Ten percent of the world’s approximately 7.5 billion people live within 10 meters of sea level, and many more live at higher elevations but close to coastlines. Protecting people, structures, and property as sea level continues to rise in the years ahead will be one of the great mega-engineering challenges of the 21st century and beyond, said Robert J. Nicholls, professor of coastal engineering at the University of Southampton, during the 2016 annual meeting of the National Academy of Engineering.
Sea level changes, as measured by tide gauges, vary from place to place. In Jacksonville, Florida, sea level rose at an average rate of 2.08 millimeters per year during the 20th century, or about 20 centimeters—equivalent to about 8 inches—over those hundred years. This “is a significant change,” said Nicholls, and “it is going to keep on going.” He cited similar developments along the East Coast: in Norfolk, Virginia, sea level rose at an average of 4.6 millimeters per year, and on Grand Isle, Louisiana, approximately 9 millimeters per year, or about 90 centimeters—3 feet—per century. In Juneau, Alaska, in contrast, sea level has been falling. The same variability can be found all around the world, Nicholls observed.
As these data demonstrate, sea level changes are determined by more than the increasing height of the world’s oceans. They are the result of several components, such as vertical movements of the land and changes in ocean currents from such phenomena as El Niño.
In Alaska, for example, the land is rebounding after being pushed down by the weight of glaciers during the last ice age. In other places, such as the cities of Bangkok and Vancouver, which are built on deltas, the land is subsiding because of groundwater extraction. In some places,
like Japan, earthquakes have caused the land to move up or down in abrupt steps. “Whenever you think about sea level, you have to take into account this local context,” Nicholls said.
To illustrate the effects of subsidence, Nicholls pointed to Tokyo, which sank as much as 4.4 meters in the 20th century. Changes to groundwater extraction practices have greatly reduced the sinking, “but once you have subsided, you cannot get it back.” Today, 2 million people in Tokyo live beneath high tide elevations because the land has sunk, said Nicholls. Especially where subsidence has been extensive, such as in the area around Tokyo’s docks, continued habitation relies on a major flood defense infrastructure.
In addition to deltas, small islands are particularly vulnerable to sea level rise, explained Nicholls. The Maldives, for example, is the lowest country on earth. It essentially consists of wave-formed coral atolls. Even a small change in sea level would make such islands dangerous places to live. “Forced abandonment is a very real possibility.”
Just a few million people live on small and vulnerable islands. But hundreds of millions of people live on deltas. Nicholls acknowledged that not all deltas are threatened by sea level rise, “but we are talking about very large populations living very close to sea level.” Small islands can be built up, as China is doing in the South China Sea. Perhaps deltas could make greater use of the immense sediment loads carried by the rivers that created them. “We have traditionally treated sediment like a pollutant,” said Nicholls. “We put up dikes, we washed it out to the sea. Can you [instead] use that sediment to build land and build elevation?”
Historical data suggest that sea level was relatively flat for several thousand years before the 19th century, when data from a variety of sources indicate that it began to rise. Global sea level rose about 17 centimeters during the 20th century, said Nicholls. Most recently, global measurements from satellites have calculated that it is rising at about 3.2 millimeters per year, with an acceleration since 1993.
The Intergovernmental Panel on Climate Change (IPCC) has estimated that sea level could rise between approximately 0.2 and 1 meter by 2100 (an estimate that includes the uncertainty incorporated in the IPCC’s projections).1 The actual amount will depend partly on how much the Earth warms during that period due to greenhouse gas emissions into the atmosphere. But Nicholls cautioned that, even if emissions stabilize, sea level will continue to rise by 30 to 40 centimeters, which is “enough to cause significant problems.”
Another critical factor in sea level rise is how warming temperatures will affect the melting of land-based ice. Particularly important are the large ice sheets on Greenland and Antarctica, including the more susceptible ice sheet on West Antarctica. “If any of that melts, that could have a significant effect on global sea level.”
