WE hear a lot about rising sea levels associated with climate change and the dangers this can pose to coastal areas and island nations such as the Maldives.
But we hear less about the dangers posed by some of the world’s biggest cities actually sinking at the same time, many of which are located in Asia-Pacific. This double-whammy of subsidence and sea level rise has the potential to cause some of Asia’s most populous cities to disappear under water if more isn’t done to curb the rising tides.
The latest report from the Intergovernmental Panel on Climate Change (IPCC) has highlighted the need for climate action and pushed for governments to stick to a 1.5-degree Celsius maximum rise or face catastrophic consequences.
Even with only a 1.5-degree increase – a target that seems likely to be exceeded within a few years – the IPCC predicts a sea rise of between 0.3 and 0.8 metres by 2100. If the current 2-degree target is met, this rises by an additional 10 centimetres.
And Asia is disproportionately at risk, with an estimated four out every five people affected globally living in the region.
Asia is the region with the largest urban population and an estimated 54 percent live in low-lying coastal zones. Add to that the fact that many cities are actively sinking, and they become particularly vulnerable to rising sea level, storm surges and extreme weather events.
Without significant climate action from governments around the world, these major Asian cities may not survive the changing tide.
Jakarta holds the dubious title of the fastest sinking city in the world, subsiding at a rate of around 25.4 centimetres a year.
According to a report by Christian Aid, entitled Sinking cities, rising seas, around 40 percent of the city now lies below current sea levels. Coastal districts, like Muara Baru have sunk as much as 4.3 metres in recent years.
Human impacts have made this already flood-prone area even more vulnerable.
The scale on which groundwater is extracted means the incoming water from rain and rivers is not enough to replenish the soils. Primarily, this is due to 97 percent of the city being covered in impermeable concrete. The result of this is subsidence across the city.
The sheer weight of the buildings and development above land exacerbates the sinking.
On top of this, many of Jakarta’s natural flood defences, such as mangroves, have been removed to make way for settlements. Building on flood plains and coastal marshes has also been allowed to continue putting these communities in real danger.
Just three years ago, Thailand’s own government released a report warning the capital city could be under water within 15 years. This might sound shocking, but with an elevation of only 1.5 metres above sea level, and sinking at a rate of 2 cm per year, it is a very real prospect.
Much like Jakarta, groundwater extraction has played a major role in Bangkok’s subsidence. Its ambitious building push hasn’t helped either.
According to the Sinking cities report, there are about 700 buildings with 20 floors or more and 4,000 buildings with 8-20 floors in the city, putting considerable pressure on the land on which they sit.
With the predicted rise in sea levels, this places large swathes of the city under greater risk. So much so that Thailand’s National Reform Council has recommended building a US$14.3 billion seawall to protect the city.
Once again, groundwater extraction is the key culprit in this city of 12.9 million sinking at an alarming rate.
It is subsiding at a worrying 10 cm per year, increasing the likelihood of flooding while also growing the area affected by floods. Higher tides are encroaching further inland and taking far longer to drain away. This has a knock-on effect on the quality and fertility of the soil needed for crops.
The growing of rice – one of the most water-intensive crops – has been blamed for the water extraction, with locals often claiming their wells run dry when major rice irrigation takes place in an area north of the city.
Unregulated use of water for golf courses and swimming pools has also been blamed.
The central business district of Shanghai has sunk a whole 2.6 metres since 1921 when the first surveys were done.
Groundwater use is again a major factor but, as is common in delta cities, it also suffers from a lack of sediment recharge as sediment is trapped by upstream dams or is extracted for building material.
The weight of infrastructure is also playing a role, and having undeniable consequences. In 2012, an 8m long crack opened up at the foot of the Shanghai Tower project.
Estimates suggest the sinking cost the city over US$2 billion between 2001 and 2010.