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8 Coastlines and Rising Seas Rising sea level may be one of the most widely felt and eas- ily recognized consequences of a warmer global climate, but the inundation it would bring is already familiar to the millions of people who live and work in the worlcl's coastal areas. Over the centuries, engineers have struggled to protect low-lying coasts from advancing waters. Their techniques range from straight- forward measures such as pumping sand onto eroded beaches, building levees, and relocating ports, to complex, multibillion dollar systems of carefully maintained dikes and sand dunes, such as the one that protects more than half of The Netherlands from inundation by the North Sea. if the earth's surface tem- perature grows warmer, as many researchers predict, the task of keeping the sea at bay will become more difficult, and more urgent. Scientists expect global sea level to rise for two reasons. First, as greenhouse gases accumulate in the atmosphere and eventually raise the earth's surface temperature, glaciers and land ice around the world will melt more rapidly, releasing water that will raise average sea level. Second, as the ocean 90
COASTLINES AND RISING SEAS 9 absorbs additional heat from the air above it, the water will expand. Like climate, sea levels have fluctuated throughout the earth's history. Eighteen thousand years ago during the last ice age, mean sea level was 100 to 150 meters (about 330 to 500 feet) lower than it is now. About 100,000 years ago during a period when the global average temperature was I° to 2°C (about 2° to 3.5°F) warmer than today and the glaciers rececled, sea level was about 5 to 7 meters higher. Even during the past 6000 years, sea level in some places has varied by a meter or more. Currently, sea level is rising. Scientists estimate that in this century, sea level has risen by about 0.15 meter or more. Though it is tempting to correlate this rise with the coincident buildup in atmospheric carbon dioxide, a cause-and-effect relationship has not been established. The reason for the rise is poorly understood and exceeclingly difficult to determine. Mark Meter, a glaciologist at the University of Colorado in Boulder, cautions that any projections for future sea level rise must be "tempered by the uncertainty of our understanding of the present." Estimates made in the last decade projected that we could expect global average sea level to rise from about 0.5 to 1.5 meters, with some estimates as high as 3 meters, in response to a 3° to 5°C warming that some estimate would be expected by 2050 with an effective doubling of carbon dioxide. Recent estimates have revised this figure downward to about a third of a meter, primarily because of new calculations on how snowfall will increase on the polar ice sheets (warmer air can transmit more moisture, resulting in more snowfall) and thus bind up the water that would otherwise go into the oceans and on how long it will take meltwater to escape from subfreezing glaciers. Scientists are still working to resolve the many uncertainties surrounding these projections. The earlier high estimates considered the possibility that the unstable West Antarctic Ice Sheet might disintegrate, releasing about 2 million cubic kilometers of ice into the ocean. Scientists increasingly believe that such a collapse, if it occurs, would be slow and that little effect would be seen during the coming cen
92 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE fury. On the other hand, as concentrations of greenhouse gases build up past the effective carbon dioxide doubling benchmark, researchers believe that global surface temperature will continue to increase. As it does, Meter explains, "Our feeling is that sea level rise will continue. In fact, it will accelerate. The problem is that we don't yet know how to estimate very well what is going to happen." About some processes and conditions related to sea level rise, scientists are confident. Worldwide average sea level de- pends simply on the volume of water resting in the ocean basins. Locally, at a particular coast or island, sea level is influenced by the characteristics of the locale, including its geology and the way humans manage the land. For this reason, global sea level may rise a little in some places and a lot in others. In some places it may even drop. The human and economic costs also would vary from nation to