Global Warming: Our Nation's Capital at Risk

Introduction

Current climate models used by the Intergovernmental Panel on Climate Change, an international group of more than 2,000 scientists, project that the Earth will warm by two to six degrees Fahrenheit by the year 2100. Sea level is projected to rise by six inches to as much as three feet during that same period. For Washington, D.C., increased sea levels may increase risks of flooding of the Potomac River, threatening such historic and culturally significant landmarks as the Mall, Georgetown, and the cherry trees surrounding the Tidal Basin near the Jefferson Memorial. Warming of the magnitude predicted would almost double the number of days over 90°F, potentially increasing heat stress and mortality. Mosquito-borne diseases, such as malaria, dengue, and eastern equine encephalitis may also reemerge or increase in incidence. Recreational beaches, such as in Ocean City, Maryland, may be lost as sea levels rise unless vigorous and expensive beach renourishment projects are continued and expanded. The shellfish, birds, and fish of the abundant Chesapeake Bay ecosystem may be decimated by a combination of sea-level rise and warmer temperatures. Some historic Chesapeake Bay island communities may be completely submerged by the end of the next century.

Flooding

The District of Columbia is extremely vulnerable to flooding. Our nation's capital is built upon former swampland bordering the Potomac River. This region of the Potomac, up to the base of Little Falls, is tidal: sea level changes in the Atlantic Ocean flow all the way upriver to Washington, so that the Potomac rises and falls with the ocean tides. Many of the District's most familiar monuments and landmarks, including the Jefferson Memorial, the Mall, the Reflecting Pool, and National Airport, are very low-lying, and are therefore susceptible to flooding. Although flooding during storms would not cause permanent inundation, large areas could be covered temporarily. Last winter, Hurricane Fran caused extensive flooding along the Potomac, including National Airport and parts of Georgetown and the Mall. In 1985, severe flooding due to heavy rains led to the deaths of at least 47 people and the region suffered more than $900 million in damage. Flooding due to heavy rains and storm surges will be exacerbated if global warming further increases sea level. Property damage and loss of life may become more common and more severe. Mitigation efforts, including increased bulkheads and retreat from vulnerable zones, may be expensive.

Disease

The increased temperatures predicted for Washington by climate models may lead to greater risk of mosquito-borne diseases, those we usually associate with much more tropical areas. Studies show that the ranges of malaria, dengue or "breakbone" fever, and eastern equine encephalitis are all expected to expand with increased warming. Higher temperatures would increase the number of mosquitoes, make each mosquito bite more infectious, and cause mosquitoes to bite more frequently. No vaccines currently exist for dengue or malaria.

While most strains of malaria can be controlled, drug-resistant strains are proliferating. Dengue, a serious illness when contracted initially, can lead to hemorrhagic (bleeding) fever in those who contract it a second time. Of this group, 1 to 5 percent would be expected to die even with adequate medical care. The incidence of eastern equine encephalitis, which attacks both horses and humans, has been increasing in parts of the United States in recent years, although transmission is still considered rare. Prince Georges County, adjacent to Washington, reported 5 cases in 1996, however, the origins of contraction are not known. Early symptoms include fever, headache, drowsiness, and muscle pain, followed by disorientation, weakness, seizures, and coma. Sixty percent of symptomatic cases are fatal, and most survivors will suffer permanent neurological damage. Mild winters and wet springs, predicted to increase with global warming, are associated with increased risk of eastern equine encephalitis.

Global warming would require mosquito-control programs in the Washington area to be initiated or expanded to address the additional risks. But even spraying to control mosquitoes may not be fully effective, as more and more mosquito species are becoming resistant to pesticides.

Heat Stress

Global warming could dramatically increase the number of days above 90°F and 95°F. Currently, Washington experiences an annual average of 36 days above 90°F. By the end of the next century, best estimates project about 60 days above 90°F though one scenario produces as many as 87 days that hot. The city would also experience more heat waves, some of which would be of much longer duration. Heat- related illness and mortality may greatly increase as a result, affecting first the most vulnerable parts of the population: the elderly, the young, and those already weakened by illness.

Warmer temperatures could also exacerbate already poor urban air quality. In the summer, high temperatures in combination with sunlight produce ground level ozone and smog, which can cause serious respiratory distress and eye irritation.

Cherry Blossoms

The blooming of the cherry trees in Washington is a national event, symbolizing the capital's natural beauty, the coming of spring, and the links between the people of our nation's capital and peoples around the world. The cherry trees, given to the people of America by Japan in 1912, are probably not in danger from projected warming. However, due to their proximity to the Potomac River, they are threatened by increased flooding from sea-level rise. Cherry tree roots are extremely sensitive to extended periods of high water. Last year's floods damaged many of the trees. New trees are being planted on mounds in an attempt to reduce their exposure to excessive moisture.

