Rising sea levels are already putting millions at risk of flooding, but a new study warns that land subsidence is making the danger even greater. Earth's sinking cities are slipping toward sea level at an alarming rate, experts from the Technical University of Munich have warned.
Subsidence More Than Doubles Sea Level Rise
In a new study, scientists found that subsidence more than doubles the rate of sea level rise for some areas of the coastline. To make matters worse, researchers say that this affects the largest and most densely populated cities more than anywhere else. Between rising seas and sinking land, heavily urbanised areas of coast experience a relative sea level increase of about six millimetres per year on average. That is three times more than the global average for relative sea level rise, which stands at 2.1 millimetres per year. Likewise, land subsidence roughly doubles the absolute sea-level rise of 3.15 millimetres per year, which measures the actual increase in volume and height of the ocean.
Global Hotspots of Subsidence
Lead researcher Dr Julius Oelsmann, of Technical University of Munich, says that this can 'significantly amplify the effects of climate-driven sea-level rise'. Scientists have found that land subsidence is doubling the rate at which the water level rises in some of the world's biggest cities, putting millions at risk of flooding. Jakarta is the world's fastest-sinking city, subsiding at a rate of 13.7 millimetres per year. This puts the megacity's 42 million residents in extreme danger of flooding. Other prominent subsidence hot spots include Tianjin (-13.5 mm/year), Bangkok (-8.5 mm/year), Lagos (-6.7 mm/year), and Alexandria (-4 mm/year).
Drivers of Land Subsidence
As the climate gets hotter, melting glaciers and the expansion of warming water are gradually raising the level of the world's oceans. However, Dr Oelsmann and his co-authors warn that the surface of the sea only tells half the story. 'If we want to understand sea-level rise along coastlines and respond effectively, we must not only observe the ocean but also the land itself,' Dr Oelsmann says. This is because a mixture of human activity and natural forces is combining to sink some of the world's biggest cities into the ocean. The biggest factors driving this subsidence are excessive groundwater and oil extraction, which remove underground resources that previously stabilised the surface. Dr Oelsmann points out that the 'sheer weight of cities' is also driving urban areas below sea level. As cities grow larger and taller, heavier buildings are constructed, which compacts the ground beneath and slowly sinks the city relative to its surroundings.
Regional Impacts and Future Risks
Countries where relative sea levels are rising the fastest include Thailand, Bangladesh, Nigeria, Egypt, China, and Indonesia, where the ocean is getting seven to 10 millimetres higher each year. The US, the Netherlands, and Italy also experience exceptionally rapid increases, with relative sea levels rising by around four to five millimetres per year. Due to the strong influence of city size, many countries also featured intense 'hot spots' of subsidence. The 42 million residents of Jakarta, Indonesia, the world's most populous city, are in particular peril as the megacity slips towards the ocean at a rate of 13.7 millimetres per year. This is closely followed by Tianjin, China, home to 13.8 million people, which is seeing 13.5 millimetres of subsidence per year. Similarly, Bangkok, Lagos, and Alexandria are all experiencing well above average rates of subsidence with 8.5, 6.7, and four millimetres per year, respectively.
Even within cities, the different rates of subsidence meant that one neighbourhood might be falling towards the ocean while others rise out of it. In Jakarta, for example, some parts of the city are sinking at a staggering 42 millimetres per year, while other regions actually see uplift. This means that millions of people in some of the world's biggest cities are being put at risk of severe flooding. Even if their homes don't fall completely below sea level, every millimetre of relative sea level increase creates a bigger risk that storms or extreme weather will lead to severe floods. This is especially concerning for Jakarta, where about 40 per cent of the city is below sea level. Studies estimate that nearly half of the city could be inundated and uninhabitable by 2050 if sea levels continue to rise at the current rate.
Contrast with Scandinavia
These dense, urban regions stand in stark contrast to Scandinavia, where natural geological processes are gradually lifting the land out of the sea. During the last Ice Age, these northern latitudes were weighed down with vast ice sheets that pushed the land into the ocean, much like coast megacities do today. As these ice sheets have retreated, that weight was removed, and the land masses are still 'rebounding' towards a stable position. That means, even as absolute sea levels increase, the relative sea level in Finland and Sweden is actually getting lower each year. Unfortunately for the rest of the world, there are no geological processes to pull cities back out of the water.
Mitigation Through City Planning
However, the researchers point out that proper city planning can dramatically slow the rate of subsidence. Co-author Professor Florian Seitz, of the Technical University of Munich, says: 'In many large coastal cities, groundwater extraction is a major driver of land subsidence. This means that local political and water-management decisions can make a significant difference.' In Tokyo, Japan, for example, subsidence rates once exceeded 10 centimetres per year, reaching peaks of 24 centimetres per year in the worst areas. However, through government intervention and the introduction of new water sources, those rates were dramatically reduced. 'Improved groundwater management, stricter regulation of withdrawals, or targeted recharge of aquifers can at least slow subsidence rates and, in some cases, largely halt them,' says Professor Seitz.
Long-Term Sea Level Rise Projections
Global sea levels could rise as much as 1.2 metres (4 feet) by 2300 even if we meet the 2015 Paris climate goals, scientists have warned. The long-term change will be driven by a thaw of ice from Greenland to Antarctica that is set to re-draw global coastlines. Sea level rise threatens cities from Shanghai to London, to low-lying swathes of Florida or Bangladesh, and to entire nations such as the Maldives. It is vital that we curb emissions as soon as possible to avoid an even greater rise, a German-led team of researchers said in a new report. By 2300, the report projected that sea levels would gain by 0.7-1.2 metres, even if almost 200 nations fully meet goals under the 2015 Paris Agreement. Targets set by the accords include cutting greenhouse gas emissions to net zero in the second half of this century. Ocean levels will rise inexorably because heat-trapping industrial gases already emitted will linger in the atmosphere, melting more ice, it said. In addition, water naturally expands as it warms above four degrees Celsius (39.2°F). Every five years of delay beyond 2020 in peaking global emissions would mean an extra 8 inches (20 centimetres) of sea level rise by 2300. 'Sea level is often communicated as a really slow process that you can't do much about ... but the next 30 years really matter,' said lead author Dr Matthias Mengel, of the Potsdam Institute for Climate Impact Research, in Potsdam, Germany. None of the nearly 200 governments to sign the Paris Accords are on track to meet its pledges.
