Rapidly growing cities are making more people vulnerable to rising sea levels. Two-thirds of the global population is expected to live in cities by 2050. Already an estimated 800 million people in more than 570 coastal cities are vulnerable to a sea-level rise of 0.5 metres by 2050.1
In a vicious circle, urbanization not only concentrates people and property in areas of potential damage and disruption, but it also exacerbates those risks—for example, by destroying natural sources of resilience such as coastal mangroves and increasing the strain on groundwater reserves. The risks of rising sea levels are often compounded by storm surges and increased rainfall intensity.
Some cities and countries started decades ago to put strategies in place to deal with accelerating sea-level rise. In the last 20 years, approaches have shifted notably towards supplementing “hard” engineering strategies with greater promotion of more “soft” nature- based approaches. In many cities, however, preparations are lagging and the need to take action is increasingly urgent.
The following sections set out the latest projections for sea-level rise, assess which parts of the world are likely to be hardest hit, and look at the potential impacts on human populations and urban infrastructure. The chapter then considers the adaptation strategies being pursued in a number of cities, highlighting the growing prevalence of holistic approaches to flood resilience.
Rising sea levels
If global warming continues at its current rate, the Intergovernmental Panel on Climate Change (IPCC) considers it likely that the rise in atmospheric temperature will reach 1.5°C degrees within the next 35 years.2 Forestalling this will require unprecedented action to drive decarbonization of agriculture, energy, industry and transport.3 It appears increasingly unlikely that the world will meet even the 2°C upper limit identified by the Paris Climate Agreement.4 The current trajectory is towards a rise of around 3.2°C.5
As global temperatures have increased, so sea levels have risen at an accelerating rate. According to the IPCC, the mean sea-level rise between 1901 and 2010 was 1.7 millimetres per year (mm/y). Between 1993 and 2010 it was 3.2 mm/y. Global sea levels will continue to rise through the 21st century and beyond, owing to increased oceanic warming and loss of glaciers and ice sheets. According to the IPCC, a 2°C increase will cause sea levels to rise between 0.30 metres and 0.93 metres by 2100.6 Other research suggests this rise could be as much as 2 metres even with warming below 2°C.7 Beyond 2100, it could eventually reach 6 metres.8 The uncertainty is due to the complex nature of the interaction of atmospheric warming, oceanic warming and ice-sheet responses: for example, the collapse of the West Antarctic Ice Sheet could push up sea levels by 3.3 metres.9
Global averages tell only part of the story. Sea-level rise will also vary regionally and locally: ice loss in the Antarctic, for example, is expected to have a disproportionate impact in the northern hemisphere, where most of the world’s coastal cities are located.10 Estimates suggest that 90% of coastal areas will experience above-average rise,11 with differentials of up to 30% relative to the mean.12
Relative sea-level rise will be even higher in the many cities that are sinking because of factors that include groundwater extraction and the growing weight of urban sprawl. Some cites are sinking faster than sea levels are rising: in parts of Jakarta, for example, ground level has sunk by 2.5 metres in the past decade.13 In addition, sea-level rise amplifies the impact of storm surges, as it takes a smaller surge to produce the same extreme water level.
