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What can nature teach Singapore about combating rising sea levels?

Audrey Tan and Mark Cheong on 19 Apr 2021

The Straits Times


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SINGAPORE - April 17, 6pm. East Coast Park. Waves lapped a beach left unusually quiet by the day's downpour. An idyllic scene for some, but also a visceral minder of Singapore's vulnerability to the impacts of rising sea levels.


While global warming and the melting of polar ice sheets mean that sea-level rise is a global problem, the issue is especially acute for places like Singapore.


Those living in places with less space constraints, said Professor Robert Nicholls, director of the Tyndall Centre for Climate Change Research at Britain's (UK) University of East Anglia, may simply allow their shorelines to retreat.


But Singapore, an island that must find the space to accommodate all the needs of a country, cannot afford to do so.


Prof Nicholls, a climate adaptation expert who has advised the Singapore Government such issues, added: "In the UK we have large lengths of coast that are retreating with little or no attempt to stop this happening. So Singapore, as far I understand, is trying to keep the shorelines where they are."


And indeed, it is, setting up new programmes to boost research into sea level science and setting aside funds for coastal protection. Other than structures like seawalls and dams, Singapore is also looking to nature in its quest to mitigate coastal erosion and rising seas.


Coastal necklace


Singapore's coast is a mosaic of colour and texture, from seawalls and mangroves to sandy beaches and rocky shores.


About 70 per cent of Singapore's coastline is currently protected by hard structures, said Ms Hazel Khoo, director of the coastal protection department at national water agency PUB.


These include concrete seawalls and stone revetment walls, as well as hard infrastructure like the Marina Barrage and dams such as the ones in Kranji and Yishun, she said.


Nature dominates the rest.


About 16 per cent of the coast are mangrove and mudflat, 12 per cent are sandy beaches, and less than one per cent are rocky shore habitats, noted Mr Lim Liang Jim, group director for conservation at the National Parks Board (NParks).


This patchwork means there is no one-size-fits-all approach to protecting the country's coastlines from rising sea levels.


"Different coastlines lead to different responses of the coast to sea-level rise and also different consequences if disaster unfolded," Prof Nicholls told The Straits Times in an e-mail interview.


"So when we assess a coastline we examine the hazard, exposure and vulnerability and how these might change to understand the scale of the problems and identify the range of potential solutions," he added.


A worsening problem


Global mean sea levels are rising today because human activity like the burning of fossil fuels is belching planet-warming gases into the atmosphere.


This global heating is driving sea level rise in two ways, said Professor Benjamin Horton, a sea level science expert and director of the Nanyang Technological University's (NTU) Earth Observatory of Singapore.


Water expands in volume when it is heated and the thermal expansion of water was the main driver of sea-level rise in the 20th century, raising water levels at a rate of 1mm to 2mm a year, he said.


But now, rising sea levels are coming from accelerating rates of ice melt dumping water into the ocean.


"What we've observed in the 21st century is the rate of sea-level rise has gone up to 3mm to 4mm a year. Thermal expansion is still occurring, but the ice sheets are now the dominant contributor to rising sea levels," Prof Horton added.


Singapore mounts a defence


In response to this threat, new developments such as the upcoming Tuas Port will be built at least five metres above mean sea level.


This photograph depicts part of a caisson, a retaining structure used as a sea wall for reclaiming land on which the port will be built.


The Maritime and Port Authority of Singapore (MPA) said it carries out regular reviews to ensure that port and maritime facilities will be well-protected against sea level rise.


"We work closely with other government agencies to understand the long-term risks the facilities will face with sea level rise and implement suitable adaptation measures as necessary with the facility operators," said a MPA spokesman.


Other than measures to protect key infrastructure, efforts are also underway to tailor protection measures to the country's different coastal areas.


"Site-specific studies to develop coastal protection measures for the various coastlines will be conducted in phases progressively, starting with the more critical segments," said PUB's Ms Khoo.


Singapore had earlier identified four areas as being vulnerable to rising sea levels - the City-East Coast stretch, Lim Chu Kang, Sungei Kadut and around Jurong Island.


Studies at the coastlines of City-East Coast and Jurong Island will start this year, while studies in the remaining areas in the north-west are expected to commence next year.


How nature can help: corals and mangroves


One thing the country is considering is combining hard coastal protection structures with the softer elements of nature.