Yet even these qualifications do not capture the full range of uncertainty, said Nicholls. He often talks with people and organizations about much larger and faster sea level rises than those projected by the IPCC.
1 Parry ML, Canziani OF, Palutikof JP, van der Linden PJ, Hanson CE, eds. 2007. Climate Change 2007: Impacts, Adaptation and Vulnerability—Appendix I: Glossary. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.
“How much might sea level rise? What is the high-end risk that we face? We do not understand that as well as we would like.”
Sea level rise has complex interrelated impacts, Nicholls explained. To begin with, it makes extreme events more extreme. Storms can cause higher levels of inundation both from surging ocean waters and from inland flooding. Erosion, sediment supply, flood management, and reclamation can all affect storm surges, while catchment management and land use can affect flooding.
Sea level rise can also cause wetland loss and change, saltwater intrusion into surface waters or groundwater, and higher water tables that impede drainage—and these impacts can be interconnected. For example, changes in coastal ecosystems such as mangrove forests and coral reefs can affect wave action, storm surges, and erosion.
Coastal development increases vulnerability to sea level rise. The population living at or near sea level has grown dramatically, with an associated increase in infrastructure. In many places, such as the East and Gulf coasts of the United States, nuisance flooding has become much more common because of the combination of sea level rise and development.
“We are increasingly seeing floods occurring under what you might call normal or blue sky conditions,” Nicholls said. Places like Atlantic City might have water in the streets 30 or 40 days per year, whereas in the 1960s that would have happened only 1 or 2 days a year. “A flood is the sum of the mean, the tide, and anything the weather is doing . . . sea level rise [reduces] the threshold that will cause events.”
Beyond nuisance flooding are major floods such as the ones that killed hundreds of people in the Netherlands in 1953, Germany and England in 1962, and New Orleans in 2005. “These are extreme events, but they make the point that as sea level rises, you need a smaller extreme event to have the same effects,” Nicholls said. Development puts more people in harm’s way and will continue to do so as the residential and commercial development of coastal areas continues.
Sea level rise does not create entirely new problems. Rather, it is “showing you the problems you already have,” Nicholls observed. “If you are getting flooded by sea
level rise, you are probably flood prone today. It also allows you to assess your current preparedness” to deal with sea level rise, “a very useful reflection on today’s risks,…which is as important as—and maybe more important than—the issue of sea level rise.”
An IPCC subgroup that tried to find any positive effects of higher sea levels was unsuccessful. But Nicholls observed that the effects can be mitigated. On the southern coast of England, where he works, the number of events that could cause floods has increased—but the actual number of floods has not, and in fact has possibly decreased.
Investments in flood defense, such as mobile gates that can be closed when water levels are high, can reduce flooding with relatively modest expenditures. “They have to depend on warnings to be operated, but they are reducing the frequency of flooding in areas that [previously] flooded very frequently.”
Some have argued that land values are going to fall as more people think about the consequences of sea level rise, making it easier to buy out those properties and move occupants to higher ground. Nicholls considers this “an interesting thesis,” but especially with more valuable property, adaptation is the more likely prospect. “It is going to come down to economic kinds of questions.”
Approaches to Adaptation
According to the IPCC, adaptation consists of “adjustment in natural or human systems in response to actual or expected climatic stimuli or their effects, which moderates harm or exploits beneficial opportunities.” It can involve a planned retreat from the coast, accommodation of higher sea levels (e.g., elevation of structures above flood levels), or protection from the sea (e.g., dikes).
Large areas of the Netherlands, for example, are below mean sea level. They are kept dry by a large system of ring dikes and other flood control measures, such as minimization of subsidence. In contrast, large parts of New Orleans are also below sea level, but subsidence is continuing, so when floods occur they are deeper, cause more damage, and pose more of a threat to life. “Trying to minimize subsidence is quite an important thing to do.”
Other steps can reinforce and complement adaptation:
- It is important to educate the public about the problem.
- Monitoring and evaluation can help determine whether goals are being met.
- Informatics and modeling can produce better warnings and evacuations.
