
Rising temperatures, exacerbated in some simulations by de-
creasing winter precipitation, produce substantial reductions in
snowpack in the Sierra Nevada Mountains, with cascading
impacts on California winter recreation, streamflow, and water
storage and supply. Snowpack SWE was estimated by using daily,
bias-corrected and spatially downscaled temperature and pre-
cipitation to drive the Variable Infiltration Capacity distributed
land surface hydrology model. The Variable Infiltration Capac-
ity model, using the resolution and parameterization also im-
plemented in this study, has been shown to reproduce observed
streamflows when driven by observed meteorology (10) and has
been applied to simulate climate change (8) in this region. April
1 SWE decreases substantially in all simulations before midcen-
tury (see Fig. 15, which is published as supporting information
on the PNAS web site). Reductions are most pronounced at
elevations below 3,000 m, where 80% of snowpack storage
currently occurs (Table 1 and Fig. 2). Interscenario differences
emerge before midcentury for HadCM3 and by the end of the
century for both models. These changes will delay the onset of
and shorten the ski season in California (see Impact of Decreasing
Snowpack on California’s Ski Industry in Supporting Text).
Water stored in snowpack is a major natural reservoir for
California. Differences in SWE between the B1 and A1fi sce-
narios represent ⬇1.7 km
of water storage by midcentury and
by the end of the century for HadCM3. For PCM, overall
SWE losses are smaller, but the difference between the A1fi and
B1 scenarios is larger by the end of the century, representing ⬎4
of storage. Reductions for all simulations except PCM under
the lower B1 emission scenario are greater than previous pro-
jections of diminishing snowpack for the end of the century (8,
21). By 2020–2049 the SWE loss is comparable to that previously
projected for 2060 (22).
Warmer temperatures and more precipitation falling as rain
instead of snow also causes snowmelt runoff to shift earlier
under all simulations (Table 1), which is consistent with earlier
studies (23). The magnitude of the shift is greater in the
higher-elevation Southern basins and under the higher A1fi
scenario. Stream inflows to major reservoirs decline because
of diminished snowpack and increased evaporation before
midcentury, except where winter precipitation increases (Ta-
ble 1). The greater reductions in inflows seen under A1fi are
driven by both higher temperatures and lower average precip-
itation as compared with B1.
Fig. 2. Average snowpack SWE for 2020 –2049 and 2070–2099 expressed as a percent of the average for the reference period 1961–1990 for the Sierra Nevada
region draining into the Sacramento–San Joaquin river system. Total SWE losses by the end of the century range from 29–72% for the B1 scenario to 73– 89%
for the A1ﬁ scenario. Losses are greatest at elevations below 3,000 m, ranging from 37–79% for B1 to 81–94% for A1ﬁ by the end of the century. Increases in
high elevation SWE for midcentury HadCM3 B1 and end-of-century PCM B1 runs result from increased winter precipitation in these simulations.
Hayhoe et al. PNAS
August 24, 2004
Earlier runoff may also increase the risk of winter flooding (7).
Currently, state operators maintain ⬇12 km
of total vacant
space in the major reservoirs to provide winter and early spring
a volume approximately equal to that stored
in the natural snowpack reservoir by April 1st. Capturing earlier
runoff to compensate for future reductions in snowpack would
take up most of the flood protection space, forcing a choice
between winter flood prevention and maintaining water storage
for the summer and fall dry period use. Flood risk and fresh-
water supply are also affected by higher sea levels, which are
projected to rise 10–40 cm under B1 and 20–65 cm under A1fi
by 2100 (Table 1; see also Fig. 16, which is published as
supporting information on the PNAS web site).
Declining Sierra Nevada snowpack, earlier runoff, and re-
duced spring and summer streamflows will likely affect surface
water supplies and shift reliance to groundwater resources,
already overdrafted in many agricultural areas in California (24).
This could impact 85% of California’s population who are
agricultural and urban users in the Central Valley, San Francisco
Bay Area, and the South Coast, about half of whose water is
supplied by rivers of the Central Valley. Under A1fi (both
models) and B1 (HadCM3), the projected length, frequency, and
severity of extreme droughts in the Sacramento River system
during 2070–2099 substantially exceeds what has been experi-
enced in the 20th century. The proportion of years projected to
be dry or critical increases from 32% in the historical period to
50–64% by the end of the century under all but the wetter PCM
B1 scenario (see Table 2, which is published as supporting
information on the PNAS web site). Changes in water availability
and timing could disrupt the existing pattern of seniority in
month-dependent water rights by reducing the value of rights to
mid- and late-season natural streamflow and boosting the value
of rights to stored water. The overall magnitude of impacts on
water users depends on complex interactions between temper-
ature-driven snowpack decreases and runoff timing, precipita-
tion, future population increases, and human decisions regarding
water storage and allocation (see Impacts on Water Supply in
Impacts on Agriculture and Vegetation Distribution
In addition to reductions in water supply, climate change could
impact California agriculture by increasing demand for irrigation
to meet higher evaporative demand, increasing the incidence of
pests (25), and through direct temperature effects on production
quality and quantity. Dairy products (milk and cream, valued at
$3.8 billion annually) and grapes ($3.2 billion annually) are the
two highest-value agricultural commodities of California’s $30
billion agriculture sector (26). Threshold temperature impacts
on dairy production and wine grape quality were calculated by
using downscaled temperature projections for key counties,
relative to average observed monthly temperatures.
