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'Zero-meter zones' expand globally as sea levels rise Millions of urban residents worldwide face growing flood risks as sea levels rise

As sea levels continue to rise with global warming, "zero-meter zones," areas below sea level, are growing. In densely built, populated urban areas, there are many constraints on strengthening flood control and evacuation measures. Looking ahead over the next 100 years, strategies used to protect cities and their residents will be put to the test.

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Tokyo and surrounding areas, world's biggest population center, at risk

The Tokyo Metropolitan Area (Tokyo, Kanagawa, Saitama and Chiba prefectures) has a population of approximately 35 million. According to a ranking by U.S. research company Demographia, it is the most populous metropolitan area in the world. Located downstream from the Arakawa and Edo Rivers, which flow through the Kanto Plain, the area has been battling water for hundreds of years since the era when Tokyo was called "Edo." While flood control measures like the construction of levees and sluice gates are progressing, the population in the zero-meter zone in the eastern part of Tokyo Prefecture continues to grow because of urbanization.

Nikkei analyzed the impact of a rise in sea level on the population in the zero-meter zones around the Arakawa, Edo and Tama rivers, which flow through Tokyo. This was done using data from the National Spatial Planning and Regional Policy Bureau, published by the Ministry of Land, Infrastructure, Transport and Tourism and population data from Docomo Insight Marketing.

As the area below sea level grows, the affected population increases significantly

The Arakawa River flows between Adachi and Katsushika wards

The levees protecting Tokyo's coasts and riverbanks are designed to withstand a storm surge equivalent to that of Typhoon Vera, which hit in 1959 and caused extensive damage. The Tokyo Metropolitan Government alone manages more than 200 kilometers of levees.

"Vertical evacuation" saves lives in emergencies

Surrounded by the Arakawa and Edo rivers, about half of Tokyo's Katsushika Ward currently is a zero-meter zone. Recognizing the difficulty of evacuating the ward's 460,000 residents before a flood disaster strikes, one option to explore is "vertical evacuation" into taller buildings.

Vertical evacuation into schools, commercial facilities and apartment complexes
Scattered evacuation sites, increasing over 30 years
  • Elementary and middle schools,
  • other public facilities

Created from a 3D city model by the MLIT and a hazard map from Katsushika Ward. ©Mapbox, ©OpenStreetMap

In 2021, the ward designated 209 ward-owned facilities, including schools and apartment buildings that would not be submerged at anticipated flood depths, as "emergency flood evacuation buildings." Ward officials also asked private apartment complexes and commercial facilities to cooperate in accepting evacuees, and it began subsidizing the related costs from fiscal 2022.

The plan, formulated in 2018, calls for a 30-year plan to increase the number of evacuation buildings and to promote the provision of power, food and water. Up to 240,000 people would be able to live as evacuees for two weeks until the waters recede.

Population of the five wards in eastern Tokyo and projection of affected population

Note: Prepared from materials from the Working Group on Large-Scale and Widespread Evacuations due to Floods and Storm Surges

No fixed answer for protecting residents

According to data released by the government in 2018 on large-scale and widespread evacuations due to floods and storm surge inundation, as of 2015, 2.55 million people lived in the zero-meter zone in the five wards of Sumida, Koto, Adachi, Katsushika, and Edogawa.

2.36 million people live in areas expected to be inundated in a scenario in which the water levels of the Arakawa and Edogawa rivers rise simultaneously.

Of these, 810,000 people live in buildings where all livings spaces will be flooded at anticipated flood depths. Where will the evacuate to and how will they get there? How are they to wait for rescue? Currently there is not a certain answer.

Population facing potential risks growing around the world

According to a report released by the United Nations Intergovernmental Panel on Climate Change in August 2021, If greenhouse gases continue to be emitted at or near current levels, sea levels are projected to rise by about 1 meter by 2100 and 2 to 7 meters by 2300. Many of the world's major cities rest on coastal flatlands, and a growing number of people will face flood risks as sea levels rise.

