Getting Bangkok ready for the consequences of climate change is crucial for saving this megacity from being submerged under rising sea levels, climate experts say.
They also called on the government and state agencies to come up with urgent efforts to reform urban planning and city development to make Bangkok more adaptive and resilient to dealing with climate hazards.
As global warming accelerates, Thanawat Jarupongsakul, president of the Thailand Global Warming Academy (TGWA), an independent climate think tank, said Thailand's climate risk has heightened during the past decade.
According to the annual Global Climate Risk Index by Germanwatch, Thailand jumped from 43rd on the ranking of most affected countries by extreme weather in 2011 to ninth in the latest edition of the analysis in 2021.
"It is apparent that human-induced climate change is already affecting many weather and climate extremes throughout the globe, as during the past 10 years we have experienced increasingly volatile and violent weather, as seasonal patterns have become more and more erratic and floods and droughts are now regularly occurring," he said.
"No place has been spared from the impacts of extreme weather events."
As the densely populated Bangkok Metropolitan area, home to over 10 million inhabitants, is situated on the low-lying Chao Phraya River delta right next to the open waters of the Bay of Bangkok, the uppermost part of Gulf of Thailand, Prof Thanawat said this geographic location makes Bangkok even more vulnerable to weather-related disasters, especially floods.
Although he said the impacts of global warming on precipitation patterns and weather-related disasters will be major concerns for Bangkok in the near future, he said the increase in the level of the world's oceans is by far the greatest threat to this megacity.
Melting water from glaciers and polar ice caps is raising the sea level and slowly submerging low-lying coastal cities globally.
According to the Intergovernmental Panel on Climate Change (IPCC), human-induced heating of the climate system has been responsible for about a 0.2 metre rise in the global mean sea level over the past century.
Under the current circumstances, IPCC warns that global mean sea level will rise at an increasingly faster rate over the 21st century and well beyond into the far future.
It is estimated the global mean sea level may soar up to 0.5-1 metres by the end of the 21st century, while in the longer term, the global mean sea level may reach over 6 metres over the next 2,000 years.
Due to the grim prospect of sea levels rising in the future, Prof Thanawat said Bangkok is apparently the most vulnerable area in Thailand to the looming threats of climate change. The low-lying Chao Phraya River delta has an average elevation of around 1.5 metres above sea level.
"Considering the rise in sea level in local waters near Bangkok is about 1.2 centimetres per year, if nothing is done to protect Bangkok's shoreline, the waterline will slowly creep further inland by about 1.3 kilometres every year," he said.
"Eventually, it is likely that most of Bangkok Metropolitan will sink beneath sea level within the next 100 years. So, governmental agencies need to carry out immediate action to ensure the integrity of the shoreline."
Meanwhile, another report on the economic impact of extreme sea level rises in seven Asian cities by Greenpeace says almost all of Bangkok will be vulnerable to floods in the scenario of an extreme sea level rise by 2030. Under this extreme projection about 96% of the city will be below the mean sea level.
Greenpeace's report said the economic damage from such extensive coastal flooding in Bangkok would be phenomenal, as the impact to GDP could reach US$512.28 billion (17 trillion baht) and affect the lives of millions of citizens.
Bangkok governor Chadchart Sittipunt said the Bangkok Metropolitan Administration (BMA) has mitigation plans in place to address the issues of coastal erosion and a rise in sea levels.
"I admit that, due to global warming, our city is facing greater flood risks from extreme precipitation and rising sea levels. Although the flood threats are real, we are not without a plan to save our city from deluges," Mr Chadchart said.
The BMA plans to build a barrier system, similar to the Thames Barrier in London, at the mouth of the Chao Phraya River to stem flows of seawater into the river. Meanwhile, the roads that run along the seashore will be elevated and have coastal dykes to protect the city from surging sea levels.
Other ideas have been suggested, for instance the Pheu Thai Party has proposed a megaproject to reclaim new land in the Bay of Bangkok as a way of keeping the encroaching sea away from the capital.
Prof Thanawat cautioned these megaprojects may do more harm than good, as the adverse impacts will destroy the fragile marine ecosystems in the Bay of Bangkok that provide bountiful fishery resources for local communities.
"We cannot simply replicate coastal protection projects that work elsewhere and expect the same result here in Bangkok. We need to understand the environmental and ecological differences of each locality to deliver an approach that is compatible with the environmental context of the area," he said.
Since Bangkok has already lost most of its original mangrove forests that acted as a natural barrier against erosion, he suggested that one measure to protect the shoreline is to install clusters of triangular concrete poles at the seafront to mimic the natural function of a mangrove forest.
This would absorb the eroding force of winds and waves and help trap sediment behind the lines of defensive structures.
Prof Thanawat said this shoreline protection technique has proven to be effective in protecting the shoreline from erosion, according to an initial trial at Ban Khun Samut Chin in neighbouring Samut Prakan province.
Apart from flood risks posed by rising sea levels, Wijitbusaba Marome, Assistant Professor at the Faculty of Architecture and Planning, Thammasat University, said rainfall and seasonal riverine flooding are two other prominent factors behind flooding in Bangkok.
"Most flooding in Bangkok is caused by the poor drainage system, which often has trouble draining water after heavy downpours, causing flash floods on the streets. This type of flood is a minor one, as it usually recedes within 24 hours though results in traffic disruption and inconvenience to commuters in the city," she said.
"Meanwhile, riverine flooding due to high water flows in the Chao Phraya River and its tributaries, which normally occurs during late monsoon season from September to November, is behind the more severe type of flood, such as the major floods which the Chao Phraya River Basin witnessed in 1983, 1995, 2006, and 2011."
Asst Prof Wijitbusaba said Bangkok's poor urban planning is also playing a major part in the exacerbation of flood risks.
"Bangkok is expanding without a plan. So, the city is now encroaching into surrounding floodways in the east and west, blocking the natural drainage pathway of seasonal flooding and causing the problem to worsen," she said.
She suggested city planning should be upgraded to ensure the city will grow properly with a more efficient infrastructure to manage floods.
State agencies also should focus on helping vulnerable people cope with the impacts of the crisis.
"Building infrastructure is just another part of preparing for the climate crisis, but the more important issue is to build climate resilience among people to let our city deal with climate impacts in the long run," she added.
Note: This story is the first in a series of four stories about urban planning reform and city development.