The warning signs are increasingly hard to ignore. Sea-level rise is real, displacing thousands of people, destroying millions of acres of land and generating billions of dollars in losses. Due to competing predictions of future global temperatures, scientists are unsure exactly how fast or high sea levels will rise. But they all agree on its principle impacts: submergence and flooding of coastal land, saltwater intrusion into surface waters and groundwater, increased erosion and overwhelmingly negative social and economic repercussions. They are also emphatic that these effects will be widespread and will accelerate with time.

Rising temperatures, rising seas

The conservative scientific consensus is that a 1.5°C increase in global temperature will generate a global sea-level rise of between 1.7 and 3.2 feet by 2100. Even if we collectively manage to keep global temperatures from rising to 2°C, by 2050 at least 570 cities and some 800 million people will be exposed to rising seas and storm surges. And it is not just people and real estate that are at risk, but roads, railways, ports, underwater internet cables, farmland, sanitation and drinking water pipelines and reservoirs, and even mass transit systems. While some coastal cities and nations will literally disappear, the rest will need to adapt, and quickly.

A sizeable number of coastal cities have yet to adequately prepare for rising sea levels. This is dangerous. As the World Economic Forum's Global Risk Report 2019 shows, around 90% of all coastal areas will be affected to varying degrees. Some cities will experience sea-level rises as high as 30% above the global mean. Making matters worse, sprawling cities are sinking at the same time as sea waters seep in. This is due to the sheer weight of growing cities, combined with the groundwater extracted by their residents. In parts of Jakarta, a city of 9.6 million people, the ground has sunk 2.5 metres in less than a decade. Sea levels have simultaneously risen by 10 feet over the past 30 years.

While all coastal cities will be affected by sea-level rises, some will be hit much harder than others. Asian cities will be particularly badly affected. About four out of every five people impacted by sea-level rise by 2050 will live in East or South East Asia. US cities, especially those on the East and Gulf coasts, are similarly vulnerable. More than 90 US coastal cities are already experiencing chronic flooding – a number that is expected to double by 2030. Meanwhile, about three-quarters of all European cities will be affected by rising sea levels, especially in the Netherlands, Spain and Italy. Africa is also highly threatened, due to rapid urbanization in coastal cities and the crowding of poor populations in informal settlements along the coast. The coming decades will be marked by the rise of ex-cities and climate migrants.

So-called “delta cities” are already bearing the brunt of rising seas. More than 340 million people live in deltas like Dhaka, Guangzhou, Ho Chi Minh City, Hong Kong, Manila, Melbourne, Miami, New Orleans, New York, Rotterdam, Tokyo and Venice. What a difference a few centuries makes. Over the past few thousand years, the 48 major coastal deltas in the Americas, Europe and Asia formed ideal sites for cities to thrive, owing to their access to the sea and fertile farmland. This explains why the Nile, Indus, Ganges, Brahmaputra and Yangtze served as cradles of major civilizations. But coastal living is becoming a liability: the costs of sea-level rise could rise to trillions of dollars a year in damages by 2100.

Seeing solutions to sea-level rise

A growing number of cities are stepping up to the challenge of sea-level rise. Most of them literally have no choice. Alongside mitigating their carbon footprints through reducing emissions, there are basically three ways that states and cities are taking action. First, they are fielding hard engineering projects like sea walls, surge barriers, water pumps and overflow chambers to keep water out. Second, they are adopting environmental approaches involving land recovery and the restoration of mangroves and wetlands to help cities cope with floodwater inundation. The third strategy involves people-oriented measures including urban design, building resilience and retreating after all other options have been exhausted.

The good news is that coastal cities are not starting from scratch – most of them have deep stores of knowledge and expertise. For centuries, cities bordering oceans and waterways have had to contend with local sea-level fluctuations and periodic storms. Many coastal cities have experimented with a combination of all three types of measures for hundreds of years. But past successes do not necessarily guarantee future safety. Today’s cities are different from their predecessors. Many of them are of an unprecedented size and complexity. Complicating matters, sea levels are rising more rapidly than in the past, in some cases overwhelming local capacities to respond.

A growing number of wealthy states and cities are making massive investments in technical solutions to keep seas at bay. It is true that large infrastructure schemes including barriers and break-walls can at least temporarily reduce the risks of losses. But an overreliance on concrete walls and pump systems to beat back rising tides, storm surges and downstream floods can only go so far. The lesson from the most successful cities is that a combination of approaches is essential. What is more, environmental-based solutions to reinforce the existing ecology’s protective capacities are not only effective, but lower cost. One country that has pioneered these multi-prong measures is the Netherlands.

A Dutch model for coastal adaptation

Dutch coastal cities are combining all three approaches to manage persistent sea level rises. They have good reason to be proactive given that over a quarter of the country is below sea-level. The national government has already decentralized many aspects of water management: flood protection is the responsibility of regional water management boards. Public authorities have also bolstered hard defences including a 3,700km network of dikes, dams and seawalls, including the famous Maeslant Barrier. Built to protect Rotterdam, which is 90% below sea level, the Barrier is the size of two Eiffel Towers on their sides.

Cities like Rotterdam offer a model for how to manage sea-level rise. Rotterdam is one of the safest delta cities in the world precisely because it has learned to live with water. This attitude can be traced back to the 13th century, when local merchants and city administrators erected a 400-metre dam to keep high waters at bay, but also to facilitate drainage. New canals were built in the 1850s to improve water quality and reduce epidemic outbreaks of cholera. Several decades after catastrophic floods killed over 1,800 people in 1953, the Maesland Barrier was constructed. Today, it protects the city’s 1.5 million people from floods with no impediments to sea traffic.

