United States
Environmental Protection
August 2016
EPA 430-F-16-005
What Climate Change
Means ^rjzona
Lake Mead viewed from Hoover Dam in 2009. The white area reveals the drop in water levels
over the last decade, during which drought has reduced flows of the Colorado River into the
lake. Low lake levels threaten water supplies and hydroelectric power. © Chris Lamie; used
by permission.
Arizona's climate is changing. The state has warmed
about two degrees (F) in the last century. Throughout
the southwestern United States, heat waves are
becoming more common, and snow is melting earlier in
spring. In the coming decades, changing the climate is
likely to decrease the flow of water in the Colorado
River, threaten the health of livestock, increase the
frequency and intensity of wildfires, and convert some
rangelands to desert.
Our climate is changing because the earth is warming.
People have increased the amount of carbon dioxide
in the air by 40 percent since the late 1700s. Other
heat-trapping greenhouse gases are also increasing.
These gases have warmed the surface and lower
atmosphere of our planet about one degree during the
last 50 years. Evaporation increases as the atmosphere
warms, which increases humidity, average rainfall, and
the frequency of heavy rainstorms in many places—but
contributes to drought in others.
Greenhouse gases are also changing the world's oceans
and ice cover. Carbon dioxide reacts with water to form
carbonic acid, so the oceans are becoming more acidic.
The surface of the ocean has warmed about one degree
during the last 80 years. Warming is causing snow to
melt earlier in spring.
As the climate warms, less precipitation falls as snow, and more snow melts during
the winter. That decreases snowpack—the amount of snow that accumulates over
the winter. Since the 1950s, the snowpack has been decreasing in Arizona, as well
as most mountainous areas in the Colorado River Basin. Diminishing snowpack can
decrease water supplies and shorten the season for skiing and other forms of winter
tourism and recreation.
Water Availability
The changing climate is likely to increase the need for water but reduce the supply.
Rising temperatures increase the rate at which water evaporates (or transpires)
into the air from soils, plants, and surface waters. Irrigated farmland would thus
need more water. But less water is likely to be available, because precipitation is
unlikely to increase enough to make up for the additional water lost to evaporation.
Annual precipitation has decreased in Arizona during the last century, and it may
continue to decrease. So soils are likely to be drier, and periods without rain are
likely to become longer, making droughts more severe.
Temperature change (°F):
-1 -0.5 0 0,5 1 1.5 2 2.5 3 3,5
Rising temperatures in the last century. The last decade
was the warmest on record in the Southwest. Source: EPA,
Climate Change Indicators in the United States.
The decline in snowpack could further limit the supply of water for some purposes.
Mountain snowpacks are natural reservoirs. They collect the snow that falls during
winter and release water when the snow melts during spring and summer. Over the
past 50 years, the snowpack throughout the Colorado River Basin has been melting
earlier in the year (see map on back page). Dams capture most meltwater and retain
it for use later in the year. But upstream of these reservoirs, less water is available
during droughts for ecosystems, fish, water-based recreation, and landowners who
draw water directly from a flowing river.

lota do
Sources: Esri, DeLorme, USGS, NPS,
Sources: Esri, USGS, NOAA
In 2011, Arizona experienced the largest wildfire in the state's recorded history,
the Wallow Fire (pictured here), it burned more than half a million acres. Two years
later, the Yarnell Hill Fire became the state's deadliest wildfire when it took the
lives of 19 firefighters. Credit: Eastern Arizona Incident Management Team.
Percent Change
-80 to -60
-60 to -40
-40 to -20
-5 to -20
-5 to 5
5 to 20
20 to 40
40 to 60
60 to 80
Warmer and drier conditions make forests more susceptible to pests.
Drought reduces the ability of trees to mount a defense against attacks
from pests such as bark beetles, which have infested 100,000 acres in
Arizona. Temperature controls the life cycle and winter mortality rates of
many pests. With higher winter temperatures, some pests can persist
year-round, and new pests and diseases may become established.
I rends in April snowpack in the Colorado River Basin, 1955-2013. Snowpack has
decreased at most sites in the basin and all sites in Arizona. Source: EPA.
increasing droughts and higher temperatures are likely to affect Arizona's
top agricultural products: cattle, dairy, and vegetables. Hot temperatures
threaten cows' health and cause them to eat less, grow more slowly, and
produce less milk. Livestock operations could also be impaired by fire,
the lack of water, and changes in the landscape from grassland to woody
shrubs more typical of a desert. Reduced availability of water would also
create challenges for irrigated farms, which account for two-thirds of the
water used in the state.
Wildfires and Changing Landscapes
Higher temperatures and drought are likely to increase the severity,
frequency, and extent of wildfires, which could harm property, livelihoods,
and human health. On average, more than 2 percent of the land in Arizona
has burned per decade since 1984. Wildfire smoke can reduce air quality
and increase medical visits for chest pains, respiratory problems, and
heart problems.
The combination of more fires and drier conditions may expand deserts
and otherwise change parts of Arizona's landscape. Many plants and
animals living in arid lands are already near the limits of what they can
tolerate. A warmer and drier climate would generally extend the Sonoran
and Chihuahuan deserts to higher elevations and expand their geographic
ranges. In some cases, native vegetation may persist and delay or prevent
expansion of the desert. In other cases, fires or livestock grazing may
accelerate the conversion of grassland to desert in response to a
changing climate. For similar reasons, some forests may change to
desert or grassland.
Human Health
Hot days can be unhealthy—even dangerous. Certain people are
especially vulnerable, including children, the elderly, the sick, and the
poor. High air temperatures can cause heat stroke and dehydration, and
affect people's cardiovascular, respiratory, and nervous systems. Higher
temperatures are amplified in urban settings where paved and other
surfaces tend to store heat. Construction crews may have to increasingly
operate on altered time schedules to avoid the heat of the day.
Rising temperatures can increase the formation of ground-level ozone, a
key component of smog. Ozone has a variety of health effects, aggravates
lung diseases such as asthma, and increases the risk of premature
death from heart or lung disease. EPA and the Arizona Department of
Environmental Quality have been working to reduce ozone concentrations.
As the climate changes, continued progress toward clean air will be more
Tribal Communities
Changing the climate threatens natural resources and public health of
tribal communities. Rising temperatures and increasing drought are likely
to decrease the availability of certain fish, game, and wild plants on which
the Navajo and other tribes have relied for generations. Water may be
less available for domestic consumption, especially for those who are not
served by either municipal systems or reliable wells, which includes about
30 percent of the people on the Navajo Nation, who must haul water to
meet daily needs. Recurring drought and rising temperatures may degrade
the land itself. In the Navajo Nation, for example, the Great Falls Dune
Field has advanced almost a mile in the last 60 years, threatening roads,
homes, and grazing areas. Extreme heat may also create health problems
for those without electricity, including about 40 percent of the people on
the Navajo reservation.
The sources of information about climate and the impacts of climate change in this publication are: the national climate assessments by the U.S. Global Change Research
Program, synthesis and assessment products by the U.S. Climate Change Science Program, assessment reports by the Intergovernmental Panel on Climate Change, and EPA's
Climate Change Indicators in the United States. Mention of a particular season, location, species, or any other aspect of an impact does not imply anything about the likelihood or
importance of aspects that are not mentioned. For more information about climate change science, impacts, responses, and what you can do, visit EPA's Climate Change website
at www.epa.aov/climatechanae.