For dairy production, losses were estimated for temperatures
above a 32°C threshold (27), as well as for additional losses
between 25°C (28) and 32°C. For the top 10 dairy counties in the
state (which account for 90% of California’s milk production),
rising temperatures were found to reduce production by as much
as 7–10% (B1) and 11–22% (A1fi) by the end of the century (see
Table 3, which is published as supporting information on the
PNAS web site). Potential adaptations may become less practical
with increasing temperature and humidity (29).
For wine grapes, excessively high temperatures during ripen-
ing can adversely affect quality, a major determinant of market
value. Assuming ripening occurs at between 1,150 and 1,300
biologically active growing degree days (30), ripening month was
determined by summing modeled growing degree days above
10°C from April to October, for both baseline and projected
scenarios. Monthly average temperature at the time of ripening
was used to estimate potential temperature impacts on quality.
For all simulations, average ripening occurs 1–2 months earlier
and at higher temperatures, leading to degraded quality and
marginal兾impaired conditions for all but the cool coastal region
See the U.S. Army Corps of Engineers Flood Control Requirements for California Reser-
voirs, Sacramento District Water Control Data System, Sacramento, CA (www.spk-
See Western U.S. Climate Historical Summaries (Western Regional Climate Center) at
Fig. 3. Statewide change in cover of major vegetation types for 2020 –2049 and 2070 –2099, relative to simulated distributions for the 1961–1990 reference
period. ASF, alpine兾subalpine forest; ECF, evergreen conifer forest; MEF, mixed evergreen forest; MEW, mixed evergreen woodland; GRS, grassland; SHB,
shrubland; DES, desert. Increasing temperatures drive the reduction in alpine兾subalpine forest cover and cause mixed conifer forest to displace evergreen conifer
forest in the Sierra Nevada Mountains and the North Coast. Mixed conifer forest in the South Coast expands because of increased humidity and reduced ﬁre
frequency. Because of drier conditions and increased ﬁre frequency in inland locations, grassland displaces shrubland and woodland, particularly in the PCM
simulations, whereas warmer and drier conditions under HadCM3 cause an expansion of desert cover in the southern Central Valley.
www.pnas.org兾cgi兾doi兾10.1073兾pnas.0404500101 Hayhoe et al.
under all scenarios by the end of the century (see Table 3, which
is published as supporting information on the PNAS web site).
As with other perennial crops, adaptation options to shift
varieties or locations of production would require significant
time and capital investment.
The distribution of California’s diverse vegetation types also
changes substantially over the century relative to historical
simulations (Fig. 3; see also Fig. 17, which is published as
supporting information on the PNAS web site). Projections of
changes in vegetation distribution are those given by MC1, a
dynamic general vegetation model that simulates climate-driven
changes in life-form mixtures and vegetation types; ecosystem
fluxes of carbon, nitrogen, and water; and fire disturbance over
time (31). Vegetation shifts driven primarily by temperature,
such as reductions in the extent of alpine兾 subalpine forest and
the displacement of evergreen conifer forest by mixed evergreen
forest, are consistent across models and more pronounced under
A1fi by the end of the century. Changes driven by precipitation
and changes in fire frequency are model-dependent and do not
exhibit consistent interscenario differences. Most changes are
apparent before mid-century, with the exception of changes in
desert cover. The shift from evergreen conifer to mixed ever-
green forest and expansion of grassland are consistent with
previous impact analyses (13), whereas the extreme reduction in
alpine兾subalpine forest and expansion of desert had not been
reported in previous impacts assessments (12, 13).
Consistent and large increases in temperature and extreme heat
drive significant impacts on temperature-sensitive sectors in
California under both lower and higher emissions scenarios, with
the most severe impacts occurring under the higher A1fi sce-
nario. Adaptation options are limited for impacts not easily
controlled by human intervention, such as the overall decline in
snowpack and loss of alpine and subalpine forests. Although
interscenario differences in climate impacts and costs of adap-
tation emerge mainly in the second half of the century, they are
largely entrained by emissions from preceding decades (32).
SRES scenarios do not explicitly assume climate-specific policy
intervention, and thus this study does not directly address the
contrast in impacts due to climate change mitigation policies.
However, these findings support the conclusion that climate
change and many of its impacts scale with the quantity and timing
of greenhouse gas emissions (33). As such, they represent a solid
starting point for assessing the outcome of changes in green-
house gas emission trajectories driven by climate-specific policies
(32, 34), and the extent to which lower emissions can reduce the
likelihood and thus risks of ‘‘dangerous anthropogenic interfer-
ence with the climate system’’ (35).
We thank Michael Dettinger and Mary Meyer Tyree for providing
assistance with data and analysis, and Frank Davis for providing review
of earlier drafts of this manuscript. PCM model results were provided by
PCM personnel at the National Center for Atmospheric Research, and
HadCM3 model results were provided by Dr. David Viner from the U.K.
Met Office’s Climate Impacts LINK Project. This work was supported in
part by grants from the David and Lucile Packard Foundation, the
William and Flora Hewlett Foundation, the Energy Foundation, the
California Energy Commission, the National Oceanic and Atmospheric
Administration Office of Global Programs, and the Department of
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