nation. Some parts of the world, such as Canada, Scandinavia, and Scotland, are rising, springing up in response to the removal of glacial ice that began with the end of the last ice age. In such places rates of uplift exceed past rates of sea level rise, so that the sea level appears to be falling. In other areas, such as Bangkok and most of the U.S. Atlantic coast, sea level appears to be rising more rapidly than the global, or eustatic, change. In fact, the land itself is subsiding in response to downwarping of the earth's crust from slow geologic processes, sudden earthquakes, or compaction of soft underlying sediments such as mud or peat. Human activities such as pumping of groundwater, oil, and gas from below the land surface can be major factors in subsidence. Because sea level in any one location is subject to these many factors, it is measured in relative terms, namely the difference between the eustatic change and any local change in land elevation. THE HUMAN COSTS OF RISING SEA LEVEL As many as one billion people, or 20 percent of the worId's population, live on lands likely to be inundated or dramatically
COASTLINES AND RISING SEAS 93 changed by rising waters. Many buildings are at elevations low enough that even at current sea levels, the safety of people during storms cannot be assumed. On the basis of current pre- dictions of sea level rise, a 1989 Resources for the Future report explains that all land currently up to 5 meters (about 16.5 feet) above mean sea level is potentially vulnerable in the next one to three centuries to upstream effects of flooding, particularly storm surges walls of water pushed by heavy winds and to intrusion of saltwater into groundwater and estuaries. The effects of sea level rise will vary with conditions along the coastlines. Sandy areas and unconsolidated cliffs of sedi- mentary material will erode quickly, whereas cliffs of resistant rock will not. Steep-sloped areas will be affected by each in- crement of rising water but less so than gently sloping coastal terrain, which would be flooded by the same sea level rise. An extreme example is the Republic of Maldives, a nation of Il90 small islands, most of which are barely higher than 2 meters in elevation. If mean sea level rises 2 meters, the entire country would be submerged. Even if mean sea level rises ~ meter, a storm surge would be, as Maldives president Maumoon Abdul Gayoom told the U.N. General Assembly in 1987, "catastrophic and possibly fatal to the nation." Some of the most visible effects of predicted changes in climate will be along the ocean coastlines, where rising seas would flood wetlands and lowlands and hasten coastal erosion. Water tables near the coast would rise, and saltwater would in- trude into rivers, bays, and the underground aquifers that sup- ply water for drinking, industry, and irrigation. In developed areas, buildings, roads, and sewage systems would be threat- ened. Gjerrit P. Hekstra, of the Ministry of Housing, Physical Planning, and Environment in The Netherlands, reports that if maximum storm surges and the upstream effects of flood- ing are taken into account, about 3 percent of the land area worldwide would be subject to inundation or made vulnerable by intrusion of saltwater. This area includes one third of the world's cropland because productive soils commonly occur in delta areas. Estuaries, where freshwater rivers flow into the sea
94 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE 7200 7100 7000 6900 - J 1 900 > CO LU 7300 7200 7100 7000 6900 6800 6700 BALTIMORE 1920 1940 1960 1980 YEAR - STOCKHOLM ' ~ Vet 1 1 1 1 1910 1930 1950 YEAR 1970 1 990 (a) (b) Tide-gauge records from the early twentieth century indicating (a) rising local relative sea level at Baltimore on the North American East Coast and (b) falling local relative sea level at Stockholm in Scandinavia, as the land rises in "rebound" from the weight of receding glaciers. (Courtesy D. G. Aubrey and A. R. Solow, Woods Hole Oceanographic Institution.) and mix with the incoming tides, and groundwater aquifers are most vulnerable to intrusion. According to lames Titus, of the EPA, increased salinity already is considered a cause of reduced oyster harvests in the Chesapeake Bay and has hastened the conversion of cypress swamps in Louisiana to open water. Perhaps no other impact of climate change would displace more people from their homes than rising seas, swelling a grow- ing class of "environmental refugees." Not surprisingly, most of these refugees would be in developing countries, where scarce
