The northern coastline on Indonesia's main island of Java is sinking annually at a fast rate, causing land subsidence and threatening residents of communities living near the shore.
Economic development and land conversion from mangrove forests to industrial or residential uses are among the factors causing the sea level to rise and water to creep farrther inland, say experts.
But in Jakarta, the nation's capital, groundwater extraction remains the main culprit behind land subsidence, as up to 65% of its residents rely on underground water sources. Land is subsiding by anywhere from 3 to 18 centimetres annually in various parts of the city and a lack of mitigation would lead to 30% subsidence in 2050, Abdul Malik Sadat Idris, an official from the National Development Planning Agency, warned in February.
"Land subsidence is obvious along the northern coastline of Java at a rate of 1 to 25cm annually," said Dr Heri Andreas, a geodesist from Bandung Institute of Technology (ITB) in West Java. "This trend is a warning for us that land subsidence will continue to happen unless we do something to stop or to withhold it, since land subsidence is like a silent killer that will affect our communities as it causes inundation and land to erode."
Indonesia's 54,700-kilometre coast line is the second-longest in the world after Canada, and its mangrove ecosystem is the largest in the world, covering 3.5 million hectares or 23% of the world's mangrove ecosystem, according to data from the Ministry of Environment and Forestry.
These mangrove forests are part of an estimated 30-million-hectare coastal area ecosystem along with peatlands, wetlands, lagoons, river deltas, sea banks, marshes and evaporation ponds, many of which are located close to human settlements that are less than 30 metres above sea level.
They support to a wealth of life as a natural habitat for various species, and also serve as a buffer to seawater intrusion. At the same time, they store fresh water and contain high carbon reserves that help mitigate climate change. A mangrove forest that extends at least 100 metres inland can suppress rising tides by 13% to 66%.
"But most of our wetlands and 52% of our mangrove forests have been destroyed. We have lost 85% of the mangrove areas on the the northern coast of Java as they have been converted to human settlements or man-made fish ponds. In some areas, the sea level has even risen," said Agung Kuswandono, a deputy minister in charge of natural resources coordination at the Coordinating Ministry of Maritime Affairs.
Nyoman Suryadiputra, director of Wetlands International Indonesia, said the only course of action now is an immediate halt to wetlands conversion.
"Many of us don't realise that Indonesia's largest fresh water reserves are in peatlands instead of lakes or rivers," he said, adding that at the same time, climate change also causes sea levels to rise and these two situations are increasing the risk of further calamities along the coastline.
Jakarta is not the only city in the region suffering from land subsidence. Other Asian megacities such as Tianjin, Shanghai, Tokyo, Osaka, Manila, Bangkok, Dhaka and Bombay also have some areas that have slipped below sea level, said Dr Athanasios Vafeidis of the Department of Geography, Coastal Risks and Sea-Level Rise at Kiel University in Germany.
Calamity could be prevented, he said, if people are willing to adapt to the sea level rise and prepare for it by looking at data and historical facts. Planning based on such knowledge is less costly compared to the damage that could result from ignoring the facts, or having to take action after the fact.
"Adaptation costs are generally lower than direct damage costs. If we include indirect impacts, the benefits are even larger," Dr Vafeidis told a group of visiting international journalists to the institution in late 2017.
"The costs depend on the timing of adaptation but proactive adaptation pays off," he said.
Dr Vafeidis added that the rise in sea levels has been accelerating for the past decade and will continue to do so. The rates are not the same but some regions could see acceleration three times faster than others. Without adaptation, he said, many areas will become unviable by 2100.
"A better understanding of adaptation and decision-making under certainty is essential, especially for vulnerable regions such as deltas and small islands," he said.
Klaus Schwarzer from the Institute of Geosciences, Sedimentology, Coastal and Continental Shelf Research at Kiel University, cited the Mekong in Vietnam, Chao Phraya in Thailand and Mahakam in Indonesia's Kalimantan province as examples of deltas that are vulnerable, based on current estimates of the relative sea level rise to 2050, including land subsidence in the deltas.
If no adaption is in place, the number of people displaced from the Mekong delta would be extreme at more than one million, while anywhere from 5,000 to 50,000 people would have to flee their communities along the Chao Phraya and Mahakam.
Mr Schwarzer said more research on coastal zones would be essential to provide knowledge that could be apllied in drafting potential future scenarios so that people can continue to make use of the coastal environment. However, such usage must be sustainable with better management of coastal ecosystem resources, given that some 2.8 billion people in the world now live within 100km of a coast.
Mangrove forests that help to keep the tides at bay continue to be cut down, but very few people are talking about the loss of this essential element of coastal protection, said Martin Zimmer, a professor of mangrove ecology at the University of Bremen in Germany.
He said researchers are developing an approach to mangrove spatial conservation that focuses on human needs while also maintaining biodiversity.
"It is an ecosystem design that focuses on what people in the area need. We implement an ecosystem that functions for them, not just to make the area look beautiful," he said. "To protect coastal areas, we can't just build dykes or other structures, but we can do that with something that is naturally there, which is mangrove."
Coastal zones serve as an essential lifeline for much of the world's population, Mr Schwarzer said, given that 95% of international trade involves marine transport, which ends up in harbours.
Oceans are also important sources of food with 90% of the world's fishery activity carried out in coastal zones.
"There is an increase in extreme storm surges and predictions of sea level rise. Coastal erosion already endangers about one third of the world's population," Mr Schwarzer said.