A key ingredient of Rotterdam’s success is attitude. The current mayor, Ahmed Aboutaleb, claims his city’s residents "do not view climate change as a threat, but rather as an opportunity to make the city more resilient, more attractive and economically stronger”. In the mayor’s view, climate adaptation is a window of opportunity to upgrade infrastructure, increase biodiversity and more meaningfully engage citizens in city life. A few years ago, the city launched a Climate Change Adaptation Strategy to make Rotterdam “climate proof” by 2025. Across the Netherlands, cities like Rotterdam are converting ponds, garages, parks and plazas into part-time reservoirs. They’re also revitalizing neighborhoods and improving equity to build social resilience to future water threats.

A Chinese way to mitigating rising seas

Chinese cities are also taking action to mitigate and adapt to sea-level rise. As in the case of the Netherlands, the Chinese were motivated in part by disaster. In 1998, floods killed roughly 4,000 people when the Yangtze River basin overflowed. A growing number of big cities such as Beijing – which more than doubled its total land coverage in the last decade – are also suffering a rise in floods. Today, roughly 641 of China’s 654 largest cities are affected by regular flooding, especially those on the coast. The Chinese government has responded with a combination of hard engineering, environmental and people-based strategies, together with the relocation of millions of citizens.

A student walks to school along a flooded street in Jakarta, Indonesia, February 2018.
A student walks to school along a flooded street in Jakarta, Indonesia, February 2018.
Image: Reuters/Beawiharta

In 2014, China launched the so-called sponge city initiative. The term actually originated in Hyderabad when the city authorities started collecting storm-water to offset water demand during planting season. Likewise, Vinh in Vietnam also adopted a "city as sponge” strategy to lessen the impacts of seasonal floods on vulnerable urban areas. In the case of China, the sponge strategy requires that 80% of all urban land is able to absorb or reuse 70% of storm-water. More than 30 cities are currently part of the initiative including Shanghai – one of the most flood-prone cities in the world. The Chinese expect that at least another 600 cities will join in the coming decade.

Shanghai’s authorities are putting enormous stock in adaptation strategies. And not without good reason – by 2050, the city is expected to experience flooding and rainfall that is 20% higher than the global average. The city is already rocked by two to three typhoons every year. Shanghai is also sinking, albeit less slowly than Jakarta. To reduce its exposure to rising seas, Shanghai has constructed 520km of protective seawalls that stretch across the Hangzhou Bay and encircle the islands of Chongming, Hengsha and Changxing. As in the case of Rotterdam, Shanghai has also installed massive mechanical gates to regulate overflowing rivers.

Fight or flight in South East Asia and the South Pacific

South East Asian cities are busily building defences against sea level rise. For example, Jakarta is building a massive sea wall with Dutch support, and is planning to relocate 400,000 people from threatened riverbanks and reservoirs. Critics, however, fear that the city is not doing enough to address groundwater issues that are causing the city to sink. Bangkok, which faces similar challenges to Jakarta, has also laid out a 2,600km canal network and central park with a capacity to drain 4 million litres into underground containers.

Some Thai parliamentarians fear that the only way forward is a managed retreat, moving the capital further inland. Singapore, too, is adopting myriad mitigation strategies including land reclamation schemes and embankments across 70% of its coastal areas.

But arguably the most dramatic responses to rising sea levels are occurring in those parts of the world that are most acutely at risk. Small island nations such as Kiribati, Tuvalu, the Marshall Islands and the Maldives could be literally wiped off the map. Kiribati is negotiating to buy 5,000 acres of land in neighbouring Fiji onto which to move its 113,000 citizens if necessary. The country’s website concedes that national survival is unlikely. The Marshall Islands face a similarly stark choice: leave or elevate. The country is looking for ways to reclaim land and build islands that are high enough to withstand rising seas. And the Maldives – the poster-child for rising sea levels – is attempting to reclaim, fortify and build new islands, and relocate when necessary.

Finally, US cities are busily investing billions of dollars to bolster their resistance to rising sea levels. New Orleans established the Hurricane and Storm Damage Risk Reduction System shortly after Hurricane Katrina killed more than 1,600 people in 2005, leaving 80% of the city underwater. The system includes a series of massive dam barriers, reinforced levees and flood-walls stretching some 560km around the city. The city also built a living water system of parks, wetlands and other existing features to reduce reliance on pumping and canals. It is one of the largest public works projects in US history and the most expensive flood-control system in the world. Boston, Houston, Miami, New York City and dozens of other places are following suit, albeit on different scales.

From Asia and Africa to Europe and the Americas, sea-level rise is inevitable. Mitigation efforts must be scaled up. But, precisely because it already poses an existential threat to coastal communities everywhere, adaption is essential. At a minimum, governments, businesses and citizens need to avoid making a bad situation worse. Adjusting zoning laws and reducing building in at-risk coastal areas and flood plains is a start. As the Global Risk Report makes clear, proactively developing strategies to relocate populations who are vulnerable to sea-level rise is no less important. Another tricky challenge relates to burden-sharing between and within nations and cities. A new mindset, innovative financing models and multi-stakeholder partnerships are critical as the seas continue to rise.