United States
Environmental Protection
Agency
Office of Policy, Planning
and Evaluation
(2111)
EPA 230-F-97-008f
September 1997
«>EPA Climate Change And Colorado
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
Solar
radiation
passes
through
the clear
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.
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, common
air pollutants, cool the atmosphere by reflecting incoming solar
radiation. However, sulfates are short-lived and vary regionally.
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
between 1890 and 1996. The 9 warmest years in this century all
have occurred in the last 14 years. Of these, 1995 was the
warmest year on record, suggesting the atmosphere has re-
bounded 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
Source: U.S. Department of State (1992)
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Global Temperature Changes (1861-1996)
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
r^ V /
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 uncertain-
ties 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 tempera-
ture 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 Fort Collins.
Colorado, has increased 4.1°F, and precipitation has decreased
by up to 20% in many parts of the state.
Over the next century, climate in Colorado 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 Colorado could increase by 3-4°F in spring
and fall (with a range of 1-8°F) and 5-6°F in summer and winter
(with a range of 2-12°F). Precipitation totals are estimated to
change little in summer, although there could be an increase in
the frequency of summer thunderstorms associated with moisture
flow from the Gulf of Mexico. In spring and fall, precipitation is
estimated to increase by 10%, and winter increases could have a
range of 20-70%. Other climate models may show different
results. 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
Precipitation Trends From 1900 To Present
Trends/100 years
+20%
+10% 0
+5% •
-5% O
-10% O
-20%
Source: Karl et al. (1996)
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trend. Although it is not clear how severe storms would change.
an increase in the frequency and intensity of winter storms is
possible.
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.
Human Health
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. Air
pollution also is made worse by increases in natural hydrocarbon
emissions during hot weather. If a warmed climate causes
increased use of air conditioners, air pollutant emissions from
power plants also will increase.
Although Colorado is in compliance with current air quality
standards, increased temperatures could make future compliance
more difficult. Ground-level ozone has been shown to aggravate
respiratory illnesses such as asthma, reduce existing lung func-
tion, and induce respiratory inflammation. In addition, ambient
ozone reduces crop yields and impairs ecosystem health.
Warming and other climate changes may expand the habitat and
infectivity of disease-carrying insects, thus increasing the
potential for transmission of diseases. A recent study has con-
cluded that a 5-9°F temperature increase would cause a signifi-
cant northern shift in Western equine encephalitis outbreaks.
Mosquitos capable of transmitting this disease already are present
in Colorado. If Colorado's climate becomes warmer and wetter.
mosquito populations could increase and conditions may become
more favorable for disease transmission.
In 1993, hantavirus pulmonary syndrome emerged in the Four
Corners area. Ecological and climatic changes may have contrib-
uted to a tenfold increase in 1992-1993 in the deer mouse
population, the primary reservoir for hantaviruses. Infection may
have occurred because more people were exposed to the virus
following the increase in the deer mouse population, although this
is still uncertain.
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
climate, it could result in lower river flow and lower lake levels.
particularly in the summer. If streamflow and lake levels drop.
groundwater also could be reduced. In addition, more intense
precipitation could increase flooding.
Colorado's rivers are used heavily both within Colorado and in
downstream states. Most of the western third of Colorado
eventually drains to the Colorado River, and the eastern two-
thirds drains more or less equally to the Arkansas and Platte
rivers. A small share drains into the Rio Grande. A warmer
climate would lead to earlier spring snowmelt, resulting in
higher streamflows in winter and spring and lower streamflows
in summer and fall. Most of Colorado's reservoirs are small in
relation to total runoff; therefore, earlier snowmelt could reduce
the reliability of many water supply systems within the state by
limiting the amount stored for use in summer. The reliability of
water systems for downstream states (e.g., Arizona and
California) in the Colorado basin, however, is protected by the
large storage capacities in Lakes Powell and Mead. These effects
could be mitigated if summer rainfall increases.
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
Changes In Agricultural Yield And Production
I
(0
30
20
10
0
o -10
5s -20
-30
-40
-50
-60
Irrigated Yield
Production
Corn Wheat Hay
• AT = 9°F; Aprecip. = 9%
Corn Wheat Hay
AT = 8°F; Aprecip. = 1%
Source: Mendelsohn and Neumann (in press); McCarl (personal
communication)
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evaporation 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 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 change modeling results substantially. Analyses that
assume changes in 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 Colorado, agriculture is a $4 billion annual industry, three-
fourths of which comes from livestock, mainly cattle. Almost
half the crop acreage is irrigated. The major crops in the state are
corn, wheat, and hay. Climate change could reduce grain yields
8-33%. Hay and pasture yields could fall by 6% or rise by 13%.
depending on whether irrigation is used, leading to changes in
acres farmed and production. For example, yields could fall while
production rises because of an increase in acres farmed.
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 range, 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 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 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.
With changes in climate, the extent of forested areas in Colorado
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. Along the Front Range, drier
conditions would reduce the range and health of ponderosa and
lodgepole forests, and increase their susceptibility to fire.
Grasslands and rangeland could expand into previously forested
areas in the western part of the state, and pinon-juniper forests in
Changes In Forest Cover
Current +10°F, +13% Precipitation
Tundra
Conifer Forest
Broadleaf Forest
Savanna/Woodland
Shrub/Woodland
Grassland
Arid
Source: VEMAP Participants (1995); Neilson (1995)
the southern part of the state could expand northward. 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 affect the character of Colorado forests and
the activities that depend on them. However, increased rainfall
could reduce the severity of these effects.
Ecosystems
Ecosystems in Colorado are diverse, ranging from grasslands and
deserts to mountain shrublands, forests, meadows, and alpine
tundra. They also include numerous wetlands and streams.
Because of elevation changes and human land use, many habitats
are fragmented and restricted in area. Changes in temperature and
precipitation caused by climate change could affect the location
and health of these ecosystems.
Warming could affect alpine areas, causing tree lines to rise by
roughly 350 feet for every degree Fahrenheit of warming.
Mountain ecosystems such as those found in Rocky Mountain
National Park could shift upslope, reducing habitat for many
subalpine species. Changes in rainfall and snowfall could alter
streamflows and affect wetlands and wildlife, and possibly
accelerate the invasion of non-native plants into streamside
habitats. Aquatic species that are sensitive to water temperature
could be affected adversely by climate change. Cold-water fish
such as trout could lose important habitat in Colorado, and
endangered species such as the Colorado squawfish could have
difficulty adapting.
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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