Cities at sea level around the world – including Bangkok, New Orleans, Shanghai and Amsterdam – are bracing themselves for rising seas and sinking ground. Populations on river deltas, atolls and islands face flooding and displacement. Sea-level rise accumulates slowly, measured in millimetres a year, but the incremental pace can deceive us. Sea-level rise, particularly when combined with sinking land, presents a growing problem.

Consider that the rate of sea-level rise is itself rising. Sea rise remained virtually zero over the last several millennia. Then, in the 20th century, the sea rose about 20 centimeters. Now, today, the rate has reached about 30 centimeters per century, and still increasing. Recently, oceanographers have boosted their predictions of 21st century sea level rise from about 20 centimetres to a metre or more.

Sea-level rise is not uniform around the world. Gravitational forces, including the gravity of ice caps themselves, cause uneven fluctuations. Meanwhile, some coastal plains sink as others rise, so exaggerating sea-level rise in some regions and cancelling it in others. Furthermore, if humanity cannot change its hydrocarbon habits quickly enough, we risk runaway warming that could accelerate sea-level rise.

In an extreme runaway scenario, a complete melting of the Greenland ice sheet would add 7 metresto the world’s oceans, and a complete melting of the Antarctic sheet would add 60 metres. Those scenarios would require a massive restructuring of human civilisation as we know it. However, even a one-to-two metre rise in sea level will inundate certain port cities, islands, atolls, flood deltas and coastal plains, obliterate vulnerable species and displace millions of people. 

Sea changes in history

Earth’s coastal plains and inland seas have dried out and flooded many times. Historically, sea-level changes disrupt marine shallows, intertidal zones and coastal ecosystems - the most productive habitats - leading to substantial species loss. About 235 million years ago a massive ecosystem collapse, associated with warming and sea rise, obliterated 95% of all living species, the greatest diversity loss event in Earth history. Sixty-five million years ago a meteorite struck the Gulf of Mexico region and initiated a long cooling trend. As water froze, the sea level dropped and 75% of all species, including the dinosaurs on land, perished.

Over the last 20 million years, during the Miocene period, the Mediterranean basin dried and flooded several times. The basin finally filled with a catastrophic flood about 5.3 million years ago, when human ancestors Kenyapithecus and ‘Toumaï’ (Sahelanthropus tchadensis) roamed the forests of east Africa. 

Later floods affected human settlements. During the most recent glacial maximum, 20,000 years ago, sea levels dropped about 125 metres. The Mediterranean basin partially dried and was re-flooded in about 16,000 BCE. The Caspian and Black Seas may have flooded later, about 13,000 BCE, from melting Scandinavia ice sheets. The Black Sea likely flooded twice again about 10,000 and 7,600 years ago as the world’s oceans rose.

The Dogger Banks and other shallows around Britain and Ireland were dry lowlands during the last glaciation. The plains provided reindeer for human hunters and a land link from the European mainland to the British Isles. Human encampments have been identified on the ocean floor. During the post-glacial melt, sea water and fresh water from ice-dammed lakes flooded Doggerland and separated Britain and Ireland from Europe. 

The lower Tigris-Euphrates valley also flooded during the post-glacial melt under the rising Persian Gulf. Lowlands around Indonesia, Australia, New Guinea and East Asia also flooded. Many of these floods submerged human settlements and hunting regions, likely inspiring the universal deluge and flood stories found in most human cultures. There may be a thousand submerged Atlantis-like cities, still undiscovered, and there may be more in the future.

Modern sea rise 

August 11 2010: After breaking off the Petermann Glacier six days earlier, a massive ice island floats slowly down the fjord toward the Nares Strait. Scientists warn that loss of the ice from this glacier is almost certain to speed up the rate at which ice from the Greenland icesheet melts into our oceans. Image: NASA

After the last ice age, as Earth warmed, melting ice raised sea levels by an average of about one metre a century, peaked at four metres a century, until about 7,000 years ago when Earth’s sea level stabilised. During the 2,000 years between 200 BCE and the year 1800, Earth’s sea level only rose about 20 centimetres, one centimetre a century, not enough to disrupt human coastal settlements.

However, after 1800 - during a century of human hydrocarbon industrialisation - the seas rose ten times faster, 10 centimetres a century. In the 20th century, this rate doubled to 20 centimetres, and now stands at 30 centimetres a century, 30 times faster than any period during the previous 7,000 years prior to 1800.

