Sea Level Rise

Introduction

Global sea level, the average height of the ocean’s surface apart from the daily changes of the tides, is rising. Both the main causes of this change are linked to anthropogenic (human-caused) global warming. First, warming of the ocean causes it to expand. This effect, confined mostly to the ocean’s top 2,300 ft (701 m), is called thermal expansion or thermosteric expansion. Second, melting of glaciers and other bodies of ice lying on land causes the total mass of water in the ocean to increase. The heat energy causing both thermal expansion and ice melting comes from warming of the atmosphere, which has in turn been caused in recent decades primarily by anthropogenic climate change. Retention of liquid water on the continents, as for example by the damming of rivers, makes a small negative contribution to sea-level rise, withholding some water from the oceans.

In the late 1800s, after two or three thousand years of stability, sea level began to rise steadily at about 0.07 in (1.7 mm) per year. From 1993 to 2007, the rate of sea-level rise increased to about 0.12 in/yr (3 mm/yr). Sea level will continue to rise for at least the next few centuries because of global-warming processes already set in motion, but how much more it rises will depend on factors that are difficult to predict, including whether human societies reduce their greenhouse-gas emissions and the degree to which the West Antarctic and Greenland ice sheets melt. The future behavior of these ice sheets is uncertain. Recent data show that melting of both sheets has recently been faster than any computer climate model had predicted. Rising sea levels harm human coastal communities and natural coastal ecosystems.

Historical Background and Scientific Foundations

istory of Sea Level Change

Earth’s sea levels have risen and fallen due to natural causes many times over the planet’s history. During the last half-million years, Earth has gone through about half a dozen ice ages alternating with warmer periods called interglacial periods. During each ice age, water evaporating from the oceans falls as snow on cold regions near the poles and stays there, locked up in the form of land-based ice sheets. This causes sea level to drop. (Ice that forms as floating sheets, on the other hand, such as the sea ice floating over the North Pole today, does not affect sea level directly, either by forming or melting.)

During the last four glacial cycles, each of which has lasted about 100,000 years, sea level has fallen about 400 ft (122 m) when the ice was at its height and then risen again by the same amount as the ice melted. During the warmest part of the last interglacial period, about 130,000-118,000 years ago, Earth’s average global air temperature was 3.6-5.4°F (2–3°C) warmer than today and sea level was 13–20 ft (4-6 m) higher than today. During the warm interglacial period of the Middle Pliocene, about three million years ago, global temperature was 6.3–9.9°F (3.5–5.5°C) warmer than today and sea level was 80–115 ft (24–35 m) higher than today.

The patterns of ancient sea-level change are therefore not a matter of merely abstract interest: Scientists study them for clues to the changes that the human race may face in coming decades and centuries. Since most of the water that is available for increasing the mass of the oceans resides in the ice sheets of Antarctica and Greenland, understanding the nature of past changes to these ice sheets is particularly important for predicting future sea-level rise and other aspects of climate change. If the ice sheets can be partly or wholly destabilized by

WORDS TO KNOW

CALVING: Process of iceberg formation in which huge chunks of ice break free from glaciers, ice shelves, or ice sheets due to stress, pressure, or the forces of waves and tides.

EUSTATIC SEA LEVEL: Change in global average sea level caused by increased volume of the ocean (caused both by thermal expansion of warming water and by the addition of water from melting glaciers). Often contrasted to relative sea level rise, which is a local increase of sea level relative to the shore.

FORAMINIFERA: Single-celled marine organisms that inhabit small shells and float free in ocean surface waters. The shells of dead foraminifera sink to the sea bottom as sediment, forming thick deposits over geological time. Because the numbers and types of foraminifera are climate-sensitive, analysis of these sediments gives data on ancient climate changes.

GREENHOUSE-GAS EMISSIONS: Releases of greenhouses gases into the atmosphere.

ICE AGE: Period of glacial advance.

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): The Intergovernmental Panel on Climate Change (IPCC) was established by the World Meteorological Organisation (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.

PALEOCLIMATE: The climate of a given period of time in the geologic past.