Using data from the United Nations for the world's 30 most populous cities, Nikkei estimated the land area and population that would be below sea level because of sea levels rising in the 17 cities that are coastal cities at low elevations.

A one-meter rise in sea levels would have the greatest impact on Guangzhou, China. The land below sea level will grow to 1155 square kilometers from 990 square kilometers, and the affected population will increase to 18% (3.31 million people) from 14% (2.62 million people).

If the sea-level rise reaches 3 meters, 28% of the population of Suzhou, China (about 3.55 million people) would be affected, the highest share. At 7 meters, that figure reaches 84% (about 10.7 million people). Other Asian cities that stand out include Jakarta, Indonesia and Mumbai, India.

Zero-meter zone expands along the River Thames

The British capital of London, home to 9 million people, is divided into 33 boroughs centered on the financial district in the City of London. There are 11 square kilometers of zero-meter zone within the city limits, near the estuary of the River Thames. A 3-meter rise in sea levels would increase that to 56 square kilometers, or 4% of the city, affecting 170,000 people.

A "guardian diety" has prevented storm damage

The Thames Barrier protects central London (back of photo) from storm surges. (Courtesy of the U.K. National Police Air Service)

The Thames Barrier is a mobile levee located 10 kilometers downstream from the city center that acts as a "guardian deity," protecting the city from storm surges. An underwater barrier rises to block the 520-meter-wide river, preventing storm surges from moving upstream. It has been in operation since 1982 and has been used more than 200 times.

An explanatory video on YouTube about the Thames Barrier from the U.K. Environment Agency

98-cm sea level rise expected in 2100

To prepare for future climate change and sea level rise, the U.K. Environment Agency (EA) published Thames Estuary 2100 (TE2100) in 2012. The EA is calling for stronger comprehensive flood control based on the assumption that sea levels will rise by 98 centimeters by the end of this century.

Looming lifespan: retrofit or new construction?

The Thames Barrier is designed to last until 2030, but replacing parts and maintenance will enable it to last until 2070 while authorities monitor rising sea levels and other conditions. The option is available to continue using the barrier with additional modifications.

If a new barrier is constructed, Long Reach, 15 kilometers downstream from the current barrier, is the most likely site because of the narrower river width there and lower construction costs. About 10 kilometers further downstream, Tilbury, known as London's outer harbor, is another option, but the impact on shipping would be an issue.

Accelerating implementation to respond to sea level rises

Source: Based on the five-year review of the TE2100 (2016)

TE2100 states that measures may be brought forward by 10 to 15 years, depending on the speed at which sea levels rise. "Through the Thames Estuary 2100 Plan we are taking an adaptive approach so we are agile to the latest climate science, growth projections, investment opportunities and changes to the local environment," said Julie Foley, the EA's director of flood strategy and national adaptation.

Frequent flooding, more to come

Shanghai sits at the mouth of the Yangtze River, facing the East China Sea. The metropolis, with a population of approximately 25 million, is still frequently flooded. If sea level rises by one meter, the zero-meter zone will expand from the current 110 square kilometers to 335 square kilometers, and the risk of flooding will further increase.

Green parks are like a "sponge"

The Starry Sky park opened in Pudong in August 2021. (Courtesy of Shanghai Municipal People's Government)

The Starry Sky, the city's biggest sponge park, which opened in August 2021 near Pudong's man-made Dishui Lake, covers more than 50 hectares. Located next to the Shanghai Astronomy Museum, one of the world's largest astronomical facilities, a promenade runs through the park, which takes advantage of the wetlands and green areas along the riverbank.

Protecting the city by "absorbing" rainwater

The park's name references its function -- it protects the city during heavy rainfalls by absorbing water like a sponge. In addition to the water retention capacity of the wetlands and green areas, the promenade and other areas have permeable pavement and underground water-storage tanks.