PUB's Ms Khoo said the efficacies of hybrid solutions combining hard and soft measures will need further study. "Upcoming site-specific studies will examine them and determine their suitability at specific segments of the coastline," she said.


In some cases, however, nature returns on its own.


Marine biologists from the National University of Singapore had earlier noticed corals growing naturally on seemingly barren artificial structures, such as seawalls, on Lazarus Island - one of the isles located south of mainland Singapore.


They later did a study that found that compared with vertical seawalls, sloping stone revetment walls can support a greater number of coral species, and in larger numbers too.


Corals are found mainly in the waters south of Singapore. But up north, there are gems too - Mangroves.


These habitats help to protect coastlines during storm surges and shield seagrass beds and coral reefs from siltation, said NParks' Mr Lim.


This video depicts how mangroves habitats can help to reduce wave action on coastlines.


Mangroves also play a role in soaking up the planet-warming carbon dioxide that is driving climate change, with the ability to capture more than three times the carbon in dryland tropical rainforests, Mr Lim added.


Earlier this month, The Straits Times got a rare opportunity to go off the boardwalk and onto the muddy mangroves at Sungei Buloh Wetland Reserve to document how NParks was working with the WILDSMarines student group from the Singapore University of Social Sciences to reforest this important habitat.


A mangrove at low tide is like a vignette of a different world filled with strange structures.


There were pencil-like forms sticking out from the ground, and woody "snakes" in a tangle. Other structures were weirder still, looking like a heap of legs dismembered from wooden marionettes, all of them bent at the knee.


These were pencil roots, and knee roots, all named for their unusual shapes, the modified roots of mangrove trees which are unique in their ability to trap sediment from the tides as they ebb and flow.


Because of this ability to keep pace with sea level rise - provided that water levels do not rise too fast - mangroves are often considered a nature-based solution to dealing with coastal inundation.


NParks' Mr Lim noted that Singapore was taking steps to conserve its mangroves.


Last year, for instance, NParks announced that the Khatib Bongsu mangroves will be conserved as a nature park. Nurseries for mangrove plants have also been set up in Sungei Buloh and on offshore Pulau Ubin to conserve rare mangrove species of Singapore, and will also help in the propagation of seedlings for mangrove restoration, said Mr Lim.


What more can be learnt from nature?


But when it comes to protecting the country against future sea level rise, Mother Nature has more tricks up her sleeve, and scientists here are trying to coax her to reveal her secrets.


Coastal habitats like mangroves stand where land meets sea.


If left untouched by development, such plots are a rich archive of how the environment has changed, as they have been keeping record long before the advent of sensors, tide gauges and other technology.


A group of researchers, led by NTU's Prof Horton, are studying mangrove sediment cores and coral microatolls in Singapore to piece together a clearer picture of how sea levels have fluctuated here over millennia. Their work is being funded under the National Sea Level Programme, coordinated by the National Environment Agency (NEA).


Prof Horton, who is principal investigator of the project, told The Straits Times: "The reason why we must study geological recorders of sea level is because the instrumental records of sea levels in Singapore, like in many tropical locations, do not go back far enough."


The technology that helps humans study sea levels using satellite data, for instance, goes back to the early 1990s, while information from tide gauges in Singapore go back to the 1970s, he added.


Sea levels have risen and fallen over the course of history. But different places experience these changes differently, because how much sea levels fluctuate in one location depends on many factors.


This could include, for instance, the elevation of land relative to the sea in one location, or how far a coastline is from a melting glacier.


Understanding how sea levels have changed in Singapore over the course of geological history will help scientists here come up with models that are more accurate at predicting sea level rise in this part of the world, instead of solely relying on global forecasts.


Today, a warming planet is causing the ice sheets in Greenland and Antarctica to melt, discharging water into the oceans and causing sea levels to rise.


But, unlike a bathtub where water level rises uniformly if a block of ice were added to it and left to melt, sea level rise will not be experienced the same way around the world.


One need only look into the history of South-east Asia to understand this.


More than 20,000 years ago, ice sheets used to cover a greater extent of the planet - locking away freshwater in ice.


Back then, South-east Asia was not the maritime continent it is today, where countries and islands are surrounded by shallow seas.


In fact, during the period known as the Last Glacial Maximum, it would have been possible to walk from Singapore to the islands of Borneo or Bali, said Assistant Professor Aron Meltzner, part of the NTU team looking into coral microatolls as an indicator of past sea level rise.


