United States
Environmental Protection
Agency
Office of Policy, Planning
and Evaluation
(2111)
EPA 230-F-97-008e
September 1997
©EPA Climate Change And California
The earth's climate is predicted to change because human
activities are altering the chemical composition of the atmosphere
through the buildup of greenhouse gases — primarily carbon
dioxide, methane, nitrous oxide, and chlorofluorocarbons. The
heat-trapping property of these greenhouse gases is undisputed.
Although there is uncertainty about exactly how and when the
earth's climate will respond to enhanced concentrations of
greenhouse gases, observations indicate that detectable changes
are under way. There most likely will be increases in temperature
and changes in precipitation, soil moisture, and sea level, which
could have adverse effects on many ecological systems, as well
as on human health and the economy.
The Climate System
Energy from the sun drives the earth's weather and climate.
Atmospheric greenhouse gases (water vapor, carbon dioxide.
and other gases) trap some of the energy from the sun, creating
a natural "greenhouse effect." Without this effect, temperatures
would be much lower than they are now, and life as known today
would not be possible. Instead, thanks to greenhouse gases, the
earth's average temperature is a more hospitable 60°F. However.
problems arise when the greenhouse effect is enhancedby
human-generated emissions of greenhouse gases.
Global warming would do more than add a few degrees to today's
average temperatures. Cold spells still would occur in winter, but
heat waves would be more common. Some places would be drier.
others wetter. Perhaps more important, more precipitation may
come in short, intense bursts (e.g., more than 2 inches of rain
in a day), which could lead to more flooding. Sea levels would
be higher than they would have been without global warming.
although the actual changes may vary from place to place because
coastal lands are themselves sinking or rising.
The Greenhouse Effect
Some solar radiation
is reflected by the
earth and the
k atmosphere
Some of the infrared radiation passes
through the atmosphere, and some is
absorbed and re-emitted in all
directions by greenhouse gas
molecules. The effect of this is to warm
the earth's surface and the lower
atmosphere.
Source: U.S. Department of State (1992)
Emissions Of Greenhouse Gases
Since the beginning of the industrial revolution, human activities
have been adding measurably to natural background levels of
greenhouse gases. The burning of fossil fuels — coal, oil, and
natural gas — for energy is the primary source of emissions.
Energy burned to run cars and trucks, heat homes and businesses.
and power factories is responsible for about 80% of global
carbon dioxide emissions, about 25% of U.S. methane emissions.
and about 20% of global nitrous oxide emissions. Increased
agriculture and deforestation, landfills, and industrial production
and mining also contribute a significant share of emissions. In
1994, the United States emitted about one-fifth of total global
greenhouse gases.
Concentrations Of Greenhouse Gases
Since the pre-industrial era, atmospheric concentrations of carbon
dioxide have increased nearly 30%, methane concentrations have
more than doubled, and nitrous oxide concentrations have risen
by about 15%. These increases have enhanced the heat-trapping
capability of the earth's atmosphere. Sulfate aerosols, a common
air pollutant, cool the atmosphere by reflecting incoming solar
radiation. However, sulfates are short-lived and vary regionally.
so they do not offset greenhouse gas warming.
Although many greenhouse gases already are present in the
atmosphere, oceans, and vegetation, their concentrations in the
future will depend in part on present and future emissions.
Estimating future emissions is difficult, because they will depend
on demographic, economic, technological, policy, and institu-
tional developments. Several emissions scenarios have been
developed based on differing projections of these underlying
factors. For example, by 2100, in the absence of emissions
control policies, carbon dioxide concentrations are projected to
be 30-150% higher than today's levels.
Current Climatic Changes
Global mean surface temperatures have increased 0.6-1.2°F since
the late 19th century. The 9 warmest years in this century all have
occurred in the last 14 years.
Several pieces of additional evidence consistent with warming.
such as a decrease in Northern Hemisphere snow cover, a
decrease in Arctic Sea ice, and continued melting of alpine
glaciers, have been corroborated. Globally, sea levels have risen
4-10 inches over the past century, and precipitation over land has
increased slightly. The frequency of extreme rainfall events also
has increased throughout much of the United States.
