BANGKOK, July 7 (CNA) - Huge swathes of land are closer to sea level across Southeast Asia than previously realised, highlighting the flood dangers faced by millions, according to a new study using advanced satellite imagery.
In low-lying countries vulnerable to sea level rise and with land prone to sinking due to subsidence, like Indonesia, Thailand, Vietnam and the Philippines, it means more of the population will face annual extreme sea level events that are progressively getting worse due to climate change.
Past coastal flood risk assessments and projections have been let down by a lack of accuracy in satellite radar data that measures land elevation levels.
However, researchers from Dutch-based research institute Deltares were able to perform the first global elevation model using new satellite Light Detection and Ranging (LiDAR) data, providing a more accurate picture.
“Most existing elevation models for the region, and indeed globally, were based on radar data which cannot fully penetrate vegetation and therefore yield a land elevation that is often too high by one or several metres,” explained Dr Aljosja Hooijer, the co-author of the report, which was published in the Nature Communications journal in late June.
“If the assumed land elevation is too high, the resulting calculated flood risk is too low,” he told CNA.
As a result, in Indonesia, the land area below 2m detected by LiDAR is more than 14 times what was previously thought, compared to the more commonly used Shuttle Radar Topography Mission (SRTM) data.
In Thailand, the amount is about five times SRTM estimates, and seven times in the Philippines.
These figures can vary at a local level, the researchers stressed, where more precise data can help planning officials reduce flooding risks.
BURDEN FALLS ON TROPICAL ASIA
The researchers found that the “burden of current coastal flood risk and future sea level rise falls disproportionately on tropical regions, especially in Asia”.
The study estimates that 157 million people in tropical Asia now live in areas below 2m above sea level, the range where impacts are predicted to be severe. This number would increase significantly if sea levels rise in the coming decades, as they have been forecasted.
The Intergovernmental Panel for Climate Change expects sea level rise to be 0.8m, possibly by 2100, and extreme flooding events that used to happen once in 100 years could occur every year by the end of the century.
If a sea level rise of 1m does eventuate, densely populated areas of major deltas will be underwater. Some 129 million globally would live at high risk of permanent flooding by the sea.
“We knew the numbers would be high, as we see flooded and nearly flooded land along many coastlines. But to find two-thirds of the lands and people most at risk in developing tropical countries was more than we expected,” Hooijer said.
“The numbers for Asia are especially large. It could be said that half the global coastal flooding problem is in tropical Asia alone – a bit more by population, less by area,” he said.
Under a 1m sea level rise scenario, forecast in the study to happen around 2100, 38 million people in Vietnam, 28 million in Indonesia and 23 million in Thailand would be living in a zone at high risk of frequent coastal flooding, a more than 21 per cent increase compared to today.
That does not account for any growth of population or movement of people towards coastal areas, making those estimates conservative, according to Hooijer. “Many scientists believe coastal populations will keep increasing,” he said.
The impact could be severe in major cities, coastal communities and agricultural heartlands. Large areas might become uninhabitable and unproductive, cascading pressures onto food systems, congested urban developments and economic systems.
A Greenpeace report released last month, using high spatial resolution data from scientific research organisation Climate Central, aimed to quantify the economic damages being faced by seven of Asia’s biggest cities, due to extreme sea level rise by 2030.
It estimated the potential damages - in urban areas alone - at US$724 billion in Bangkok, Jakarta, Manila, Taipei, Seoul, Tokyo and Hong Kong. It found that 96 per cent of the Thai capital’s annual GDP could be threatened and 10 million of its population affected.
“Climate change impact is not only an environmental problem. It devastates the economy and, as a result, will create nationwide social problems,” said Tata Mustasya, Greenpeace Southeast Asia’s regional climate and energy campaign strategist.
“Millions of vulnerable people live in the flooding areas. They will be displaced and lose their livelihoods.”