RELATIVE SEA LEVEL: Sea level compared to land level in a given locality. Relative sea level may change because land is locally sinking or rising, not because the sea itself is sinking or rising (eustatic sea level change).

THERMOSTERIC EXPANSION: Expansion as a response to a change in temperature, also called thermal expansion.

TREE RINGS: Marks left in the trunks of woody plants by the annual growth of a new coat or sheath of material. Tree rings provide a straightforward way of dating organic material stored in a tree trunk.

TIDE GAUGE: A device, usually stationed along a coast, that measures sea level continuously. Measurements from tide gauges were the main source of sea-level data prior to the beginning of satellite measurements in the 1970s.

global warming and melt relatively rapidly—that is, over decades and centuries rather than over thousands of years—then sea-level rise over the next century might turn out to be significantly larger than the half-meter to a meter predicted by the Intergovernmental Panel on Climate Change (IPCC). Many scientists—for example, Jonathan Overpeck and colleagues (2006)—have argued that standard models of ice-sheet instability and melting are inadequate, and that sea-level rise may happen larger and faster than the IPCC’s cautious predictions allow.

For example, warming of the polar regions—especially the north—by the end of the twenty-first century may raise air temperatures to levels not seen for at least 118,000 to 130,000 years, during the last major interglacial period, when sea level was 13 to 20 ft (4–6 m) higher than today. If sea-level rise corresponds to temperature increase, equally large increases in sea level may happen again, though not all in the coming century. This is not, however, a worst-case scenario. In 2007, the IPCC estimated that unless successful efforts are made to curb greenhouse-gas emissions, by 9.9°F (5.5°C), a temperature associated three million years ago with sea levels up to 115 ft (35 m) higher than today’s. At the end of the last interglacial period, sea levels rose at speeds of between 0.43–0.78 in/yr (11–20 mm/yr). Although such an increase would be slow by everyday human standards, it is much faster than the 0.102±0.002 in/yr (2.6±0.04 mm/yr) that was occurring as of 2006. Since many coastal lands are nearly level with the sea, a 0.5 in/yr (1.2 cm/yr) sea-level rise would cause them to retreat by several feet to many feet annually. This would disrupt coastal human settlements and natural coastal ecosystems at a pace that would be hard to adapt to.

The most recent interglacial sea-level rise happened from about 7,000 to 20,000 years ago. After that time, sea levels rose slightly or not at all until the late 1800s. Starting then, the oceans began to rise at a rate of about 0.07 in/yr (1.7 mm/yr). Recently, as a result of global climate change, sea-level rise has accelerated: from 1993 to 2007, the oceans rose at about 0.12 in/yr (3 mm/yr).

Measuring Sea Level Changes

Ancient climate, called paleoclimate from the Greek palaois, is revealed by the various marks it left on the land, the sea bottoms, the ice caps, and elsewhere. For example, average global air temperatures up to about 900,000 years ago can be inferred from the atomic composition of air bubbles trapped in ancient ice layers in Antarctica and Greenland. These layers record the date of their formation much as tree rings record the age of a tree. Also, cylinders of layered muck drilled from the ocean floors contain trillions of shells of single-celled organisms called foraminifera, the atomic composition of whose shells records water temperature. Fossil corals found above or below present-day sea level record ancient sea levels, since corals grow only in a narrow range of water depth, and these fossil corals can also be dated by their atomic composition (the ratios of certain atomic isotopes in their skeletons). Sea levels from the present back to about 500,000 years ago can be inferred from fossil corals. More ancient sea levels are shown by other evidence, such as fossils marking ancient inland shorelines.

Modern sea levels are measured both on-site and remotely. On-site measurements are made by tide gauges, instruments attached to posts standing in the water near shore. High-quality tide-gauge data from around the world date back to about 1950. Lower-quality data from a smaller number of gauges, mostly in the Northern Hemisphere, go back to 1870.

Tide gauges have several disadvantages. First, the gauges must be checked regularly to make sure that the pilings they are attached to are not sinking into the ground, giving a false impression of sea-level rise. Second, their readings must be corrected for changes of local altitude due to post-glacial rebound. Third, tide gauges measure sea level only at a series of unevenly distributed points around the edges of the ocean.