- Insurance can cover worst-case losses.
Many kinds of actions can “get us to a better place,” said Nicholls.
An important point, he added, is that “adaptation is a process.” Because of the momentum inherent in the climate system, humans will need to continue to adapt to high sea levels even if global temperatures are stabilized at a certain level.
“There is no magic silver bullet here that you can [use to] solve this problem and walk away. The time scales of sea level rise seem to be very long. We expect it to rise for the 21st century, and probably the 22nd century and beyond—beyond any kind of timeframe we think about. We can expect that our children and grandchildren will have to do more than we have done.”
Flood Protection on the Thames
Another example of adaptation that Nicholls described is the system of flood barriers in place to protect London. When a storm tide is forecast in the North Sea, floodgates in the Thames River are closed to keep London dry. They are part of a much larger system of defenses against flooding, including 337 kilometers of dikes.
Many of these protections already take higher sea levels into account. The Thames floodgates, for example, were built 50 centimeters (about 20 inches) higher than would be needed if sea level were constant. However, adapting the overall system as sea level continues to rise will require improving all the flood control systems on the Thames even as the original structures are aging.
One possible approach to future sea level rise is to make major investments to protect against events that might not happen or might happen more slowly than expected. A better option, according to
Nicholls, is adaptive management that responds on an ongoing basis to rising sea level and increasing risk. Under this approach, “we only do things when we actually need to,” he said.
Nicholls was once asked by a government agency in the United Kingdom by how much sea level would have to go up to force drastic actions, such as the abandonment of coastal cities. “They were expecting a number like 80 centimeters, [but] that misses the whole point about how we can adapt,” he said.
“I do not think there are any physical or technical limits at the present time to adaptation as long as you have enough resources.… If you really wanted a wall 40 meters high, we could build it. The question is, Do you want that wall? Probably you do not. But it could be provided if you did.”
As sea level continues to rise, the choices for protecting vulnerable areas become starker. For example, the amount of sea level rise that would require unavoidable choices between building massive and expensive new barriers or abandoning parts of London is about 3 meters, Nicholls said, which is above the levels projected for the 21st century. “But the science can change,” he added. “These numbers can move.”
Under an adaptive management response, if sea level rise is overestimated, investments can be cut back, and if it is underestimated they can be increased. One challenge is to link sea level projections with the investments needed to protect coastlines to give officials enough time to respond. A project in Southampton is working to do just that, Nicholls said.
Economic and financial limits will be key issues. What can be afforded? Social and political limits, such as attitudes toward risk and confidence, also will play a role, and may vary from one place to another. “What works in Europe may not work in the United States.”
Adaptation can be spontaneous or planned, private or public, proactive or reactive, or combinations of these. Adaptation options also can be combined into reinforcing bundles that are consistent with the idea of resilient coasts.
“Sometimes the solutions will be easy and cost [very little], so it is a no-brainer. Sometimes there may be real challenges. But at least you start to think about those challenges today so you can be more prepared for them if they do eventually emerge.”
With regard to proactive or reactive adaptations, Nicholls noted that “we are very good at responding to what we observe,” whereas being proactive rather than reactive remains a major challenge. “How do we deploy all our technical knowledge and understanding in a joined-up manner? We still tend to move toward a reactive approach.”
Simulations can provide information about disasters without having to experience them in real life. The Delta Commission in the Netherlands and the Thames Estuary 2100 project looked into the distant future and considered major changes in sea level and the built environment. “They argued for intelligent defense,” said Nicholls. “It was not about just pulling back and retreating. It was often about intelligent attack and recognizing that adaptation is never completed. It is a process.”
Another important step would be to conduct major planning studies for all coastal cities and other threatened locations around the world. “If you ignore sea level rise, it will come back to bite you,” Nicholls concluded on a cautious note.
