Some of Southeast Asia’s largest cities could be hit disproportionately hard by rising sea levels and be underwater by the end of the century, a new study said.
Researchers mapped sea level hotspots worldwide and combined the effects of climate change on sea levels with natural oceanic fluctuations to show how millions of people in coastal cities could be impacted. The study was published on March 2 in the scientific journal Nature Climate Change.
The research showed that internal climate variability could amplify or reduce the impact of climate change on sea level rise along certain coastlines by up to 30% more than would result from climate change alone, exponentially increasing extreme flooding.
By 2100, if variability reaches its upper limit due to high levels of greenhouse gasses, new sea level rise hotspots would appear in Southeast Asian megacities, including Yangon, Bangkok, Ho Chi Minh City, and Manila, as well as Chennai and Kolkata in India.
The population of Metro Manila is 13 million; Bangkok has at least 11 million; Ho Chi Minh City surpasses 9 million; and Yangon has roughly 5 million inhabitants.
The study differs from previous research by incorporating naturally occurring sea level fluctuations, such as El Nino, along with changes in the water cycle, known as internal climate variability, to determine the impact of climate change on sea level rise.
Internal climate variability refers to the natural fluctuations in the earth’s climate system that arise from internal processes such as ocean currents, atmospheric circulation patterns and variations in the earth’s orbit and tilt.
That could result in changes in temperature, precipitation and other climate variables lasting from months to decades.
Variability is not influenced by external factors, including human activities like greenhouse gas emissions.
“The internal climate variability can greatly reinforce or suppress the sea level rise caused by climate change,” said Aixue Hu, a co-author of the paper.
“In a worst-case scenario, the combined effect of climate change and internal climate variability could result in local sea levels rising by more than 50% of what is due to climate change alone, thus posing significant risks of more severe flooding to coastal megacities and threatening millions of people.”
Massive risk for Southeast Asia
Southeast Asia has one of the longest coastlines in the world at 234,000 kilometers (154,400 miles), with an estimated 77% of the population living in coastal areas, making it one of the world's most vulnerable regions, according to a 2021 ASEAN State of Climate Change report.
Last year, the Intergovernmental Panel on Climate Change said in a report that Southeast Asia is at serious risk of losing infrastructure and low-lying coastal settlements due to flooding caused by unavoidable sea level rise.
Sea levels are already on the rise due to an increase in ocean temperatures and melting ice sheets caused by global warming, according to scientists and climate change experts.
Previous studies said coastal flooding in Manila is predicted to occur 18 times more often by 2100 than in 2006, based solely on climate change.
However, the new study showed that in a worst-case scenario, such floods could occur 96 times more often based on climate change and internal climate variability.
Similarly, Yangon was projected to see such rare events rise from 148 to 471 times more frequently, while in Ho Chi Minh City, the increase was from 77 times to 2,882 times more often.
The study said that, considering the internal climate variability impact, “significant changes in frequency of episodic floodings by the end of the century are expected” in low-lying areas that are less than 10 meters (33 feet) above sea level and densely populated regions, such as the Ayeyarwady (Irrawaddy) delta, the Mekong delta, and on the low-lying islands in the tropical Pacific, placing millions of people at risk.
The researchers drew on a set of simulations and statistical analysis with the assumption that greenhouse gasses are emitted at a high rate. The estimates come with considerable uncertainty due to the earth’s complex and unpredictable climate system, they said.
However, they warned, it's still important to be aware of the potential risks to coastal megacities of rising sea levels in order to develop effective adaptation strategies based on projections from continuously improving climate models.
Last year, climate change triggered unprecedented extreme flooding in the Asia-Pacific region, according to an analysis by the European Union’s Copernicus Climate Change Service. The report dubbed 2022 a year of climate extremes, which included deadly floods in Pakistan and large parts of Southeast Asia.
Ocean temperatures have reached their highest-ever level and are expected to continue rising. The world's oceans "were again the hottest in the historical record and exceeded the previous 2021 record maximum," according to a study published in January in the journal Advances in Atmospheric Sciences.
It said the five hottest years for oceans were in the past six years.