Since 1992, data from satellite radar altimeters aboard the TOPEX/Poseidon and Jason satellites have allowed precise global measurement of sea levels. Satellite radar altimeters send pulses of radio energy toward Earth’s surface and measure the time until an echo is received. The distance from the satellite to the surface can be calculated from the delay. Satellite data have shown faster sea-level rise since 1993, up to about 0.12 in/yr (3 mm/yr) from 0.07 in/yr (1.7 mm/yr). This observation has been confirmed by the tide-gauge data.

Global Sea Level Budget

Thermal expansion and ocean mass are the two biggest contributors to global sea-level change. Thermal expansion of the ocean (thermosteric expansion) occurs mainly as its top 2,300 ft (700 m) or so swells because of warming. For most of the twentieth century, thermosteric expansion was the main cause of sea-level rise: The ocean’s heat content increased by about 2×10²³ joules, about 20 times as much as the atmosphere’s heat content increased during that time. (The ocean can absorb more heat than the atmosphere because it weighs far more.) The expansion caused by this much heat corresponds to a sea-level rise of 1.2 in (3 cm). Today, this warmth-driven, expansive sea-level rise has accelerated with increased warming of the atmosphere to about 2.4 in (6 cm) per century.

The other major cause of sea-level rise is the melting of land-based ice. Ice on land is transported to the land by two processes, namely melting followed by runoff and dumping of glacial ice into the sea. Mountain glacial ice and ice around the edges of Greenland has been melting and running off as water,

IN CONTEXT: SEA LEVEL RISING, BUT BY HOW MUCH?

In early 2008, ongoing independent research projects confirmed that global sea level—the average height of the ocean’s surface apart from the daily changes of the tides—is rising. Sea-level rise threatens freshwater supplies, coastal land use, and vital economic activity. For some islands, significant sea-level rise could be devastating. In 2007, the Intergovernmental Panel on Climate Change (IPCC)—an international body of hundreds of government-appointed scientists and economists representing as broadly as possible the world community of climate and weather scientists—foretold that by 2100, sea levels would rise between 7 and 23 in (18 and 59 cm). It was widely reported that the upper end of this estimate’s range had been reduced from the IPCC’s 2001 estimate of 3.1–34.7 in (8–88 cm). However, no other figure in the IPCC report has been so disputed by scientists as the prediction for sea-level rise. The range calculated by the IPCC for its 2007 report did not include the possibility of increased contributions to sea-level rise from accelerated glacial melting. A number of scientists have argued in the world’s leading scientific journals that future sea-level rise may be greater than the IPCC’s 2007 forecast, but is unlikely to be less.

while glaciers in southern Greenland and along the edges of the West Antarctic Peninsula have recently accelerated their slide to the sea. As chunks of glacial ice break off into the water, a process called calving, they raise sea level at once; when they finally melt, there is no further change. Since about 1993, because of accelerated loss of mountain glaciers around the world and of parts of the Greenland and West Antarctic ice sheets, land-based ice has been responsible for about half of the observed sea-level rise, more than in the earlier part of the twentieth century.

The Twentieth-Century Sea Level Enigma

Scientists have puzzled over what they call the enigma of twentieth century sea-level rise, first pointed out by American oceanographer Walter Munk in 2002. The enigma or mystery is that the known sources of sea-level rise do not seem to add up to the observed rise. From 1961 to 2003, sea level rose at about 0.017 in/yr (0.43 mm/yr) due to thermal expansion. In many reports, a “±” sign indicates a range of uncertainty around such data (e.g., 0.028 ±0.002 in/yr). For simplicity and readability, the uncertainty figures are often omitted and so such values should be considered approximate. Sea levels rose at approximately 0.020 in/yr (0.51 mm/yr) due to the melting of glaciers and other minor bodies of land-based ice, 0.002 in/yr (0.05 mm/yr) due to melting of the Greenland ice sheet, and 0.006 in/yr (0.15 mm/yr) due to melting of the Antarctic ice sheet. The sum of these contributions to sea-level rise was approximately 0.043 in/yr (1.0 mm/yr), but the observed rise was 0.071 in/yr (1.8 mm/yr), leaving a difference of 0.028 in/yr (0.7 mm/yr) unexplained. For the period 1993 to 2003, the unexplained remainder was smaller, only 0.012 in/yr (0.3 mm/yr).

