Imagine the seaside was nearer. Not just a little bit nearer, but lapping uncomfortably close to your doorstep and rising. If you're one of the 10% of the world's population living in a low-lying coastal area, there’s a chance the ocean might get a little too familiar in the coming decades.
Humanity’s greenhouse gas emissions have been building up in the atmosphere and warming the planet for the past 150 years, dragging us into the uncharted Anthropocene, or age of man. Almost all of the planet's tropical mountain glaciers have retreated or disappeared in recent decades, including those in the South American Andes, Asian Himalaya, and African Rwenzoris.
The resulting meltwater is finding its way into the oceans. Globally, they are rising at an average of 3.5 millimetres per year – roughly twice the rate seen during the 20th Century. Sea levels are expected to rise by around 2.3 metres (7.6 feet) for every 1C of warming in the coming decades, according to a study published by scientists at the Potsdam Institute for Climate Research last month. Most of the rise in the past decade was thanks to thermal expansion – at higher temperatures the water takes up a greater volume because its molecules move about more. Now however glacial melt has overtaken thermal expansion as the leading cause of rising sea levels.
At the poles, change is underway of a magnitude so extreme that Earth hasn’t experienced its like for over 10 million years. The Arctic is warming at least twice as fast as anywhere else on the planet. One estimate suggests future average global warming of 2-3C, for example, would mean Arctic warming of 6-8C. This is partly because of the hot southerly winds bearing soot pollutants converging on the region. These particles absorb the sun’s rays and when they land on the surface attached to snowflakes they darken it and speed up melting.
Almost all the Arctic glaciers are shrinking. The Greenland ice sheet, the second largest in the world, is losing 200 gigatonnes of ice a year, four times more than a decade ago. In 2011 researchers found that the area occupied by crevasses in one part of the ice sheet had increased by 13% since 1985, speeding its flow into the sea. If the ice sheet melts completely, it would cause a global sea level rise of 7m (23feet). Its melting alongside the Antarctic ice sheet, the largest of all, would clearly be a global transformer of epic proportions.
Climate models predict that the melting of ice sheets in Greenland and Western Antarctica will reach a tipping point after which the rate will suddenly accelerate. This is because meltwater lubricates their faster break-up. Also, being darker than ice and snow, the water now covering a greater proportion of these areas will absorb more of the sun’s heat, speeding up melting even more.
The various models differ on the rate of sea level rise they predict. James Hansen's group at NASA Goddard predicts a global rise of as much as 7m (20ft) by 2100. More conservative models, such as that of Stefan Rahmsdorf’s group, at the Potsdam Institute in Germany, peg it at 1.4 metres (4.7ft) by the end of the century. That would be enough to inundate low-lying islands like the Maldives, along with many of the world’s major cities. Even Rahmsdorf says that sea level rise of metres rather than centimetres cannot be ruled out if tipping points not included in his model occur.
Many of the world's greatest cities would become unliveable long before the most extreme sea-level rise occurred. Here's a world map showing the landmasses most vulnerable to rising seas. As the waters rise, high tides, ocean swell, and storm surges will cause growing numbers of deaths as well as increased property and infrastructure damage.