SINGAPORE - As the world warms and ice caps melt, global sea levels are rising including around Singapore.
The island's average sea level today is 14cm above pre-1970 levels, said the Meteorological Service Singapore (MSS) on Monday (March 23) in its annual climate assessment report.
Global warming has led to several consequences, noted the weatherman, with rising sea levels being one of them.
"Warming oceans cause sea level to rise," said the MSS report. Water expands when heated, and this contributes to sea-level rise.
"Similarly, melting of glaciers, Greenland and Antarctica ice sheets ultimately reach the ocean, thus further increasing sea level," it added.
Data from tide gauges has shown that in the 20th century, global sea level has been rising at a rate of 1.2mm to 1.9mm a year, said the MSS report.
The thermal expansion of water was the main driver of sea-level rise in the 20th century, said sea level rise expert Benjamin Horton from the Nanyang Technological University (NTU).
But accelerating rates of ice melt from the world's ice sheets were exacerbating the situation in the 21st century, he added.
MSS said that since 1993, when high-precision satellite data was available for sea-level monitoring, the global average sea level rose at a rate of about 3.24mm per year. Sea levels reached about 90mm above the 1993 level - the highest recorded so far - in 2019, noted the MSS report.
Professor Horton said: "If all of Greenland is melted, it will contribute 6m to sea-level rise." But the impact is much greater for Antarctica, he added - if the Antarctic Ice Sheet melts completely, sea levels could go up by about 60m.
The MSS report follows another released by the United Nations climate science body last September, which also found that if warming continued unabated, sea levels may rise by several metres in the centuries ahead. Earlier estimates had said sea levels could rise by about 1m by 2100.
Severe sea-level events, which could overwhelm low-lying areas in Singapore, could also start happening once every year by around 2055 to 2065, instead of once a century, said the Intergovernmental Panel on Climate Change (IPCC)in a special report on the oceans and cryosphere - the frozen parts of the planet.
Sea level variability in Singapore
But the study of how sea levels can impact coastlines is a complex one. For one, sea level rise is not uniform, but varies regionally and at different time scales.
"In fact, in a number of regions, sea level trends deviate significantly from the global mean," said the MSS.
Singapore, for example, might experience higher rates of sea-level rise than the global mean, said Prof Horton.
"If an ice sheet melts, its gravitational attraction decreases and sea levels around it can go down," he explained. "Conversely, regions far from a melting ice sheet, such as Singapore, will see a rise in sea level greater than the global average.
And while IPCC reports had projected global mean sea level rise by 2100 and beyond, sea levels can also vary over shorter periods.
For example, during the north-east monsoon - usually the region's rainy season for the year - sea levels can rise by up to 0.2m, said MSS.
During this period, winds blow into Singapore from the north-east or north-west. As these winds blow over the South China Sea, they gather moisture before dumping it as rain in the equatorial South-east Asia region.
These "atmospheric flows" also interact with the surrounding seas and oceans, which has an impact on the height of the sea surface, said MSS.
Sea levels can also vary during the occurrence of natural climate phenomena, such as El Nino events, which can occur every two to seven years. The last major El Nino event occurred in 2015.
El Nino is a climate phenomenon that draws rainfall away from countries in the western Pacific, such as Singapore, towards the central or eastern ends of the Pacific Ocean.
This change in rainfall patterns occurs due to changes in atmospheric pressure and sea surface temperatures across the Pacific Ocean, and these changes can have an impact on the strengths of the winds blowing across the ocean basin.
Under normal conditions, trade winds blow from east to west, causing water to "pile up" around South-east Asia. But during El Nino events, these winds relax, causing water to "slosh" back towards the east, and resulting in lower sea levels around Singapore.
The MSS said it is studying how sea levels around Singapore were affected by the 2015 El Nino event, as well as the positive phase of the Indian Ocean Dipole - another climate phenomenon similar to El Nino, but centred on the Indian Ocean - which occurred last year.
"This is currently being studied and will be part of a soon-to-be published journal paper," said the MSS spokesman, adding that more details will be released later.
Singapore, as a low-lying nation with about 30 per cent of the island less than 5m above the mean sea level, is well aware of its vulnerability to sea-level rise and is taking steps to mitigate the impact.
Last August, Prime Minister Lee Hsien Loong said Singapore would need to spend $100 billion over the long term to buffer its coast from the rising tides. Last month, Deputy Prime Minister Heng Swee Keat said Singapore's new coastal and flood protection fund will get its first injection of $5 billion.
Scientific research is also ongoing.
The MSS spokesman: "The National Sea Level Programme will use this information to better understand the sea level variability. This is crucial in order to better assess future projections of sea level rise, since the long-term trends are superimposed onto this variability."