A number of explanations for the discrepancy have been suggested, including shifts in Earth’s rotational axis (polar wander), overestimation of sea-level rise, and underestimation of either the thermal expansion or ice melting. In 2007, European researchers claimed to have resolved the enigma of excess sea-level rise noted by Munk in 2002. According to G. Woppelmann and colleagues, the apparent shortfall in the sea-level rise budget is whittled away from both ends at once: 1) Since 2002, estimates of the anthropogenic greenhouse contribution to sea-level rise have been upped to 0.06 in/yr (1.4 mm/yr) from Munk’s figure of 0.02 in/yr (0.7 mm/yr); and 2) Woppelmann and colleagues’ corrections to tidal-gauge data using the Global Positioning System (GPS) reduce, they say, the amount of rise to be explained from 0.07 in/yr (1.8 mm/yr) to 0.05 in/yr (1.3 mm/yr). Thus, the enigma is resolved. However, the new claim will have to be verified by other scientists before agreement can be reached that the global sea-level budget is closed. Other explanations for the enigma, including polar wander, have also been strongly defended.

Impacts and Issues

Effects of Sea Level Rise

In some parts of the world, the land is still rising after the removal of the weight of the ice sheets at the end of the last glacial period about 10,000 years ago: The land bobs upward after the ice melts off, like a cork pushed partly under water and then released. This effect, termed post-glacial rebound, is causing the relative sea level in some places to shrink faster than eusta-tic (actual, global) sea-level rise is making it grow. In Stockholm, Sweden, for example, post-glacial rebound is causing the relative sea level to decrease at about 0.16 in/yr (4 mm/yr). Such locations will experience stable sea levels if eustatic sea-level rise increases to an equal rate, and in that case will see no direct negative effects from sea-level rise.

However, some other locations, such as Chesapeake Bay in the eastern United States, are experiencing the opposite effect. There, the southern edge of the area pushed down by the last Ice Age, formerly raised, is sinking again like one end of a see-saw after the rider on the other end gets off. Chesapeake Bay is therefore seeing faster relative sea-level rise (about twice the global average) than the eustatic rise. Most of the planet, including the areas inhabited by most of the 100 million or so people who live within several feet (a meter) of the present-day sea level, are seeing neither form of post-glacial rebound and will experience the effects of eustatic sea-level rise directly. Those effects will be almost universally negative. They include flooding of river deltas, erosion of shores, infiltration of saltwater into island groundwater supplies, disappearance of small, low-lying islands, increased vulnerability of large, low-lying cities to violent storms, and more.

Disputed Projections

In 2007, the IPCC—an international body of hundreds of government-appointed scientists and economists representing as broadly as possible the world community of climate and weather scientists—foretold that by 2100, sea levels would rise between 7 and 23 in (18 and 59 cm). It was widely reported that the upper end of this estimate’s range had been reduced from the IPCC’s 2001 estimate of 3.1-34.7 in (8-188 cm). It was apparently reassuring news: Sea-level rise might not be as bad as we thought.

However, no other figure in the authoritative IPCC report has been so disputed by scientists as this one. The range calculated by the IPCC for its 2007 report did not include the possibility of increased contributions to sea-level rise from accelerated glacial movement in the Greenland and West Antarctic ice sheets. The West Antarctic ice sheet contains enough water to raise sea level by 16 ft (5 m) and Greenland contains enough to raise it by 23 ft (7 m). During most of the twentieth century, these bodies of ice contributed only a few tenths of a millimeter per year to sea-level rise; as of 2006, however, scientists estimated that due to a combination of surface melting and accelerated glacial flow, Greenland was now contributing over 0.02 in/yr (0.5 mm/yr) to global sea-level rise, more than double previous estimates.

