United States
Environmental Protection
What Climate Change
moot* for Tennessee
Tennessee's climate is changing. Although the average
temperature did not change much during the 20th century,
the state has warmed in the last 20 years. Average
annual rainfall is increasing, and a rising percentage of
that rain is falling on the four wettest days of the year. In
the coming decades, the changing climate is likely to
reduce crop yields, threaten some aquatic ecosystems,
and increase some risks to human health. Floods may be
more frequent, and droughts may be longer, which would
increase the difficulty of meeting the competing demands
for water in the Tennessee and Cumberland rivers.
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 (F) 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.
Natural cycles and sulfates in the air prevented much of
Tennessee from warming during the last century. Sulfates
are air pollutants that reflect sunlight back into space.
Now sulfate emissions are declining, and the factors that
once prevented Tennessee from warming are unlikely to
Temperature change (°F):
-1 -0.5 0 0.5 1 1.5 2 2.5 J 3.5
Rising temperatures in the last century. Tennessee has warmed
less than most of the United States. Source: EPA, Climate Change
Indicators in the United States.
Changing Water Availability
Annual precipitation in Tennessee has increased approximately
5 percent since the first half of the 20th century. But rising tem-
peratures increase evaporation, which dries the soil and decreases
the amount of rain that runs off into rivers. Although rainfall during
spring is iikely to increase during the next 40 to 50 years, the
total amount of water running off into rivers or recharging ground
water each year is likely to decline 2.5 to 5 percent, as increased
evaporation offsets the greater rainfall. Droughts are likely to be
more severe, because periods without rain will be longer and very
hot days will be more frequent.
increased Flooding
Flooding is becoming
more severe in the
Southeast. Since
1958, the amount of
precipitation falling
during heavy rain-
storms has increased
by 27 percent in
the Southeast, and
the trend toward
increasingly heavy
rainstorms is likely to
continue. To prevent
serious floods, the
Tennessee Valley
Authority (TVA) and
the U.S. Army Corps
of Engineers release
water from the
reservoirs behind
dams they operate
before the winter
flood season. Doing so lowers water levels and provides a greater
capacity for the reservoirs behind those dams to prevent flooding.
Nevertheless, the dams cannot prevent all floods. In May 2003,
for example, heavy rains exceeded TVA's dam capacity, flooding
low-lying areas in Chattanooga and other parts of Hamilton County;
in 2010, high flows in the Cumberland River flooded Nashville.
The Cumberland River flooded parts of Nashville in
2010, damaging many businesses, including the
Grand Ole Opry. Credit: USGS.

Droughts, Navigation, and Hydroelectric Power
Droughts also pose challenges for water management. If the
spring is unexpectedly dry, reservoirs may have too little water
during summer. During droughts, TVA and the Corps of Engineers
release water from dams to keep the Tennessee and Cumberland
rivers navigable. These rivers support $35 billion in annual
shipping. The agencies try to keep channels at least eleven
feet deep, because lower river levels can force barges to carry
smaller loads, which increases transportation costs. During the
drought of 2007, however, TVA could only release enough water
to keep some channels nine feet deep. This release meant that
lake levels were lowered tens of feet, which caused problems for
recreational swimming and boating. If droughts become more
severe, TVA and the Corps of Engineers will face this type of
problem more often.
Dry years diminish the amount of electricity that TVA can produce
from its 19 hydroelectric dams in Tennessee, which provide
12 to 15 percent of the electricity produced in the state.
During the 2007 drought, TVA's hydroelectric plants produced
30 percent less than normal, which forced TVA to meet demand
by using more expensive fuel-burning power plants.
Two views of a boat ramp in Douglas Lake during the 2.007 drought The lake
is nearly dry and the 330-foot ramp is completely out of the water
Credit: NOAA.
Aquatic Ecosystems
Changing the climate can harm aquatic ecosystems. Warmer
water lowers the level of dissolved oxygen in surface water,
which can severely limit fish populations. Because fish cannot
regulate their body temperatures, warmer water can make a
stream uninhabitable for fish that require cooler water. Warmer
temperatures can also increase the frequency of algal blooms,
which can be toxic and further reduce dissolved oxygen. Summer
droughts may amplify these effects, while periods of extreme
rainfall can increase the impacts of pollution on streams.
Changing the climate will have both beneficial and harmful
effects on agriculture. Longer frost-free growing seasons and
increased concentrations of atmospheric carbon dioxide tend to
increase yields for many crops during an average year. But more
severe droughts and more hot days are likely to reduce yields,
especially in the western half of Tennessee: 70 years from now,
that part of the state is likely to have 15 to 30 more days with
temperatures above 95°F than it has today. Even on irrigated
fields, higher temperatures are likely to reduce yields of corn,
and possibly soybeans. Warmer temperatures are also likely to
reduce the productivity of dairy and other cattle farms.
Forest Resources
Higher temperatures and changes in rainfall are unlikely
to substantially reduce forest cover in Tennessee, but the
composition of those forests may change. Forests cover about
half the state, dominated by oak and hickory trees, and the
forest products industry employs 180,000 people. Although more
droughts would reduce productivity, longer growing seasons
and increased carbon dioxide concentrations could more than
offset those losses. Nevertheless, climate change is likely to
increase the damage that certain insects and diseases cause in
Tennessee's forests.
Human Health
Hot days can be unhealthy—even dangerous. High air tem-
peratures can cause heat stroke and dehydration, and affect
people's cardiovascular and nervous systems. Certain people are
especially vulnerable, including children, the elderly, the sick,
and the poor. Warmer air can also 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 Tennessee Department of
Environment and Conservation have been working to reduce
ozone concentrations. As the climate changes, continued
progress toward clean air will become more difficult.
The Smoky Mountains have always had a natural blue haze. But air pollution
has increased that haze, and higher ozone levels could increase it further.
This photo shows how haze obscures the view from the Look Hock Tower in
Great Smoky Mountains National Park. Credit: National Park Service,
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.