Estimates suggest that 90% of coastal areas will experience above-average rise
Uncertainties surround the precise interactions of regional sea-level rise and patterns of urban demography and development. However, it is clear that Asia will be the worst- affected region as a result of a combination of hydrology, population density and asset concentration.14 Asia is home to four-fifths of the people who are expected to be flooded if there is a 3°C rise in global temperatures.15 China alone has more than 78 million people in low-elevation cities, a number increasing by 3% each year.16
The World Bank notes that 70% of the largest cities in Europe have areas vulnerable to rising sea levels.17 Africa has at least 19 vulnerable coastal cities with a population of more than 1 million, including Abidjan, Accra, Alexandria, Algiers, Casablanca, Dakar, Dar es Salaam, Douala, Durban, Lagos, Luanda, Maputo, Port Elizabeth and Tunis.18 In the United States, East Coast cities including Norfolk, Baltimore, Charleston, and Miami have already experienced “sunny day” flooding due to the rising sea levels.19 One study suggests that a sea-level rise of 0.9 metres by 2100 would expose 4.2 million people to flooding, while a rise of 1.8 metres over the same period would affect 13.1 million people—equivalent to 4% of the current population.20
Deltas are home to more than two-thirds of the world’s largest cities and 340 million people.21 These delta cities are particularly vulnerable to land subsidence. Relative sea-level rise poses the highest risks for the Krishna (India), Ganges-Brahmaputra (Bangladesh) and Brahmani (India) deltas.22 In Bangladesh, a rise of 0.5 metres would result in a loss of about 11% of the country’s land, displacing approximately 15 million people.23
S/Amit Dave
Potential damage
Sea-level rise threatens significant damage to property—not only homes and businesses but also public assets and critical infrastructure, which adds significant contingent liabilities to the taxpayer. Research suggests that economic impacts are highly concentrated geographically, where sea-level vulnerabilities interact with high-value property and infrastructure. Just four cities account for 43% of average annual losses: Guangzhou, Miami, New Orleans and New York.24 The researchers note that because “coastal flood risks are highly concentrated, flood reduction actions in a few locations could be very cost-effective.”25
Existing protection already reduces these losses significantly. The same research compares cities’ recorded average annual losses with their expected exposure to a 100-year flood event—that is, a flood with a severity that would be statistically expected once every century. The results vary hugely. For example, Amsterdam’s exposure to a 100-year flood event is more than double that of Guangzhou—an estimated US$83 billion versus US$38.5 billion. But the strength of Amsterdam’s protection means its average annual losses to date are just US$3 million, compared with US$687 million for Guangzhou.26
In the United States, a study found that between 2005 and 2017 sea- level rise wiped US$14.1 billion off home values in Connecticut, Florida, Georgia, New Jersey, New York, North Carolina, South Carolina and Virginia.27 In developing countries, the threat to property is often exacerbated by coastal erosion as rising sea levels, sand- mining and built infrastructure disrupt the flow of coastal sediment. Some coastal communities in Sub- Saharan Africa are already being washed away, losing up to 30–35 metres of land each year, with thousands more at risk.28
A study by the UK National Oceanographic Centre projects the global cost of rising sea levels at US$14 trillion per year in 2100.29 It found that China would face the biggest costs in absolute terms, while as a percentage of GDP the impacts will be highest for Kuwait (24%), Bahrain (11%), the United Arab Emirates (9%) and Viet Nam (7%).30
More people will be crammed into shrinking tracts of habitable urban space
Various forms of infrastructure and economic activity are at risk from rising sea levels:
- Roads: A study of coastal roads on the US East Coast estimates that high tide flooding already causes 100 million vehicle-hours of delay every year, which could rise to 3.4 billion hours by 2100.31
- Railways: Researchers predict that a 4.5 kilometre stretch of coastal railway in the United Kingdom would be disrupted on 84 days each year with a 0.55 metre sea-level rise, and the line would cost hundreds of millions of pounds to divert.32
- Ports: The World Bank has identified 24 port cities in the Middle East and 19 in North Africa at particular risk of sea- level rise.33 Rising sea levels will lead to a greater frequency of disruptive events such as Hurricane Florence, which closed North Carolina’s port to trucks for 10 days in September 2018.34
- Internet: In the United States, more than 4,000 miles of underground fibre optic cable and 1,100 nodes are projected to be underwater within 15 years, with New York, Miami and Seattle at greatest risk.35 Unlike submarine internet cables, these are not designed to be waterproof.