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Global Temperature Changes (1861-1996)
Year
Source: IPCC (1995), updated
A new international scientific assessment by the Intergovern-
mental Panel on Climate Change recently concluded that "the
balance of evidence suggests a discernible human influence
on global climate."
Future Climatic Changes
For a given concentration of greenhouse gases, the resulting
increase in the atmosphere's heat-trapping ability can be pre-
dicted with precision, but the resulting impact on climate is more
uncertain. The climate system is complex and dynamic, with
constant interaction between the atmosphere, land, ice, and
oceans. Further, humans have never experienced such a rapid rise
in greenhouse gases. In effect, a large and uncontrolled planet-
wide experiment is being conducted.
General circulation models are complex computer simulations
that describe the circulation of air and ocean currents and how
energy is transported within the climate system. While uncertain-
ties remain, these models are a powerful tool for studying
climate. Scientists are reasonably confident about the ability of
models to characterize future climate at continental scales.
Recent model calculations suggest that the global surface temper-
ature could increase an average of 1.6-6.3°F by 2100, with signif-
icant regional variation. These temperature changes would be far
greater than recent natural fluctuations, and they would occur
significantly faster than any known changes in the last 10,000
years. The United States is projected to warm more than the
global average, especially as fewer sulfate aerosols are produced.
The models suggest that the rate of evaporation will increase as
the climate warms, which will increase average global precipita-
tion. They also suggest increased frequency of intense rainfall as
well as a marked decrease in soil moisture over some mid-
continental regions during the summer. Sea level is projected to
increase by 6-38 inches by 2100.
Calculations of regional climate change are much less reliable
than global ones, and it is unclear whether regional climate will
become more variable. The frequency and intensity of some
extreme weather of critical importance to ecological systems
(droughts, floods, frosts, cloudiness, and associated fire and pest
outbreaks) could increase.
Local Climate Changes
Over the last century, the average temperature in Fresno.
California, has increased from 61.9°F (1899-1928 average) to
63.3°F (1966-1995 average), and precipitation has decreased by
up to 20% in many parts of the state.
Over the next century, California's climate may change even
more. Based on projections given by the Intergovernmental Panel
on Climate Change and results from the United Kingdom Hadley
Centre's climate model (HadCM2), a model that accounts for
both greenhouse gases and aerosols, by 2100 temperatures in
California could increase by about 5°F (with a range of 2-9°F) in
the winter and summer and slightly less in the spring and fall.
Appreciable increases in precipitation are projected: 20-30%
(with a range of 10-50%) in spring and fall, with somewhat larger
increases in winter. Little change is projected for summer.
The amount of precipitation on extreme wet days most likely
would increase, especially in the winter and fall, and there could
be a decrease in the number of long dry spells and an increase in
the number of long wet spells.
Climate Change Impacts
Global climate change poses risks to human health and to
terrestrial and aquatic ecosystems. Important economic resources
such as agriculture, forestry, fisheries, and water resources also
may be affected. Warmer temperatures, more severe droughts and
floods, and sea level rise could have a wide range of impacts. All
these stresses can add to existing stresses on resources caused by
other influences such as population growth, land-use changes,
and pollution.
Similar temperature changes have occurred in the past, but the
previous changes took place over centuries or millennia instead
of decades. The ability of some plants and animals to migrate and
adapt appears to be much slower than the predicted rate of
climate change.
Precipitation Trends From 1900 To Present
Trends/100 years
+20% •
+10% •
+5% •
-5%o
-10%0
-20% O
Source: Karl et al. (1996)
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Human Health
Higher temperatures and increased frequency of heat waves may
increase the number of heat-related deaths and the incidence of
heat-related illnesses. Cities such as Los Angeles that experience
occasional very hot, dry weather may be especially susceptible.