But, as the planet wobbled on its axis and made its way around the sun, these movements caused Earth's climate to warm (though not as fast as the warming that is happening today due to human activity) and the ice sheets to melt, releasing all the water into the oceans.


This caused the sea to encroach onto the Sunda shelf, creating islands and weighing down the continent with the influx of water.


On the other hand, parts of North America and Europe that were once covered by ice sheets recorded an opposite uplift effect, as the weight of the ice dissipated and the land rebounded.


But how much would these ancient processes of uplift and subsidence - which continue today albeit in infinitesimal degrees - affect how fast sea levels will rise in this part of the world?


"We don't yet understand which processes are important in South-east Asia," said Prof Meltzner.


"Because these processes operate over hundreds to thousands of years, it is insufficient to just look at recent decades to get a good sense of what's important here."


What the natural archives can offer is data over thousands of years, which will help the scientists quantify the contribution of the various processes to sea level rise.


Sediment cores taken from mangroves in Singapore, such as the one shown below, can provide sea level fluctuation data going back thousands of years.


Prof Meltzner added: "Once we can do that, we can build better models that are calibrated to the past. And we can use those models to get better projections for the future."


Of pollen, forams and corals


Mangrove sediment accretes layer by layer. So to study sea levels of the past, scientists must dig deep - literally.


This photograph shows NTU's Earth Observatory of Singapore scientists Dr Tim Shaw and Dr Geoff Richards using augering and soil sampling equipment to retrieve a sediment core from a mangrove forest on offshore Pulau Ubin, located to Singapore's north-east.


By studying pollen and microfossils of foraminifera - tiny organisms found in marine environments - contained within the layers of ancient mud, researchers can look into the past to see how sea levels have fluctuated in Singapore, Prof Horton said.


For instance, the presence of foraminifera in one part of the core indicates that seawater had likely inundated the area then.


On the other hand, if pollen from trees were found in another segment of the core, seawater may not have reached that far inland at that point.


Radiocarbon dating - a technique used for archaeological discoveries - can be used to determine the exact age.


While mangrove sediment cores can go back thousands of years, they are unable to reflect environmental changes at a finer resolution.


Coral microatolls, however, can.


What microatolls can tell us


These refer to corals with a unique growth form: dead flat-tops, with living coral tissue only found along its perimeter.


This is an example of a fossilised coral microatoll found in Indonesia.


These corals tend to grow sideways, as upward growth is limited by exposure to air, heat and sunshine.


Microatolls form annual growth bands, similar to rings found in the trunks of trees in temperate climates. But where tree rings record changes in rainfall and temperature, the bands on coral microatolls record sea level change.


They usually start growing below the intertidal zone, where they remain completely submerged even at low tide.


But they begin to take on their unique form when the top part of the structure gets exposed to air. This can happen when the coral grows high enough, or when relative sea level falls such that their surface is exposed to air. This causes the topmost part of the colony to die, and they stop growing upward.


But the rest of the coral tissue that remains submerged continues to grow sideways.


When sea levels rise again to completely submerge the coral, living tissue can continue to grow upward, forming another ring.


Possible drivers of sea level changes over shorter time scales can include earthquakes in the region, said Prof Meltzner.


For instance, studies of coral microatolls off Indonesia's Sumatra island following the 2004 earthquake and tsunami in Aceh showed that sea levels could change after such events.


Coral microatolls near one village there died after the site they were sitting on were uplifted by over 1.3m following the event, he said.


His research team had last July gone on field trips to Singapore's southern islands to identify coral microatolls. Here they are looking at one on St John's Island.


They have since collected small core samples from several fossil microatolls on Sentosa and sent them off for dating, with results expected back around July.


Such information will allow the researchers to determine if recent earthquakes had an impact on sea levels here.


The Meteorological Service Singapore (MSS), a unit under the NEA, said sea level history is valuable for several reasons.


For instance, data on past sea level changes will provide a better understanding of the situation before human activity started influencing the climate. This will help scientists and policy-makers assess recent trends.


"The enhanced understanding of long term historic sea level variability will allow tuning and refinement of models that predict future changes and their spatial variability," said the MSS spokesman.


Knowledge developed under research initiatives like the National Sea Level Programme will aid in national projections of future sea level.


These projections will in turn be key inputs used by national water agency PUB's Coastal-Inland Flood model to assess combined flood risks from extreme sea-level rise and rainfall.


Source: The Straits Times © Singapore Press Holdings Limited. Reproduced with permission.



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