A number of scientists have argued on the basis of paleoclimatic data that the ice sheets are capable of much more rapid change than the IPCC has reported—change that could cause sea-level rise of up to 0.78 in/yr (20 mm/yr). The IPCC acknowledges uncertainties about ice-sheet instabilities. Future sea-level rise may be greater than the IPCC’s 2007 forecast, but is unlikely to be less. The group underestimated sea-level rise in its 2001 report, predicting a maximum rise of no more than 0.08 in/yr (2 mm/yr) from 1993 to 2006: The actual rise turned out to be 0.13 in/yr (3.3 mm/yr). German climatologist Stefan Rahmstorf commented in the journal Science in 2007, “In a way, it is one of the strengths of the IPCC to be very conservative and cautious and not overstate any climate change risk.”

Primary Source Connection

The following news article recognizes the Dutch as true innovators of controlling water through dikes, levees, and dam systems, and are now moving forward with “climate-adaptation plans.” Such plans are being drafted and acted on in hopes of preventing flooding as a response to climate change, which could cause sea levels to rise significantly each year.

HOW TO FIGHT A RISING SEA

The Dutch enjoy a hard-earned reputation for building river dikes and sea barriers. Over centuries, they have transformed a flood-prone river delta into a wealthy nation roughly twice the size of New Jersey.

If scientific projections for global warming are right, however, that success will be sorely tested. Globally, sea levels may rise up to a foot during the early part of this century, and up to nearly three feet by century’s end. This would bring higher tidal surges from the more-intense coastal storms that scientists also project, along with the risk of more frequent and more severe river floods from intense rainfall inland.

Nowhere does this aquatic vise squeeze more tightly than on the world’s densely populated river deltas.

So why is one of the most famous deltas—the Netherlands—breaching some river dikes and digging up some of the rare land in this part of the country that rises (barely) above sea level?

In the Biesbosch, a small inland delta near the city of Dordrecht, ecologist Alphons van Winden looks out his car window at a lone excavator filling a dump truck with soil. He considers the question and laughs. “We do have a hard time explaining this to foreigners,” he says.

The work here represent a keystone in the country’s climate-adaptation plans, Mr. van Winden says. Indeed, nowhere are adaptation planning efforts to address rising sea levels and flooding more advanced than in the Netherlands.

To be sure, the country’s economic wealth and long experience dealing with threats from seas and rivers give it an advantage over other low countries that face rising waters, such as Bangladesh, Vietnam, and the tiny tropical island nation of Tuvalu in the South Pacific. But many of the approaches the Netherlands is taking can and are being slowly adopted even in countries far poorer, specialists say.

The excavation work here is one example of what van Winden calls “soft approaches” to flooding in this small nation where competing interests jostle for every square foot of land. By buying out the few farmers remaining in this region, breaching the dikes they built to protect their land, and digging additional water channels, the Dutch government aims to reduce peak flood flows at Dordrecht and other cities downstream. No longer will tightly constricted river and canal channels hold high water captive. Big floods will overspread the Biesbosch, reducing the threat of water spilling over the top of levees that guard densely populated cities to the west.

The Biesbosch may also be critical to the future of farming on the productive southwest coast. There, most of the area’s fresh water sources are close to the coast—and vulnerable to salt-water contamination from a rising North Sea. This could make farming difficult, if not impossible. The Biesbosch, however, hosts three large reservoirs, each surrounded by a 20-foot-high dike. Fresh water piped from these reservoirs, some 50 miles inland, could keep coastal areas supplied.

1.4 billion live near seacoast

Globally, some 21 percent of the world’s 6.6 billion people live within 20 miles of a seacoast—and nearly 40 percent within 60 miles, says Robert Nicholls, a professor of civil and environmental engineering at the University of Southampton in England.

Seacoast populations who face the greatest risk from floods, storms, and sea-level rise live on river deltas, says the UN-sponsored Intergovernmental Panel on Climate Change.

In the IPCC’s latest set of reports on the impact of global warming, released earlier this year, scientists looked at data from 40 of the globe’s river deltas, home to 300 million people. If current trends continue through 2050, flooding in the Nile, Mekong, and Ganges-Brahmaputra river deltas could each displace more than 1 million people. Up to a million more may be forced to head for higher ground in each of another nine deltas, including the Mississippi River delta. Up to 50,000 could be forced to relocate in each of 12 other deltas, including the Rhine River delta—an area known more widely as the Netherlands.