One study estimates that a 3°F warming could almost double
heat-related deaths in Los Angeles, from about 70 today to 125
(although increased air conditioning use may not have been fully
accounted for). Little change in winter mortality is expected in
Los Angeles. The elderly, particularly those living alone, are at
greatest risk.
There is concern that climate change could increase concentra-
tions of ground-level ozone. For example, high temperatures.
strong sunlight, and stable air masses tend to increase urban
ozone levels. Air pollution also is made worse by increases in
natural hydrocarbons emissions during hot weather. If a warmed
climate causes increased use of air conditioners, air pollutant
emissions from power plants also will increase.
In the Bay Area and the Central Valley, with no other changes in
weather or emissions, a 7.2°F warming would increase ozone
concentrations by 20% and almost double the size of the area not
meeting national health standards for air quality. Currently, the
national standards for ozone are not attained throughout much of
the state. Ground-level ozone has been shown to aggravate
existing respiratory illnesses such as asthma, reduce lung func-
tion, and induce respiratory inflammation. In addition, ambient
ozone reduces agricultural crop yields and impairs ecosystem
health.
Agriculture
The mix of crop and livestock production in a state is influenced
by climatic conditions and water availability. As climate warms.
production patterns will shift northward. Increases in climate
variability could make adaptation by farmers more difficult.
Warmer climates and less soil moisture due to increased evapora-
tion may increase the need for irrigation. However, these same
conditions could decrease water supplies, which also may be
needed by natural ecosystems, urban populations, and other
economic sectors.
Understandably, most studies have not fully accounted for
changes in climate variability, water availability, and imperfect
responses by farmers to changing climate. Including these factors
could substantially change modeling results. Analyses based on
changes in average climate and which assume farmers effectively
adapt suggest that aggregate U.S. food production will not be
harmed, although there may be significant regional changes.
In California, agriculture is about a $19 billion annual industry.
one-third of which comes from livestock. About 3% of total U.S.
farm acres is in California. The principal crops are cotton, wheat.
hay, tomatoes, and oranges, as well as many other vegetables and
fruits. About 87% of the acres farmed is irrigated. While climate
change could decrease cotton yields by 9-17% and wheat yields
by 48-66%, hay, orange, and tomato yields could increase. Total
Changes In Agricultural Yield And Production
Yield Production
100
33
_n
O
Cotton Hay Oranges Cotton Hay Oranges
Wheat Tomatoes Wheat Tomatoes
DT = 8°F; Dprecip. = 3%
DT = 7°F; Dprecip. = 16%
Source: Mendelsohn and Neumann (in press); McCarl (personal
communication)
acres farmed and irrigated acres could fall slightly, and farm
income could remain unchanged or increase by up to 40%
because of possible price increases.
Coastal Areas
Sea level rise could lead to flooding of low-lying property, loss
of coastal wetlands, erosion of beaches, saltwater contamination
of drinking water, and decreased longevity of low-lying roads.
causeways, and bridges. In addition, sea level rise could increase
the vulnerability of coastal areas to storms and associated
flooding.
Along much of California's coast, sea level already is rising by
3-8 inches per century (3 inches at Los Angeles, 5 inches at
San Francisco, and 8 inches at San Diego), and it is likely to rise
by another 13-19 inches by 2100. The beaches stretching from
Santa Barbara to San Diego have been replenished with sand.
and undoubtedly will be replenished further or protected with
structures if threatened by sea level rise. Cumulative costs for
sand replenishment to protect California's coastline from a
20-inch sea level rise through 2100 could be $174 million to
$3.5 billion.
San Francisco Bay contains the most extensive salt marshes on
the West Coast, most of which have been modified dramatically
by dredging and filling activities. A 1-3 foot increase in sea level
may move the existing salt marshes in the bay to nearby lowlands
and freshwater marshes, but development probably will limit the
extent to which these marshes can "migrate" to new areas.