The US Army Corps of Engineers has seen structures that were built in the 1960s lose a foot due to sea level rise, reported Kathleen D. White, a civil engineer who leads the Corps’s Climate Preparedness and Resilience Community of Practice. The resulting chronic flooding is more than a nuisance, she said. “You lose your transportation links; there are…public health and safety issues. You can’t get the police, emergency medical [personnel]. You can’t evacuate.”
A major part of the Corps of Engineers’ work involves making sure that the country’s ports and commercial navigation system function in all kinds of situations—and this infrastructure is typically right at sea level. The Corps also protects and restores ecosystems such as the Everglades, where saltwater intrusion is decimating native species and allowing invasive species to spread. It is charged with protecting threatened and endangered species, which often requires stopping the loss of habitat. It seeks to reduce the risk of coastal storms, including the risk of shoreline erosion. And it supports the Department of Defense and other services,
many of which are also at sea level. All of these responsibilities are complicated by sea level rise, said White. “We need to manage them. That is what engineering is about.”
The Corps of Engineers’ Resilience Initiative seeks to use engineering to prepare for threatening events. Its goals are to prepare both the infrastructure and the people around it to absorb disruptions as much as possible and recover quickly from an event. Adaptation “is not rebuilding,” said White. “It is moving forward to a more resilient, better prepared,…and easier to recover” state.
To increase resilience, the Corps uses infrastructure such as coastal levees, storm surge barriers, seawalls and revetments, and detached breakwaters. It also uses natural and nature-based features to attenuate the wave energy that reaches the shore and causes damage; these include engineered dunes and beaches, maritime forests and shrub communities, barrier islands, oyster and coral reefs, and vegetated features. One difficulty with these natural features, according to White, is that they often lack engineering reliability and performance information, making it hard to use them to achieve a reliable degree of risk reduction.
“We can’t engineer our way out of every challenge,” she acknowledged, whether because a solution is impracticable or economically or
politically unviable. In these cases, policy and management are required. Relocation, flood proofing, and impact reduction are among possible solutions to deal with risk. Engineers can support this work by identifying where events will occur in the future and the kinds of policies and management that can make a difference.
With regard to relocation, for example, “typically, the American response is, ‘No, I am going to stand right here. I am going to rebuild. I am going to do the same thing I always have. I am going to hold the line.’” But holding fast is not always the right solution, said White, and piecemeal approaches can make things worse. The relocation of houses or businesses one by one can lead to public safety issues, difficulties supplying power or transportation, and other problems.
In contrast, the relocation of entire communities can keep them intact. White pointed to the Biloxi-Chitimacha-Choctaw tribe in Isle de Jean Charles, Louisiana, which recently received a Disaster Resilience Grant from the US Department of Housing and Urban Development (HUD) to relocate away from flood-prone areas. In Kivalina, Alaska, a new Corps of Engineers revetment has given the community about 15 more years before it has to move. And the Quinault Indian Nation in Washington state is moving a community from an exposed beach to a higher area, where the new residents are designing their new community to be what they want it to be.
“These people are making a more sustainable home that pays homage to their culture and allows them to thrive in the 21st century rather than spending a lot of time recovering from different kinds of events,” said White.
The idea of being proactive has taken hold in the Corps, especially since Hurricane Katrina, said White. After Katrina, a series of reports called for the Corps and other government agencies to be prepared for foreseeable future conditions such as drought, intense rainfall, and sea level rise. The Corps has developed policies and technical guidance to do that, and it has sought to incorporate those policies and technical guidance into project planning. “To the extent that we can, as a federal agency that is authorized and appropriated by Congress, we are trying to make our approach more proactive.”
Rear Admiral Bret Muilenburg, commander of Naval Facilities Engineering Command and chief of civil engineers, is responsible for 22,000 personnel around the world who take care of Navy and Marine Corps infrastructure at over 100 major installations. They plan, design, build, and maintain facilities and deliver environmental, utility, and other services.
“Sea level rise poses a real and unconventional threat to us and our installations,” he said. “We must continue to take measures that we know and understand today while accelerating our efforts to learn more about what our next steps must be.”