COASTLINES AND RISING SEAS 95 economic resources cannot accommodate the costly remedies required to keep the sea at bay. The system of wetlands that grace many stretches of the worIct's coastlines may be a prominent victim of sea level rise. Coastal wetlands inclucle marshes, swamps, and salt-tolerant mangrove swamps in the tropics. Wetlands are nurseries for many terrestrial animals and commercial fish species, and the ecosystems they support are among the most productive on earth. They help maintain water quality and protect the shore- lines from erosion and the full brunt of coastal storms. For centuries people have drained and filled wetlands, con- verting them to cropland or urban areas. Where navigational channels transect wetlands, or where flood control structures have been installed, wetlands have been converted to open wa- ter. In the 1960s, however, environmental awareness began to increase, and many governments have taken steps to curtail wetlands destruction. Titus explains that in the United States these restrictions have substantially reduced the conversion of coastal wetlands to drylands but that the conversion of wet- lands to open water continues. Louisiana, for example, loses 50 square miles per year to open water. How the wetlands respond to sea level rise will depend in part on the degree to which they have been developed. To pro- tect their property from the sea, people in many areas have built bulkheads just above the wetlands. Many coastal ecologists fear that these obstacles squeeze the wetlands against the advancing sea. In the absence of human activity, coastal ecosystems are pre- served by a highly effective mechanism that helps compensate for fluctuating sea levels. The rising waters flood areas that are now dry land, creating new wetlands, which can grow upward as sediment and organic material accumulate. Where coastlines are heavily populated and developed, this mechanism may have only limited potential to prevent a major loss of wetlands in the coming century. Titus cautions that the area of wetlands today is far greater
96 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE than the area that would be available for new wetlands as sea level rises. In its report to Congress on the Potential Effects of Global Climate Change on the United States, the EPA reports that a 1-meter sea level rise by 2100 could drown 25 to 80 percent of U.S. coastal wetlands. Their survival depends on whether they can migrate inland, or if levees and bulkheads built to protect coastal properties block their path. Nor is the future bright for the string of barrier islands that adorns the eastern United States and Gulf coasts. Barrier islands such as Hatteras Island, North Carolina, and Long Beach Island, New Jersey, are long, narrow islands and peninsulas only a few feet above sea level in elevation, with one side facing the ocean and the other side facing a bay that separates the island from the mainland. Left on their own, barrier islands keep pace with sea level rise by migrating, rolling landward as storms wash sand from the oceanside beach to the bayside marsh. Even with- out the obstacles presented by human-built structures, barrier islands may disintegrate as sea level rise speeds up. Coastal Louisiana, for instance, is subsiding so rapidly that relative sea level is rising by ~ centimeter (about 0.4 inch) per year, and its barrier islands are breaking up. (This already serious situation will worsen if the rate of sea level rise accelerates in the next century.) While rising waters may inundate the baysides of the barrier islands, the oceansides of the islands ant! the mainland coastal beaches are subject to scouring wave action and are particularly at risk for runaway erosion. Surveys indicate that during the past century, less than 10 percent of the length of the worId's sandy shorelines advanced seaward, while more than 60 percent retreated. In the United States, a 1-foot rise in sea level could cause beaches from the Northeast to the mid-AtIantic to erode 50 to 100 feet. Along the Carolinas, the sea would advance 200 feet; In Florida, where shoreline protection already costs millions yearly, the shoreline would retreat from 100 to 1000 feet; and in Louisiana, several miles. EPA notes that at high tide, most U.S. recreational beaches are less than 100 feet wide.