 The rate of sea rise continues to increase, and this acceleration makes predictions challenging. We do not know how fast the sea may rise in the future. Most oceanographers last century believed the rise would be about 20 centimetres a century. By 2007, the IPCC assumed an average rate during the 21st century of 50 centimetres. Oceanographers now estimate that the seas will rise between one and two metres this century.

But we must keep in mind that the seas won’t suddenly stop rising in 2100, and the rate could be much higher by then. Sea-level change follows what mathematicians refer to as ‘compound integration’. First, human carbon emissions drive temperature change, which in turn melts ice and drives sea-level change. This double integration means that there could be a centuries-long lag between the initial carbon emissions and the final sea-level effect.

Furthermore, global heating from greenhouse gasses can be jolted by non-linear effects, dramatic jumps in impact from relatively small carbon emissions. One such non-linear effect is dynamic ice response, whereby melting creates cavities in ice sheets that increase the melt rate. Other ‘runaway’ factors include methane released from permafrost, the reflective power of water versus ice, forests dying in the heat, and so forth. If humanity triggers runaway global warming, then Earth could enter a long warming period independent of human mitigation efforts. If such a heating period melts the world’s glaciers and both poles, the seas would rise by some 70 metres, creating another thousand Atlantises and a billion displaced people. 

Sinking Cities Today

Anjana Koyal lives in Satjellia island, India and is one of the many people affected by sea level rise: "I am a student and my school is flooded with water. There are too many mosquitoes, flies, and a bad smells comes from the water." Image: Peter Caton / Greenpeace

There is a big difference between a 1-metre and a 70-metre sea-level rise, but even at current sea-level rise rates, some major cities already face urgent and expensive adaptations. Water does not negotiate. It does not compact, and it relentlessly obeys the law of gravity. Even one millimetre of sea water over the top of a levee can fill a vast coastal plain or flood a city. The citizens of the Netherlands, New Orleans and Bangladesh can attest to this.

Amsterdam and Venice, for example, face both rising seas and sinking land. Venice has sunk about 30 centimetres over the past 100 years, doubling the effect of the Mediterranean sea-level rise. The Italian government has budgeted several billion euros to preserve Venice with flood defences, but engineers warn that even this may not be enough to save Venice in its current location.

The Netherlands land base is sinking, as deep mantle rock flows from this region, adding to the effect of sea-level rise. Amsterdam sits four metres below sea level. The Dutch Veerman Committee for coastal maintenance expects the sea level to rise between 65 and 130 centimetres over the next century, requiring a billion-euro annual budget in coastal maintenance and dam construction. Each year, crews deposit some 14 million cubic metres of sand on the intertidal zones just to combat erosion.

Other cities, such as Houston, Texas and Shanghai, China, battle rising seas and sinking ground caused by human activity. Houston is sinking from both groundwater and oil extraction, which undermines the coastal substrate foundation. Shanghai, onthe Yangtze River delta, grew from a fishing village to a city of over 20 million people. The city is simply too heavy for its swampland foundation. Aggravated by water extraction, the city sank 2.5 metres between 1921 and 1965, and continues to sink. According to China's State Oceanic Administration, ‘Sea level rises worldwide cannot be reversed’ so China must ‘adapt to the change’ by building levees and dykes.

A topographical study at the University of Colorado, concluded that ‘most of the world’s low-lying river deltas are sinking from human activity ... putting tens of millions of people at risk’. Vulnerable river deltas include the Ganges-Brahmaputra in Bangladesh, Pearl River in China, and the Mekong in Vietnam, and 24 of the worlds’ 33 major deltas. Regions such as Florida, Belize, the Bahamas and the Maldives, and cities such as Trieste, Bangkok and Dacca, also remain vulnerable.

Sinking cities and rising seas are symptoms of unsustainable human activity and habitat overshoot. Humanity has grown beyond the biological and physical limits of its Earth habitat. These limits manifest as global warming, rising seas, sinking cities, drained aquifers, disappearing species, dying forests, human starvation and islands of floating plastic.

Meanwhile, the cost of adapting – billions of dollars for new levees, dams and climate change mitigation – demands scarce resources that are needed to expand education, food production and social services. To meet the costs of adaption, industrial nations will push their economies to grow, adding to the root problem of habitat overshoot. Every millimetre of rising sea water, every drop of water from the melting ice, is a message from Earth to humanity. 

-Rex Weyler
October 2010

Deep Green is Rex Weyler's monthly column, reflecting on the roots of activism, environmentalism, and Greenpeace's past, present, and future. The opinions here are his own.