The Navy protects and defends America, its allies, and its partners from harm around the world. With its Coast Guard and international partners, it ensures that the United States has access to international waters, keeps shipping lanes open, provides humanitarian and disaster relief, deters aggression, and encourages peaceful conflict resolution in accordance with international law and norms. “Should that fail, your Navy is ready to fight,” said Muilenburg. “Think of your Navy as America’s away team, where we never want a home game.”
As an example of the extent of the Navy’s responsibilities, Muilenburg cited the one quarter of US jobs—38 million—that are directly or indi-
rectly tied to international trade. He also noted that 95 percent of all international phone and internet traffic travels via underwater cables. “The Navy, operating for and with our partners, ensures this free flow of goods and communication and that it remains unimpeded.”
The Navy has installations around the world that mostly are located at sea level, and many of its coastal facilities will be dramatically affected by sea level rise, Muilenburg reported. They are exposed to flooding, storm surge, erosion, and saltwater intrusion. “We have seen these effects already. We are believers. We understand what we are facing, to a degree. We need to understand a lot more.”
He mentioned a recent study by the Union of Concerned Scientists that studied 18 military installations on the East Coast and the Gulf of Mexico, 12 of which were Navy facilities. “It causes me great concern,” said Muilenburg. By the end of the century, 70–95 percent of the Naval Air Station Key West is projected to be inundated with daily flooding.
Muilenburg said that he “would love to be able to engineer our way out of this with your help,” but doing so requires narrowing the uncertainty associated with sea level rise. “Are we talking about 3 feet? Are we talking about 6 feet? That is very important for all of us engineers to understand. The better the science, the better the decision making and the better that I can take care of…your tax dollars.”
Today the Navy is improving both its knowledge—for example, through vulnerability assessments of its bases—and risk management plans on how to prepare for and respond to events. It is mitigating hazards for current installations, and has improved its design and construction criteria. “We have a lot of infrastructure and a lot invested in our installations. It is not practical to start over in very many places. We must protect what we have through means that we understand.”
He acknowledged that there is much more the Navy should do. First, it needs to work with the communities surrounding its installations. “Water does not honor the fenceline. It affects the community.” Multidisciplinary coordination and cooperation will be critical to tackling this problem, he said. That is happening, but the effort is “in its infancy,” he said. “With this challenge, there is no rest for the weary.”
Muilenburg also pointed out that in the future the Navy may be responding to more crises because of increased flooding, making it all
the more important to have resilient bases so that the Navy can launch responses from them. Already it is responding to more humanitarian crises and disaster relief, “whether it is…the northern Japan tsunami, Philippine typhoon, or Indonesia earthquake.” To fulfill these obligations, the military has to be forward deployed. “You can’t do it from Norfolk and San Diego.”
One of the great benefits of the Navy, he said, is to be “out there every day—sailors and marines are ready to perform those missions.” But fulfilling its responsibilities also requires allies and partners. No nation can do it alone. “It takes a network of like-minded navies and nations that are performing this mission together.”
The electric sector tends to think about sea level rise as part of the broader topic of climate change, said David Pearce, manager of Manhattan operations and engineering at Consolidated Edison. Climate change affects individual power plants, the distribution infrastructure, and electric power generation. In the future, higher temperatures will be an issue for utilities everywhere as the demand for electricity surges. Droughts could lead to less hydroelectric power generation.
As with sea level rise, impacts vary based on location and topography. When Hurricane Sandy came through New York in 2012, what was
projected to be the impact of a 100-year storm turned out to be that of a 500-year storm. A major generating station was lost for a week, which put more than half a million people in Manhattan in the dark.
“When you lose electricity in this day and age, it is not simply a matter of, ‘Well, I can’t turn on my television, I can’t turn on my radio.’ In a city, it is pumping water up to higher floors. It is people [having] to walk up and down multiple stairs. Gas stations couldn’t pump gasoline, so the transportation system fell apart. The cell phone towers ran out of power, so communication fell apart.”