COASTLINES AND RISING SEAS 97 THE PLIGHT OF THE DELTAS A rise in global sea level would be most deeply felt in river deltas, the wetlands that form when a river carries more sedi- ment into a body of water than can be carried away by currents and waves. This sediment builds up to form the delta, which acts as a barrier between the lanct and the sea. Under natural conditions, deltas form and break down continuously as they accrete and subside. But when humans intervene by damming, diverting, or channeling rivers, the amount of sediment reach- ing the delta is reduced. The delta is thrown out of equilibrium, with the accumulation of sediment no longer offsetting the sub- sidence. The Mississippi River delta in Louisiana is the most vul- nerable area to relative sea level rise in the continental United States and provides a ready example of the changes uncler way in many of the world's deltas. The fine-grained sediment car- riect by the river to the sea tends to compact and subside in the manner typical of deltas. Normally this effect is counteracted by annual flooding, cluring which the Mississippi overflows its banks and supplies sediment to the wetlands. But measures to trap sediment upstream in order to reduce dredging costs at the mouth of the river, and levees built for flood control, prevent sediment, fresh water, and nutrients from reaching the wetlands. Titus estimates that if current management practices continue and sea level rises as projected, most of Louisiana's wetlands will be destroyed in the next century. Low-lying countries in the developing world such as Egypt and Bangladesh, where rivers are large and the deltas extensive and densely populated, will be hardest hit by sea level rise. Where the rivers are dammed, as in the case of the Nile, the ef- fects of inundation and coastal erosion will be especially severe. John Milliman, lames Broadus, and colleagues at the Woods Hole Oceanographic Institution studied the economic and hu- man consequences of sea level rise for Egypt and Bangladesh. They estimate that the combined effects of sea level rise anct sub- sidence caused by extraction of groundwater could threaten the
98 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE ~ ~ , ~ , ~ = \~: I At' Aft _: hi' ~- ' ~ `_' "Lamentable News out of Monmouthshire": Old woodblock illustration of fourteenth- century coastal flooding in England. (Courtesy of University of East Anglia, Norwich, United Kingdom.) homes and livelihoods of 46 million people in these two coun- tries alone. in their analysis, the researchers used two estimates for sea level rise: a minimum of 13 centimeters by 2050 and 28 centimeters by 2100, and a maximum rise of 79 centimeters by 2050 and 217 centimeters by 2100. They then developed three possible scenarios. The "best case" assumes that the minimum rise in global sea level occurs and that the delta is in equilibrium. The "worst case" assumes the maximum rise in sea level and the complete damming or diversion of the sediment carried by the river ciraining into the delta (in this case the sediment would not be able to replenish the delta so that natural subsidence would increase the relative rate of sea level rise). The "really worst case" assumes that the effects of natural subsidence are compounded by excessive removal of groundwater as well as by the maximum increase in global sea level. Almost all of Bangladesh lies in the massive Bengal delta formed where the Ganges, Brahmaputra, and Meghna rivers converge after traversing India, Nepal, Bhutan, Tibet, and China.
COASTLINES AND RISING SEAS 99 Half of the country is at elevations of less than 5 meters. The Woods Hole researchers find that in the best case, sea level would rise about 13 centimeters by 2050 and less than ~ percent of the nation's total land area wouIc! be lost to advancing seas. In the worst case, 18 percent of the land, which currently supports about 15 percent of the nation's people, would be lost. In the really worst case, by 2100 waters would cover an area currently supporting 35 percent of the nation's population. The land area that supports nearly one third of the nation's current gross national product would vanish into the sea. Global warming could further compound the miseries of Bangladesh if the hypothesized possibility of more frequent storms occurs. Currently, an average of one and one half severe cyclones hit Bangladesh each year, and the storm surges act- vance up the rivers as far as 200 kilometers inland. If the storms increase in frequency and sea level rises, the storm surges will reach even further toward the main centers of population. Egypt too would experience massive change with global sea level rise. Though Egypt is much larger than Bangladesh, its people live on only about 3.5 percent of the land. Within this area, population density IS00 people for every square kilometer is twice that of Bangladesh. Much of Egypt's pro- ductive agricultural land is within the Nile delta. The Nile River has been completely dammed since comple- tion of the High Dam at Aswan in 1964. For this reason the Nile delivers no sediment and little fresh water to the Mediterranean, where the delta fans out into the sea. This loss of sediment com bines with subsidence to cause dramatic erosion of the delta shores. Between 1966 and 1974, many areas eroded more than ~ meter per year, and some locations lost more than 100 meters per year. Egypt is precluded from the best case scenario because the Aswan Dam already traps the sediment. The Woods Hole re- searchers report that the worst and really worst cases are more likely. In the worst case, local sea level would rise between 13 and 133 centimeters by 2050, which would affect as much as 19 percent of the currently habitable land. In the really worst case,