Water Resources
Water resources are affected by changes in precipitation as well
as by temperature, humidity, wind, and sunshine. Changes in
streamflow tend to magnify changes in precipitation. Water
resources in drier climates tend to be more sensitive to climate
changes. Because evaporation is likely to increase with warmer
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climate, it could result in lower river flow and lower lake levels.
particularly in the summer. In addition, more intense precipitation
could increase flooding. If streamflow and lake levels drop.
groundwater also could be reduced.
The seasonal pattern of runoff into California's reservoirs could
be susceptible to climatic warming. Winter runoff most likely
would increase, while spring and summer runoff would decrease.
This shift could be problematic, because the existing reservoirs
are not large enough to store the increased winter flows for
release in the summer. Increased winter flows to San Francisco
Bay could increase the risk of flooding. The fragile environment
of the bay's delta islands could be at risk from increased flooding
and the upstream movement of saltwater from the bay.
Because the Colorado River, the major water source for southern
California, has extensive storage capacity, the reliability of this
water supply to California is not vulnerable to seasonal changes.
However, it is not known if flow in the Colorado River will
increase or decrease under climate change.
California's groundwater supplies are likely to be affected by
climate change as well. Unless precipitation increases, the
increased evaporation that would accompany warmer tempera-
tures probably would reduce groundwater supplies.
Forests
Trees and forests are adapted to specific climate conditions, and
as climate warms, forests will change. These changes could
include changes in species, geographic extent, and health and
productivity. If conditions also become drier, the current range
and density of forests could be reduced and replaced by grass-
lands and pasture. Even a warmer and wetter climate would lead
to changes; trees that are better adapted to these conditions, such
as oaks and redwoods, would thrive. Under these conditions.
forests could become more dense. These changes could occur
during the lifetimes of today's children, particularly if they are
accelerated by other stresses such as fire, pests, and diseases.
Some of these stresses would themselves be worsened by a
warmer and drier climate.
With changes in climate, the extent of forested areas in California
could change little or decline by as much as 25-50%. The
uncertainties depend on many factors, including whether soils
become drier and, if so, by how much drier. Hotter, drier weather
could increase the frequency and intensity of wildfires, threaten-
ing both property and forests. Along the Sierras, drier conditions
could reduce the range and productivity of conifer and oak
forests. Farther north and along the northern coast, drier condi-
tions could reduce growth of the Douglas fir and redwood forests.
A significant increase in the extent of grasslands and chaparral
throughout the state could result. These changes would affect
the character of California forests and the activities that depend
on them.
Changes In Forest Cover
Current +10°F, +13% Precipitation
Tundra
Conifer Forest
Savanna/Woodland
Shrub/Woodland
Grasslands
Arid Lands
Source: VEMAP Participants (1995); Neilson (1995)
Ecosystems
California is an ecologically diverse state, with 134 endangered
and threatened species, including the sea otter, the California
condor, and the American bald eagle. California's unique
ecosystems include 25,000 square miles of desert (such as the
Mojave desert and the Colorado desert). California's mountain
ecosystems in the Sierra Nevada, including Yosemite National
Park's 1,200 square miles, contain alpine wilderness areas with
large numbers of sequoia trees. The ranges of many species of
plants and animals are restricted and fragmented because of both
natural and human causes. Many invading species have colonized
large areas and displaced native species in the wake of environ-
mental changes in recent centuries.
Climate change could have an impact on many of California's
species and ecosystems. For example, between 1992 and 1996
the range of the bay checkerspot butterfly shifted 130 miles to the
north and to higher altitudes as a result of climate change.
Without natural corridors to allow migration, isolated species
could be limited in their ability to adapt to climate change. Plant
and animal species near the borders of their ranges are likely to
be most affected. Climate change could create more opportunity
for the establishment and spread of weeds and pests. Increased
fire from climate change could further threaten species in
California.
Cold water species such as mountain whitefish and brook trout
could lose all of their habitat because of climate change. Other
cold water species such as cold water guild, chinook salmon, and
kokanee salmon could lose most of their habitat.
For further information about the potential impacts of climate
change, contact the Climate and Policy Assessment Division
(2174), U.S. EPA, 401 M Street SW, Washington, DC 20460.
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