United States
Environmental Protection
Agency
Office of Policy
(2111)
EPA 236-F-98-007C
September 1998
Climate Change And Arizona
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 enhanced by
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
Solar
radiation
passes
through
the clear
atmosphere
Some solar radiation
is reflected by the
earth and the
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 busi-
nesses, 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. Forexample, 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
between 1890 and 1996. The 9 warmestyears in this century all
have occurred in the last 14 years. Of these, 1995 was the warmest
year on record, suggesting the atmosphere has rebounded from
the temporary cooling caused by the eruption of Mt. Pinatubo in
the Philippines.
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
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Global Temperature Changes (1861-1996)
Year
Source: IPCC (1995), updated
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.
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 uncertainties
remain, these models are a powerful tool for studying climate. As
a result of continuous model improvements over the last few
decades, scientists are reasonably confident about the link
between global greenhouse gas concentrations and temperature
and 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, the frequency of hot or cold
spells, and the intensity of associated fire and pest outbreaks)
could increase.
Local Climate Changes
Over the last century, the average temperature in Tucson.
Arizona, has increased 3.6°F, and precipitation has increased by
up to 20% in many parts of the state, except in the northwest part
of the state where precipitation has fallen by 20%. These past
trends may or may not continue into the future.
Over the next century, climate in Arizona may change
even more. For example, based on projections made 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 Arizona could increase by 3 -4°F in spring
and fall (with a range of 1 -6°F), and by 5°F in winter and summer
(with a range of 2-9°F). Precipitation is estimated to decrease
slightly in summer (with a range of 0 to -15%), to increase by 20%
in spring (with a range of 10-40%), to increase by 30% in fall (with
a range of 10-50%), and to increase by 60% (with a range of 30-
100%) in winter. Other climate models may show different results.
especially regarding estimated changes in precipitation. The
impacts described in the sections that follow take into account
estimates from different models. The amount of precipitation on
extreme wet or snowy days in winter is likely to increase. The
frequency of extreme hot days in summer would increase because
of the general warming trend. It is not clear how the severity of
storms might be affected, although an increase in the frequency
and intensity of winter storms is possible.
Precipitation Trends From 1900 To Present
Trends/100 years
+20%
+10%
+5%
-5% O
-10% O
-20%
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. The elderly, particularly those living alone.
are at greatest risk. These effects have been studied only for
populations living in urban areas; however, even those in rural
areas may be susceptible.
Climate change could increase concentrations of ground-level
ozone. For example, high temperatures, strong sunlight, and
stable air masses tend to increase urban ozone levels. Currently.
the Phoenix metropolitan area is classified as a "moderate"
nonattainment area for ozone. Increased temperatures could
make attaining compliance more difficult. Ground-level ozone
is associated with respiratory illnesses such as asthma, reduced
lung function, and respiratory inflammation.
Infected individuals can bring malaria to places where it does not
occur naturally. Also, mosquitoes in Arizona can carry malaria. If
conditions become warmer and wetter, mosquito populations
could increase, thus increasing the risk of transmission if this and
other diseases are introduced into the area. In Arizona, water for
air conditioning is stored on roofs, so a 2°F warming even
without increased precipitation could increase mosquito popula-
tions. Increased runoff from heavy rainfall could increase water-
borne diseases such as giardia, cryptosporidia, and viral and
bacterial gastroenteritides. San Joaquim Valley fever is present in
the deserts of southern California, Arizona, and Nevada. There is
evidence that populations of this soil-based organism increase
when extreme rainfall follows periods of drought.
In 1993, hantavirus pulmonary syndrome emerged in Arizona, and
the deer mice that are the primary reservoir for hantaviruses are
prevalent in Arizona. Long droughts punctuated by heavy rains
can decrease the predators (owls, snakes, and coyotes) of
rodents, and the heavy rains can provide the rodents with added
food supplies (grasshoppers and pinon nuts).
Developed countries such as the United States should be able to
minimize the impacts of these diseases through existing disease
prevention and control methods.
Water Resources
Runoff in Arizona primarily results from summer thunderstorms
and winter precipitation. In the mountains, most of the winter
precipitation falls as snow. A warmer climate could mean less
winter snowfall, more winter rain, and a faster, earlier snowmelt.
This could lead to higher winter and spring flows and the passing
of flood waters that are usually stored for use later in the summer.
Additionally, without large increases in rainfall (approximately 15-
20%), higher temperatures and increased evaporation could lower
lake levels and streamflows in the summer. Less water would be
available to support important uses such as irrigation, hydro-
power production, public and industrial supply, fish and wildlife
habitat, and recreation. Lower streamflows would also concen-
trate pollutant levels and increase salinity, a critical water problem
in Arizona. Although Arizona has an active groundwater manage-
ment program, declining water levels are a serious problem in
parts of the state where withdrawals for irrigation and municipal
uses are large. Less spring and summer recharge could result in
lower groundwater levels and exacerbate this situation. The
surface waters in Arizona are fully allocated through water rights
agreements and legal compacts with other states and Mexico.
Tribal water rights are also being resolved. Because the state's
population is growing rapidly, there is concern that water demand
will exceed the available supply. Changes in water availability
would complicate these issues.
More rain could ease competition for water, but it also could
increase flooding. Earlier, more rapid snowmelts are likely to
contribute to winter and spring flooding. Flood flows on the
upper Colorado River can necessitate large releases from Lakes
Powell and Mead, resulting in flooding along the lower Colorado
in Arizona. The Salt-Verde systems primarily store water for
irrigation and have minimal flood control capacity. Severe
flooding can occur in Phoenix when large volumes of water are
released from the Salt River reservoirs. In a warmer climate, the
intensity of summer storms could increase. Cities such as
Tucson, as well as nearby smaller towns, are vulnerable to flash
flooding caused by intense, summer thunderstorms.