- Sanitation: A 2018 study found that in the United States, a sea- level rise of just 30 centimetres will expose 60 wastewater treatment plants, which serve more than 4.1 million people.36 Water treatment facilities in Benin and other countries in West Africa are already threatened by the sea.37
- Drinking water: Pollution of aquifers will be exacerbated by declines in streamflow: by the 2050s, more than 650 million people in 500 cities are projected to face declines in freshwater availability of at least 10%.38 As rivers and streams contain some groundwater, salination could also affect surface- level fresh water.
- Energy: The C40 Cities initiative has identified 270 power plants that are vulnerable to a sea-level rise of 0.5 metres; these plants provide power to 450 million people mostly in Asia, Europe, and the east coast of North America.39
- Tourism: In many cities, coastal areas are a source of revenue from tourism and business. In Egypt, for example, the IPCC has estimated that a 0.5 metre rise in sea levels would destroy Alexandria’s beaches, leading to losses of US$32.5 billion.40
- Agriculture: Sea-level rise can lead to increased salination of soil and of water sources used for irrigation, particularly in delta regions. In Bangladesh, the World Bank estimates salination could cause a 15.6% decline in rice yield.41
In 2017, 18.8 million people were newly displaced by weather-related causes, including floods and coastal storms.42 The intensifying impact of sea-level rise on coastal cities and plains will render an increasing amount of land uninhabitable or economically unviable.
This is likely to lead to population movement within and from large cities. More people will be crammed into shrinking tracts of habitable urban space, and more are likely to move to other cities, either domestically or in other countries. These movements have the potential to cause spillover risks—for example, they could result in heightened strain on food and water supplies and in increased societal, economic and even security pressures. According to the World Bank, climate change could force 86 million people in Sub-Saharan Africa, 40 million in South Asia and 17 million in Latin America to permanently relocate internally by 2050.43
Coastal adaptation
Cities faced with the risk of damage from rising sea levels can adapt either by trying to keep water out or learning to live with water at higher levels. Some strategies and technologies are new, but the basic idea is not: “[C]oastal societies have a long history of adapting to environmental change and local sea-level rise because coasts are amongst the most dynamic environments on Earth. For example, a number of coastal megacities in river deltas have experienced, and adapted to, relative sea-level rise of several metres caused by land subsidence during the twentieth century.”44
There are three main strategies. The first involves “hard” engineering projects to keep water out of cities, such as sea walls, storm-surge barriers, water pumps and overflow chambers. The second involves nature-based defences—for example, conserving or restoring mangroves and salt marshes—or seeking to shape how floods will affect cities, rather than always trying to prevent them. The third strategy involves people—for example, moving households and businesses to safer ground, or investing in social capital to make flood-risk communities more resilient. An appropriate mix of coastal adaptation measures can potentially “reduce some coastal impacts by several orders of magnitude.”45
Spending on disaster recovery is almost nine times higher than on prevention
The Netherlands is at the forefront of coastal adaptation because of its existential exposure to rising sea levels—two-thirds of the country is vulnerable to flooding. The importance of water management is recognized in the country’s administrative structures—regional water boards levy their own taxes for flood protection rather than depending on government.46 The Netherlands pursues a mix of the three strategies. Its highly developed hard infrastructure includes an extensive system of dikes and the world’s largest storm-surge barrier. However, inland floods in the early 1990s, in which 200,000 people were evacuated, led to a shift of approach. Instead of continuing to build ever-higher dikes—which means greater damage is done if they are breached—the “room for the river” programme lowered some dikes to allow farmland to be inundated in flood events to protect towns. Farmhouses in affected areas were demolished and families moved to new homes built on artificially created mounds, 8 metres high.47
In Rotterdam—where 90% of land is beneath sea level—a programme called “the Sand Engine” involved dredging sediment from the North Sea and depositing it off the city’s shore to prevent waves from eroding the coastline.48 Rotterdam is also home to numerous urban water innovations, such as floating houses and city squares designed to collect millions of litres of water in flood conditions.
