Q COA united states	November 2016
Environmental Protection
Agency	EPA 430-F-16-064
What Climate Change
Means for the District of Columbia
The District of Columbia's climate is changing. The
region has warmed by more than two degrees (F) in
the last century, hot days and heavy rainstorms are
more frequent, and the tidal Potomac is rising about
one inch every eight years. In the coming decades,
changing climate is likely to increase tidal flooding,
cause more heavy rainstorms and sewer overflows,
and increase some risks to human health.
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,
and mountain glaciers are retreating. Even the great
ice sheets on Greenland and Antarctica are shrinking.
Thus the sea is rising at an increasing rate.
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 District of
Columbia has warmed more than most of the nation. Source:
EPA, Climate Change Indicators in the United States.
Increasing Temperature and Changing Precipitation Patterns
Summer, fail, winter, and spring have all become warmer since the 1940s in the District of
Columbia. Five of the six hottest summers on record have occurred since 2010. This trend
is very likely to continue.
Rising temperatures and shifting rainfall patterns are likely to increase the intensity of both
floods and droughts. Average annual precipitation in the DC area has increased by
5 to 10 percent in the last century, but precipitation from extremely heavy storms has
increased by more than 25 percent across the eastern United States since 1958. Over
the next century, average annual precipitation and the frequency of heavy downpours are
likely to keep rising. During winter and spring, average precipitation is likely to increase.
During summer and fall, precipitation is unlikely to change significantly, but rising
temperatures will increase evaporation and thereby dry the soil. As a result, changing
climate is likely to intensify flooding during winter and spring, and intensify drought during
summer and fall.
Severe Storms, Flooding, and Wind Damage
As severe rainstorms become more frequent, flood damage to homes and other buildings
is likely to increase. Most homes along the north side of Watts Branch—as well as Mayfair
on the south side—are in the 100-year floodplain, and several homes and businesses
along Oxon Run are also vulnerable to flash flooding. Almost all of the land in and adjacent
to Federal Triangle is low-lying and vulnerable to flooding from severe rainstorms or high
water levels in the Potomac River. During June 2006, a severe rainstorm flooded several
federal buildings and museums around Federal Triangle and caused $10 million in damage
across the region.
More heavy storms could also harm wastewater and other types of infrastructure. One-
third of the District is served by a combined sewer system designed before 1900, which
carries both sewage and stormwater in the same system of pipes. Heavy rainfall can
overwhelm the system, forcing it to discharge raw sewage into the Anacostia River, Rock
Creek, or the Potomac River. DC Water's Clean Rivers project is building wastewater treat-
ment and stormwater storage facilities to reduce these sewer overflows. But capturing or
treating all of this water will become more difficult if heavier storms release more water
onto the city's streets, rooftops, and other impermeable surfaces. Storms can also harm
transportation systems: for example, flash flooding temporarily dosed the Cleveland Park
Metro station in June 2016.
The changing climate may also increase damage caused by winds. Hurricanes and other
tropical storms have become more intense during the past 20 years. Although warming
oceans provide these storms with more potential energy, scientists are not sure whether
the recent intensification reflects a long-term trend. Nevertheless, wind speeds and rain-
fall rates during hurricanes are likely to increase as the climate continues to warm. These
storms can damage homes and disrupt power supplies and transportation networks. In
2003, Hurricane Isabel knocked down many trees throughout the District, and half of
PEPCO's customers lost electric power—some for as long as a week. Downed power lines
made many roads impassable in adjacent counties in Maryland. In 2011, Hurricane Irene
also left hundreds of thousands of people without electric power in the Washington area.

Rising Sea Level and Tidal Inundation
Sea level is rising more rapidly along the shores of the Potomac and
Anacostia rivers than along most shores because the land here is sinking.
At the official tide gauge along the Southwest Waterfront, sea level has
risen six or seven inches during the last 50 years. If the oceans and
atmosphere continue to warm, sea level in the District is likely to rise
sixteen inches to four feet in the next century.