Agriculture
The mix of crop and livestock production in a state is influenced
by climatic conditions and water availability. As climate warms.
production patterns could 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, industry, and
other users.
Understandably, most studies have not fully accounted for
changes in climate variability, water availability, crop pests.
changes in air pollution such as ozone, and adaptation by farmers
to changing climate. Including these factors could change
modeling results substantially. Analyses that assume changes in
Changes In Agricultural Yield And Production
Irrigated Yield
Production
Cotton Hay
Wheat Oranges
Cotton Hay
Wheat Oranges
• AT = 8°F; Aprecip. = 11% • AT = 7°F; Aprecip. = 6%
Sources: Mendelsohn and Neumann (in press); McCarl (per-
sonal communication)
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average climate and effective adaptation by farmers suggest that
aggregate U.S. food production would not be harmed, although
there may be significant regional changes.
In Arizona, production agriculture is a $ 1.9 billion annual indus-
try, one-half of which comes from livestock, mainly
poultry. Almost all of the farmed acres are irrigated. The major
crops in the state are cotton, wheat, hay, and oranges. Climate
change could have a significant effect on crop production by
reducing cotton yields by 5-11% and wheat yields by about 70%
as temperatures rise beyond the tolerance levels of the crop. On
the other hand, hay and pasture yields, however, are estimated to
rise by 3 -12%. Farmed acres could fall by 4-20%, and the share of
irrigated acres could rise. Livestock and dairy production may
not be affected, unless summer temperatures rise significantly
and conditions become significantly drier. Under these condi-
tions, livestock tend to gain less weight and pasture yields
decline, limiting forage.
Forests
Trees and forests are adapted to specific climate conditions.
and as climate warms, forests will change. These changes could
include changes in species composition, geographic range, and
health and productivity. If conditions also become drier, the
current range and density of forests could be reduced and
replaced by grasslands and pasture. Even a warmer and wetter
climate could lead to changes; trees that are better adapted to
these conditions, such as fir and spruce, would thrive. Under
these conditions, forests could become more dense. These
changes could occur during the lifetimes of today's children.
particularly if the change is accelerated by other stresses such
as fire, pests, and diseases. Some of these stresses would
themselves be worsened by a warmer and drier climate.
desert could expand into some of these previously forested areas.
Milder winters could increase the likelihood of insect outbreaks
and of subsequent wildfires in the dead fuel left after such an
outbreak. These changes would significantly affect the character
of Arizona forests and the activities that depend on them.
However, increased rainfall could reduce the severity of these
effects.
Ecosystems
Arizona supports a rich diversity of habitats, from deserts to
mountain highlands. Narrow strips of riparian vegetation are
extremely important to wildlife, especially birds such as the
endangered cactus ferruginous pygmy owl, southwestern willow
flycatcher, masked bobwhite quail, Mexican spotted owl, and the
bald eagle. Riparian habitat along the San Pedro River, for
example, hosts hundreds of bird species, more than any other
U.S. river. Riverine habitats running through the desert
environment are critically important habitat at the southern end
of winter migration routes for geese, ducks, sandhill cranes, and
shorebirds such as dowitchers and sandpipers. Within the rivers
and streams, 24 of Arizona's 28 native fish are threatened or
endangered, including the Colorado squawfish, razorback sucker.
desert pupfish, Gila topminnow, Yaqui chub, spikedace, and
Apache trout. The Sonoran Desert of southern Arizona has the
highest plant diversity of the southwestern desert, and is home to
fan palms, saguaro cacti, palo verde, and mesquite. Endangered
mammals found here include jaguar, ocelot, Mexican gray wolf.
Sonoran pronghorn, and Sandborn's lesser long-nose bat. The
biological communities of the isolated mountaintops within the
desert have diverged remarkably over time. More than 2,000 plant
species thrive in this area at the intersection of Arizona, New
Mexico, and Mexico, nearly 10% of all the species found in the
United States.
With changes in climate, the extent of forested areas in
Arizona could change little or decline by as much as 15-30%. The
uncertainties depend on many factors, including whether soils
become drier and, if so, how much drier. Hotter, drier weather
could increase the frequency and intensity of wildfires, threaten-
ing both property and forests. Grassland, rangeland, and even
Changes In Forest Cover
Current +10°F, +13% Precipitation
I Conifer Forest
Broadleaf Forest
Savanna/Woodland
• Shrub/Woodland
Grassland
Arid Lands
Sources: VEMAP Participants (1995); Neilson (1995)
The decline of riparian areas threatens many species offish
and wildlife in Arizona. Here, more species are threatened
with extinction than nearly any other state. For example, the
Gila River is the only U.S. river basin with all 47 of its freshwater
fish species either extinct, listed as threatened or endangered.
or recommended as candidates of such listings. Climate change
could exacerbate these existing threats. Narrow riparian habitats
could be greatly influenced by decreasing water availability. This
could ultimately alter the number of bird species and community
composition, with the loss of many species that rely on riparian
vegetation for nesting and food resources, such as the endan-
gered southwestern willow flycatcher and Mexican spotted owl.
Higher water temperatures could adversely affect habitat condi-
tions for already endangered fish, and favor the spread of exotic
species that already imperil the survival of Arizona's native fish
species. In desert areas, many plants and animals already live
near their tolerance limits and may be unable to survive under
hotter conditions. Mountaintop island habitats are especially
vulnerable to climate change.
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, or
visit http://www. epa.gov/globalwarming/impacts.
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