REUTERS/Lucas Jackson
Managed retreat
Like the Netherlands, China’s approach to flood management changed in the 1990s in response to major flooding. The 1998 Yangtze River Basin floods killed 4,000 people and prompted a shift away from reliance on hard infrastructure projects. Nature- based measures were prioritized and more than 2 million people were relocated to higher ground.49 However, the rapid pace of urbanization has continued to increase flooding risks in many coastal areas by destroying natural flood defences: in Shenzhen, for example, around 70% of mangrove coverage has been destroyed.50 In 2015 a new “sponge city” initiative was launched to offset this process by introducing urban features such as permeable pavements, new wetland areas and green roofs; the 30 cities in the programme include Shanghai, which is particularly vulnerable to sea-level rise. The target is for 80% of urban land to be able to absorb or re-use 70% of stormwater by 2030.51
Many cities and countries have struggled to cope with the mounting challenges posed by rising sea levels. In Indonesia, Jakarta is building a massive sea wall—with Dutch help—and has also launched a five-year project to relocate around 400,000 people away from riverbanks and reservoirs under threat from rising sea levels.52 However, some critics argue that the authorities should also be doing more to prevent the city from sinking.53 This debate over the right course of action highlights the institutional complexity of getting flood management right: often success depends on legacy infrastructure issues that are hugely expensive to resolve. Jakarta’s system of water pipes reaches only one-third of residents, leaving two-thirds reliant on the groundwater extraction that is weakening the city’s foundations.54
In Thailand, Bangkok is low lying and sinking, its natural coastal defences have been eroded, and the nearby Gulf of Thailand is rising faster than the global average.55 Bangkok’s surface area is also one of the world’s most impervious—it averages just 3.3 square metres of green space per resident, compared with 66 square metres in Singapore.56 Extreme weather patterns are intensifying, leaving the city vulnerable to rising sea levels from the south and increasingly severe monsoon rains from the north.57 The government’s response includes constructing a 2,600 kilometre canal network, as well as a central park that can drain 4 million litres into underground containers.58
In 2011, severe flooding in Bangkok prompted some authorities to suggest moving the capital city.59 The idea of “managed retreat” is likely to become an increasingly familiar feature of adaptation plans as sea levels rise and extreme weather intensifies. One study identifies 27 cases across 22 countries that have already occurred.60 Elsewhere, plans are in preparation. The Maldives intends to build artificial islands, fortified with 3 metre high sea walls and financed by renting out islands and boosting tourism.61 In the Pacific Ocean, Kiribati has purchased land in Fiji as a potential new home for its citizens. And in the United States, US$48 million has been allocated to relocate the entire community of the Isle de Jean Charles in Louisiana, which has lost 98% of its land since 1955.62 The complex task of resettling these residents while keeping their sense of community will serve as a test case for the future.
No time to waste
As sea levels rise and urban vulnerabilities increase, the urgency of the need to respond to these changes is going to intensify. Beyond adaptation measures, addressing urban vulnerability to sea-level rise will require households, businesses and governments to avoid exacerbating dangers. There is little point putting new flood defences in place, for example, if existing defences are undermined through continued development of homes and businesses in coastal areas and on floodplains.
The affordability of flood resilience is set to become an increasingly important issue. Robust risk financing strategies will be required, both to fund investment in adaptation and to pay for recovery when floods occur. At present, spending on recovery is almost nine times higher than on prevention.63 Turning that around will not be easy: building support for pre-emptive spending and action—particularly if it involves major disruptions such as relocation—can take many years of dialogue and planning. There is no time to waste.
As adaptation becomes more costly, questions of burden-sharing will arise—for example, between the public and private sectors, and between municipal and national authorities. Burden-sharing may also be needed between countries. Failure to prepare for sea-level rise will create cross-border spillovers, and some of the cities most at risk are in countries that may struggle to find the resources to adapt. Innovative and collaborative approaches may be needed to ensure that action is taken globally before it is too late.