Besides global warming, scientists say the challenges these regions face have other causes as well. Levees, sea walls, drainage canals, dams, and other land-use patterns have taken a toll. Deltas tend to subside (sink) naturally, accentuating the rise in sea level. Past engineering projects can actually limit the ability of natural processes to replenish the landmass of deltas.

A patch of the Netherlands between Rotterdam and Gouda, called Zuidplaspolder, highlights the issue in a way that New Orleans might recognize. The 19-square-mile area is bounded by dikes and the Gouwe River. Face the river, and the landscape looks like a typical river plain. But turn and face Zuidplaspolder, and you see a steep decline dropping more than 20 feet. The huge dimple in the delta stretches as far as the eye can see.

It’s the lowest spot in Europe, some 23 feet below sea level.

“And it’s all subsidence,” says Willemien Croes, a planner with the provincial government of South Holland. Over the centuries, residents dug up thick layers of peat to warm their homes in Gouda, Rotterdam, and Amsterdam, she says. Much of Zuidplaspolder then filled with water. Farmers pumped it dry, grew crops, and raised dairy herds on the rich clay and peat. When the soil settled, farmers ringed the area with dikes for protection.

The area’s low elevation and the anticipated increased future risk of floods, combined with development pressures from Rotterdam and Gouda, have turned this area into one of the country’s biggest adaptation challenges. But it’s hardly alone: Some 60 percent of the country, accounting for 70 percent of its gross domestic product, lies below sea level.

These sinking lowlands are protected along the coast by sand dunes, dikes, and sea barriers that stretch across the mouths of estuaries. These natural and engineered defenses have protected millions from the North Sea since a devastating storm surge hit the country in 1953. But these defenses have come at an ecological cost. Unlike river deltas such as the Mississippi’s, which grew as sediment washed downriver from deep in the North American interior, the Dutch delta was built by the sea. Currents swirling through the Strait of Dover since the end of the last ice age eroded the white cliffs and deposited the material along the Dutch coast.

That process has slowed substantially, says van Winden, who works for Stroming, an environmental consulting firm in Nijmegen. Although the delta drains three of Europe’s major rivers—the Rhine, Meuse, and Scheldt—the rivers never carried enough sediment to build the delta, and don’t carry enough silt to maintain it today. From that standpoint, he says, over the long term “we are living beyond our means.”

Dutch humble in face of rising threat

Faced with the twin threats of increased river flooding from inland storms and higher ocean storm surges as the climate warms and sea levels rise, the country aims to meet these challenges with a variety of approaches, ranging from complex engineering to “natural.” But it’s doing so with increased humility, given the levee failures in New Orleans after hurricane Katrina in 2005.

“If you want a caricature of the Netherlands, it’s: ‘We have the dikes; we are 100 percent safe. So just go on with your life,”’ says Pieter Bloemen, who runs the government’s Adaptation Program for Spatial Planning and Climate. But these days, “even we proud Dutch, with climate change in the back of our heads, have to think about broken dikes. That’s a big paradigm shift.”

Zuidplaspolder is a case in point. As the lowest real estate in one of the Netherlands’ most vulnerable provinces, it has become a test bed for factoring water and climate change into zoning and development plans. In the next 20 years, 15,000 to 30,000 new housing units will be built here. Anticipating this growth, in 2004, officials from provincial and local governments joined with nongovernmental organizations to develop a master plan for the polder. (A polder is a large tract of land containing farms and villages encircled by dikes. The dikes offer flood protection, but they also turn the polders into enormous bathtubs with bottoms that slowly, inexorably sink.)

The new homes that rise in the polder may look nothing like those in the villages the Dutch are used to, Mr. Bloemen says. To deal with floods, homes on this higher ground could be designed to float in place or built on stilts. They may sport tall ground floors, with living space and utilities placed on higher floors. Entire villages might be built to float in place, linked by buoyant sidewalks and roads.