80% sponge city
  • Proposed construction in 2016 plan

In its main 2018 urban development plan, Shanghai plans to strengthen flood control measures by developing other sponge parks and drainage systems, setting a goal of turning 80% of its urban area into a "sponge city" by 2035.

The concept of the sponge city, which utilizes wetlands and green areas to enhance the city's drainage and water storage capacity, was reportedly proposed by landscape architect Professor Yu Kongjian of Peking University. 30 cities in China, including Shanghai, are in pilot projects to transform into sponge cities as a part of a state project.

Growing zero-meter zone in a city of 10 million

There are 10.56 million people in Jakarta, the capital of Indonesia in the western part of the island of Java. The zero-meter zone currently covers 30 square kilometers, and it will expand to 51 square kilometers if sea levels rise 1 meter, and 122 square kilometers if sea levels rise 3 meters.

Vanishing land, rising risk

A woman pushes a motorcycle during a flood in November 2021. (AP)

Jakarta experienced eight floods in 2020 alone. According to a paper published in 2019 by a research group from Indonesia's Bandung Institute of Technology, the ground around Jakarta is sinking 1 to 20 centimeters per year. Together with rising sea levels, this puts the area at an increased risk of flooding.

Capital relocation

In 2012, the country's National Development Planning Agency announced plans to relocate the capital city, noting Jakarta's disaster risk, including a 25-50 centimeter rise in sea levels in in by 2050. In January 2022, the legislature passed a bill to build a new capital, Nusantara, in eastern Kalimantan (Borneo), 1,200 km from Jakarta.

New government facilities by 2024

Image of the new capital city of Nusantara (Courtesy of Indonesia's Ministry of Public Works and Housing)

Although close to the sea, the site for the new city has a plateau-like topography with elevations exceeding 30 meters in some places. The first phase, which runs until 2024, will see the construction of government buildings like the presidential palace, office areas, and residential areas. From 2025, construction will begin on industrial parks, universities and research centers.

$8.36 trillion in damages in 136 major cities

Top 20 cities with the greatest economic damage from rising sea levels as of 2100
  • With ice sheets melting
  • No countermeasures

(Source: Estimates by the Basque Center for Climate Change and other research groups in Spain. Costs are median. Billions of dollars.

A group of researchers, including the Basque Center for Climate Change in Spain, in 2020 estimated the economic damage associated with rising sea levels for 136 major coastal cities around the world. In the scenario with no greenhouse gas emission reductions, the total (median) damages for all cities in 2100 would be $8.36 trillion. The biggest loss was in Guangzhou, China, which saw damages of $1.39 trillion. In Japan, damages in Osaka/Kobe were estimated to be $342.3 billion, Tokyo was $270.4 billion and Nagoya was $204.1 billion.

In a scenario in which sea levels rise significantly due to melting ice sheets in Antarctica and elsewhere, the total damage to the 136 cities swells to $11.51 trillion.

Preparing for 'adaptation'

"By mid-century, more than a billion people living in low-lying coastal cities and settlements globally are projected to be at risk from coastal-specific climate hazards," according to a report released by the Intergovernmental Panel on Climate Change (IPCC) at the end of February. "Many of those will be forced to move to higher ground, which will increase competition for land and the probability of conflict and forced relocation," it warned.

Rising sea levels threaten to change the geographic conditions on which cities formed and developed. To maintain their prosperity and security, cities must be prepared to adapt to the changing environment and rebuild.

Methodology

In analyzing the growing zero-meter zone around Tokyo due to rising sea levels, the average elevation was calculated by dividing the target area into 250-meter squares based on topographical data by the Ministry of Land, Infrastructure, Transport and Tourism, and the area that would be below sea level if the sea level rose 1, 3 or 7 meters. The population affected by the zero-meter zone was determined using estimated population data (as of February 15) calculated by Tokyo-based Docomo Insight Marketing based on cell phone location data.
The analysis of the 17 coastal cities worldwide used NASA's three-dimensional topographic data and WorldPop's publicly available global spatial demographics (2020) to determine the area of the zero-meter zone and the population affected.

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