The ripple effect of the city losing electricity had widespread and dramatic impacts. And these impacts in cities close to the sea are different from those elsewhere, Pearce observed.
Utilities have an array of mitigation strategies to counter the effects of climate change, including—for flooding—barriers, relocation, and hardening of the infrastructure. In response to Hurricane Sandy, Pearce reported, equipment that was low to the ground was elevated or relocated, equipment that could not be elevated was hardened, and existing civil infrastructure was retrofitted to prepare for storms.
As in other sectors, collaboration is essential. In some cases a city or state may put up a barrier that protects utility equipment, while in other cases the utility itself may have to take action.
Utilities also have a role in altering the overall trajectory of climate change, according to Pearce. In New York state, the Public Service Commission is leading the renewable energy division, which is setting targets for greenhouse gas reductions and the architecture of the electric grid.
As an industry, the electricity generating sector has promoted energy efficiency and end-use management and invested in low-carbon or carbonless technologies for electricity generation. New technologies in building sciences and in architecture have significantly reduced the use of electricity per person and per square foot in Manhattan. All these steps and more will be necessary to counter the effects of climate change.
The Netherlands has been fighting back the sea for more than a thousand years, said Bart de Jong, the country’s counselor for infrastructure and the environment. Today, a quarter of the country—with all its major population centers and 60 percent of its population—is below sea level. Another 60 percent of the country is prone to flooding. One-third of the coastline is defended by natural dunes, the other two-thirds by dikes.
Much of the Netherlands is an alluvial plain—consisting of the delta of the Rhine, Scheldt, and Meuse rivers—that is susceptible to flooding and subsidence. A sizable portion of the country is also built on peat and clay, which tends to subside when the land is cultivated. The stereotypical image of the Netherlands as a country dotted with windmills to keep its fields dry is a product of this geohistory.
Flooding became more pronounced in the 20th century, with severe floods in 1916, 1926, and 1953. The country’s first reaction was to build more and higher dams, levees, and dikes. Thus the Delta Project—which began after the flood of 1953 caused 1,800 deaths—cut off all the estuaries in the southwest of the country, and river levees elsewhere were continually heightened.
Sea level rise is clearly a severe challenge for the Netherlands. More extreme storms are expected to increase erosion of the dunes protect-
ing the coastline. Both increased river discharges expected in a warmer world and decreased river discharges during times of drought could cause problems. Furthermore, the country is sinking because of subsidence and because melting ice caps in Scandinavia and Greenland are causing the newly unburdened land to rebound and the Netherlands to lower. Finally, the country’s population is growing, and the Netherlands is already one of the most populous countries in the world.
In the 1990s threats of severe river flooding led the country to begin rethinking its flood policy. Instead of flood protection, policies began to emphasize flood management. “We had been building dams and levees and dikes and pumping water out. Suddenly, we realized there is no end to that. You can’t just keep raising your dikes forever and ever. We realized we had to accommodate water.”
One way to accommodate water is to open up more space for river overflows. To illustrate, de Jong showed an old branch of the Rhine River that has been reopened, giving the river a new place to flow during floods. Similar actions have been taken in many other places in the country, he said.
Another adaptation has been to create places in cities where excess water can pool without causing damage. Areas along canals and in city squares can be engineered to be covered by water during floods and then drained when floodwaters recede. Underground parking garages and other structures have been built with massive water basins to hold floodwaters. Houses, greenhouses, and other structures can be built to float on water and not be threatened by flooding, an approach that is “definitely worth exploring further.” Through careful planning, a multilayered approach can yield multiple and overlapping protections.
In addition, working with the US Army Corps of Engineers and other US agencies, the Netherlands has been taking steps to improve crisis management as a way to limit both the risks of floods and the consequences of flooding. The government is also raising public awareness about flooding and how to respond to floods. Residents can now enter their postal code in an app to find out whether they are in a flood-prone area and, if so, how they should prepare. And the country is prepared for large-scale evacuations—it has acknowledged the necessity of retreating from areas that are no longer considered viable for habitation.