In addition, he adds, officials may ask developers to use a technique that dates back centuries: building houses, even whole villages, on mounds. That low-tech approach is appearing in other parts of the world, too. Oxfam International is working with villages in Bangladesh to build individual homes and even small villages on flood-resistant mounds.

In the Netherlands, river floods are a top item on the climate-change adaptation must-fix list. To be sure, the country has tried to be forward-looking in tackling flood control and sea-level rise, notes Hans Balvoort, with the Netherlands’ Ministry of Public Works, Transport, and Water Management. It typically uses a 50-year planning horizon. But a wake-up call came in the 1990s, “when, for the first time, rainfall was so heavy and intense that our pumping systems could not cope,” he says.

Powerful pumps long ago replaced the signature windmills as the way to keep the polders from flooding. “On such a large scale,” he says, the inability of pumps to keep pace with rainfall was “something we had not experienced before.”

Moreover, for two winters during that decade, flooded rivers rose so high that officials evacuated some 250,000 people out of concern that levees might not hold. Instead of building large numbers of new levees, he continues, scientists, engineers, and officials looked for other ways to store flood waters over the short term to reduce the risk.

The Biesbosch project, with its dike removal, or “depoldering,” is one approach. The government also is working on a range of other strategies to give flooded rivers more room to flow. They might spread dikes farther apart, excavate land between river and dikes (to capture overflow), deepen central river channels, remove jettylike groins that now force most of the flow into the center of a river, remove other obstructions, and even add new channels to the flood plain or restore old ones.

Storm surge is biggest coastal worry

The government plans to spend €2.2 billion ($3.2 billion) to make these changes to its rivers. Meanwhile, along the coast, the big worry is not about any average increase in sea level, which scientists project to rise here between 35 and 85 cm (14 to 33 inches) by 2100. Instead, the biggest concern is the change in storm-surge patterns that will ride atop that rise, says Pier Vellinga, who heads the climate program at Wageningen University.

As if to highlight this point, last weekend Britain and the Netherlands closed their sea barriers in the face of a storm in the North Sea that sent a 13-foot surge bearing down on their coasts.

Planners in other countries often design for a once-every-hundred-years storm. While that approach can be useful, the challenge is that climate change may throw those projections out of whack. For example, some researchers say that in the U.S. Northeast, midcentury coastal winter storms could lead to flood levels every three or four years—floods of a severity that used to occur only once every 100 years. Netherlands planners aim for a 10,000-year storm for the country’s most vulnerable areas. And even that may be inadequate, Dr. Vellinga says.

“When you do an economic assessment of the damage,” he says, “and what you can afford to [spend to] avoid that damage, a better safety level would be a recurrence of 1 in 100,000 years.” One storm like that could cost the country up to a year’s worth of gross domestic product—€ 500 billion ($730 billion).

In 1990, the government decided to maintain the country’s existing coastline by replenishing its extensive phalanx of coastal dunes using enormous deposits of sand that lie far offshore—another geological gift delivered over millennia from the English Coast to the Netherlands.

Three years ago, the government added that it will strive not only to maintain the coastline at its current position, but also to maintain the shape of the current offshore slope to a depth of about 130 feet. Today, that means dredging and depositing nearly 16 million cubic yards of sand along the coast each year. So, as the sea level rises, the dunes will, too, says Joost Stronkhoorst, with the National Institute for Coastal and Marine Management at The Hague.

Offshore sand deposits are large enough to allow the Dutch to accommodate a rise in sea level up to 16 feet, he says. But the line of coastal dunes is not unbroken. The gaps are spanned by barriers that in some cases will require 20 feet added to their height given sea-level-rise scenarios out to 2100.

In some cases, that’s not possible. The northern coastal town of Petten shows why. It’s tucked hard against the back of a sea dike that traces its origins to the Middle Ages—and sits 14 feet below the level at which waves crash on the other side.

To build up a dike, you must expand its base, explains Roel Posthoorn, with the Dutch nature trust Natuurmonumenten, as he stands on the crest of the dike on a blustery fall afternoon. The presence of the village eliminates the chance to expand the dike’s base inland. And churning North Sea currents already sweep away precious coastal sand from the seaward edge of the dike’s base, preventing planners from trying to expand the dike seaward.