As sea level rises, the lowest dry lands are submerged and become
either tidal wetland or open water, while lands that are rarely flooded
by the tides become flooded more frequently. The District has between
one and two square miles of land within about three feet of the average
high tide. These areas include most of Kenilworth Aquatic Gardens, the
northern portion of Joint Base Anacostia-Bolling, and about half of East
Potomac Park.
As sea level rises, occasional extreme high tides are able to reach farther
inland. Most of the sidewalks along the Tidal Basin and part of the road to
Hains Point in East Potomac Park, for example, are about one foot above
the average daily high tide. This water level was only reached about six
times per year during the 1950s, but now it is exceeded more than
30 times per year. By the time sea level rises one foot, these areas will be
flooded during half the days of the year.
Although few homes in the District are close enough to sea level to be
permanently submerged, sea level rise can exacerbate damage caused
by storm surges and river flooding. A higher average water level in the
Potomac and Anacostia provides a higher base for storm surges and
river surges, which means that water pushed inland by storm winds
could penetrate farther into adjacent dry land, as could flood waters from
upstream. A higher water level also makes storm drains less effective.
Fishing at Hains Point in East Potomac Park (left) and flooding in the same area in
September 2015 (right), Hains Point is one of several areas along the Potomac that
could be regularly inundated at high tide as sea level rises. Credit: (left) James G.
Titus, EPA; (right) NOAA,
The tidal wetlands in the Washington area build their own land by
capturing floating sediments, and they are generally likely to keep pace
with the rising sea during the next century. Nonetheless, rising sea level
could alter wetland habitat and harm fish and birds that depend on it
for food or shelter. Areas at risk include freshwater marshes along tidal
portions of the Anacostia River,
where surveys have found
nearly 200 bird species;
marshes on Roosevelt
Island; and other marshes
downstream along the
Potomac. The rise in sea
level may also submerge
parts of the swamp forest on
Roosevelt Island.
Flooding at the Tidal Basin during cherry
blossom season. /Is the climate changes,
tidal flooding is occurring more often, while
Washington's famous cherry trees are
blooming earlier in the spring.
Credit: National Park Service.
Rising temperatures are
lengthening the growing
season in the Mid-Atlantic
region, and they could change
the composition of woodlands
and the timing of ecological
processes. In many parts of our nation, wildflowers and woody perennials
are blooming—and migratory birds are arriving—sooner in spring.
Not all species adjust in the same way, however, so the food that one
species needs may no longer be available when that species arrives on
its migration. Washington's cherry trees are blooming earlier: since 1921,
peak bloom dates have shifted earlier by approximately five days. The
timing of the peak bloom is important to tourism and the local economy
because the cherry blossoms draw more than one million people each
year, many of whom are from out of town.
Human Health
Hot days can be unhealthy—even dangerous. Rising temperatures will
increase the frequency of hot days and warm nights. High air tempera-
tures can cause heat stroke and dehydration and affect people's cardio-
vascular and nervous systems. Warm nights are especially dangerous
because they prevent the human body from cooling off after a hot day.
Certain people are especially vulnerable, including children, the elderly,
the sick, and the poor. Because the District of Columbia is warmer than
surrounding areas, Washingtonians face a greater risk of heat-related
illnesses—especially residents without air conditioning.
Warmer temperatures can increase the formation of ground-level ozone,
a component of smog that can contribute to respiratory problems. Rising
temperatures may also increase the length and severity of the pollen
season for plants such as ragweed, which has already been observed in
other regions.
The risk of some diseases may also increase. West Nile virus, transmitted
by mosquitoes, could become more common due to rising temperatures,
which speed up the mosquito life cycle and increase biting rates, as
well as dry periods, which benefit the type of mosquito that transmits
West Nile. But the effects are still uncertain and likely to vary by region.
Increased flooding from more intense storms could lead to more
indoor dampness and mold, which contribute to asthma, allergies, and
respiratory infections.
The sources of information about ciimate 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