100 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE sea level would rise between 28 and 332 centimeters by 2100 and would affect up to 26 percent of the habitable lanct. Several brackish lakes behind the coastal dunes supply approximately half of the nation's fish catch and would be enciangered. Other areas with low-lying deltas and large rivers could experience similar fates. The researchers suggest that although there may be little that individual developing countries can do to control global climate change, they can control groundwater extraction and the trapping of sediments in dams. These factors are perhaps more responsible than global climate change for local changes in relative sea level. PREPARING FOR RISING SEAS In many cases, decisions about how to prepare for rising sea levels come down to stark economic facts that will vary with specific countries and localities. A 1987 report from the National Research Council on the engineering implications of sea level rise suggests that "holding back the sea as water levels rise will almost always be feasible," but that in some cases it may not be economically or environmentally sound. Responses fall into three main categories. Humans can retreat from the shoreline, use structures that prevent flooding and shoreline recession, or raise the land. For some cities the choice whether to protect is obvious: Manhattan will never surrender to rising seas. In fact, most well- cleveloped coastal communities will probably decide that protec- tive measures such as bulkheads, levees, and pumping systems, and the high costs of maintaining them, are worthwhile, whereas sparsely developed areas may continue to adapt as the shoreline retreats. The EPA estimates that low-lying coastal cities in the United States could be protected from even a 2-meter (about 6.6 feet) sea level rise. A conservative cost estimate ranges from $30 billion to $100 billion, but these sums would still be a small fraction of the values of the coastal cities. Cost estimates for raising the nation's recreational barrier islands, which cover a combined area of 100 to 150 square miles, range from $50 billion
COASTLINES AND RISING SEAS 101 to $100 billion for a I-meter rise and from $135 billion to $215 billion for a 2-meter rise. The trade-offs between protection from rising seas and economic costs of protection will be difficult to balance. In addition, levees and seawalIs hold back the sea but also block waterfront views, cause the beach to erode, and pre- vent the migration and preservation of wetlands. The choice of response should be based on a sound understanding of coastal processes. If people opt to stem destruction of wetlands, they may have to surrender control of these areas to natural forces. This would mean removing artificial structures that thwart sed- imentation and migration of wetlands, or moving them back from the shore as the waters advance. Coastal scientists, engi- neers, and policy analysts suggest that some communities may plan an orderly retreat from the shoreline rather than retreat- ing in response to future rises. The National Research Council report cautions that because methods for dealing with coastal erosion are evolving rapidly, communities should periodically reconsider any plans to retreat. The projected sea level rise should cause neither alarm nor complacency, researchers stress. The severity of impacts from sea level rise depends on how society responds and adapts. People are adept at using and advancing technology when nec- essary and appropriate. So far, the projected acceleration in the rate of sea level rise has not been detected. If the rate does accelerate, the increase initially would be so slow that we might not detect it until the coming century. But some responses, especially those requiring construction of costly and massive structures, can take three to four decades from planning stages to completion. Some deci- sions must be made soon if defenses are to be ready in time. No one can anticipate all the changes that a warmer climate will cause. Of the changes that are expected, rising sea level could have the most visible effects on society, literally forcing people to abandon their homes or to build virtual fortresses around their cities. Whereas the industrialized countries of the world that have contributed most heavily to the increase in greenhouse gases conceivably could afford the technological
102 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE remedies to cope with rising waters (though it will cost them dearly), many developing nations couIcl not. As Tom Goemans, a consultant with KPMG in The Hague, notes, accelerated sea level rise is a "manmade phenomenon that presents a perfect example of external costs, that is costs not included in the present price of energy from fossil fuel burning."