Interestingly, the date of peak blooming for cherry trees is being pushed earlier, in part due to warmer temperatures. Between 1921 (when peak bloom dates were first recorded) and 1970, the average date of blooming for Yoshino cherry (Prunus x yedoensis) was April 5. But between 1970 and 1997, the average date of peak bloom was April 3. This period included the earliest peak blooming date ever recorded, March 15, 1990. The peak blooming date (defined as the date when 70% of the single, white blossoms of the Yoshino cherry surrounding the Tidal Basin are open) is dependent to a large extent on spring temperatures. Warmer springtime and nighttime temperatures tend to lead to an earlier bloom date. Both warmer average springtime temperatures and warmer nighttime temperatures are predicted to occur with global warming. The earlier blooming of the cherry trees is indicative of the trend that could occur with global warming.

Beach Erosion - Ocean City, Maryland

Sea-level rise threatens beaches and resort areas on the Atlantic coast in the Washington- Baltimore region. Ocean City, Maryland, located about 150 miles from Washington and 135 miles from Baltimore, is an excellent example. Built on a barrier beach island, Ocean City is one of the primary resorts on the East Coast, boasting a temperate climate, lovely bathing beaches, and a variety of hotels, restaurants and amusements that attract more than eight million visitors each year. Visitor spending in Ocean City contributed an estimated $1.5 billion to the local and state economy in 1996. Nearby Assateague Island National Seashore (MD, VA), Assateague State Park (MD), and Chincoteague National Wildlife Refuge (VA) provide excellent opportunities to enjoy and explore nature. The broad beaches, secluded coves, and marshes of these barrier islands teem with nature and are home to crabs, plovers, and wild horses.

Sea-level rise caused by global warming would threaten the barrier islands, their natural resources, and their economies. An Environmental Protection Agency study in 1985 estimated that a one-foot rise in sea level at Ocean City could double the rate of beach erosion, causing the loss of more than 200 feet of shore. To maintain current shorelines would require expensive, ongoing projects, dredging sand from offshore, transporting the sand to the islands, and spreading the sand to fill in the beaches. The 1985 EPA study estimated that, for a one-foot rise in sea level, maintaining the beach at Ocean City alone would cost a total of $60-$85 million. High-rise condominiums and hotels built only a few hundred feet from the water's edge will prevent migration of the barrier island inland, so without continuous sand replenishment, inundation and erosion could lead to beach loss.

During storms, increased sea levels would lead to greater flooding due to storm surges and thus greater levels of storm damage. Drinking water supplies could be threatened as higher sea levels increase the salinity of surface and ground water. Higher sea levels also push up water tables and could lead to the inundation of low-lying areas, including beachfront development. In the nearby national parks and seashores, where beach nourishment and other human interventions would probably be too expensive, marshes would have to migrate landward or, if sea-level rise were too rapid, they would be inundated.

Chesapeake Bay

Chesapeake Bay is an immense body of water bordered by Baltimore, Maryland and Norfolk, Virginia, and fed by the Potomac, James, and Susquehanna Rivers. The Bay's watershed is home to thirteen million people. Chesapeake Bay is the nation's largest estuary, formed by the confluence of fresh water from rivers and streams and salt water from the ocean that creates a nutrient-rich zone which supports a huge diversity of fish, shellfish, mammals, shorebirds, and migratory birds. Sea-level rise threatens this rich ecosystem as well as numerous Bay Island settlements, some dating from the 19th century.

Sea level has been rising at a rate of about 3 feet per 1,000 years for the last 5,000 years. In the last 100 years, that rate accelerated to 1 foot per 100 years. In the next 100 years, it is predicted that a combination of subsidence and global warming will cause this area to experience a relative sea-level rise of from 2 to 3 feet. Impacts from this accelerated sea- level rise are expected to include greater damage from wave action on the eastern shore of the Bay, intrusion of saline waters into freshwater marshes, and an inundation of wetlands. Some islands in the Bay,including historic Poplar Island, could become completely submerged.

Higher sea levels would cause the Bay to become saltier, causing damage to shellfish, such as oysters and soft shell clams. The soft shell clam, already at the southernmost point of its range, could be eliminated due to a combination of salinity changes and warming waters. Saltwater intrusion could cause large-scale plant die-off, which is already occurring at Chesapeake's Blackwater National Wildlife Refuge as a result of current rates of sea-level rise. Although sea-level rise will convert some low-lying areas to marshes, that process is much slower than the rate at which existing marshes are lost. Onshore human development will constrain the amount of land that can be converted to wetland.