Possible solution: artificial reefs

Here, Mr. Posthoorn says, the long-term solutions may lie in building an offshore reef to reduce the height of the waves slamming into the dike. Or, as some are now beginning to suggest, perhaps the large deposits of sand offshore should also be used to build the country’s coast westward by nearly a mile.

In the meantime, groups like Natuurmonumenten are working to meet two of the country’s adaptation goals by trying to prevent further development behind sea dikes like this one and converting the land to nature reserves. These “climate buffers” are another tool in the Netherlands’ kit for coping with global warming.

Adaptation experts generally agree that scientists, engineers, and policymakers already know what needs to be done to adapt to global warming. For the most part, they say, it means doing what they already know how to do to reduce risks from natural hazards—it’s just doing more of it and a better job of it.

As if to underscore the point, Henk Wolfort, a researcher at Alterra, an institute at Waganingen University that focuses on sustainable development, shows a set of maps illustrating the evolution of watery areas and polders in the country since the 14th century.

“Our problems are not so very different from the problems the people in the Middle Ages had,” he says. Even back then, techniques like building on mounds or widening the space between river dikes to accommodate flooding were well understood. The lesson? In a high-tech age, some of the effective adaptation approaches may come from a decidedly low-tech time.

“I think the people in the Netherlands have forgotten about those old ideas because they have relied on technological solutions,” he says. “Now they see that technical solutions don’t provide 100 percent safety. So perhaps we should think about the old solutions again.”

Peter N. Spotts

SPOTTS, PETER N. “HOW TO FIGHT A RISING SEA.” CHRISTIAN SCIENCE MONITOR (NOVEMBER 15, 2007).

See Also Climate Change; Global Warming; Intergovernmental Panel on Climate Change; IPCC 2007 Report; Oceans and Coastlines

BIBLIOGRAPHY

Books

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Alley, Richard B., et al. “Ice-Sheet and Sea-Level Changes.” Science 310 (2005): 456–460.

Dowdeswell, Julian A. “The Greenland Ice Sheet and Global Sea-Level Rise.” Science 311 (2006): 963–964.

Ericson, Jason P. "Effective Sea-Level Rise and Deltas: Causes of Change and Human Dimension Implications.” Global and Planetary Change 50 (2006): 63-82.

Gregory, J. M., and P. Huybrechts. “Ice-Sheet Contributions to Future Sea-Level Change.” Philosophical Transactions of the Royal Society A 364 (2006): 1709–1731.

Meehl, Gerald A., et al. “How Much More Global Warming and Sea Level Rise?” Science 307 (2005): 1769–1772.

Munk, Walter. “Twentieth Century Sea Level: An Enigma.” Proceedings of the National Academy of Sciences 99 (2002): 6550–6555.

Oppenheimer, Michael. “The Limits of Consensus.” Science 317 (2007): 1505–1506.

Overpeck, Jonathan T. “Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise.” Science 311 (2006): 1747-1,750.

Rahmstorf, Stefan. “A Semi-Empirical Approach to Projecting Future Sea-Level Rise.” Science 315 (2007): 368-370.

Shepherd, Andrew, and Duncan Wingham. “Recent Sea-Level Contributions of the Antarctic and Greenland Ice Sheets.” Science 315 (2007): 1529–1532.

Web Sites

Goddard Institute for Space Studies, NASA. “Coastal Populations, Topography, and Sea Level Rise.” http://www.giss.nasa.gov/research/briefs/gornitz_04 (accessed April 2, 2008).

National Aeronautics and Space Administration (NASA). “NASA Satellites Measure and Monitor Sea Level.” http://www.nasa.gov/home/hqnews/2005/jul/HQ_05175_sea_level_monitored.html (accessed April 2, 2008).

Woods Hole Oceanographic Institution. “Rising Sea Levels and Moving Shorelines.” http://www.whoi.edu/page.do?pid=12457&tid=282&cid=2484 (accessed April 2, 2008).

Larry Gilman