Migration to coastal areas is increasingly common in many countries around the world, developed and developing alike. Settlements and urban centres in coastal regions have expanded more rapidly than elsewhere. Of the 39 big metropolitan areas with a population over 5 million, sixty percent are located within 100km of the coast. These include twelve of the biggest 16 with populations exceeding 10 million although the great majority of people live in smaller settlements in the coastal zone. High densities of people are also found in delta regions, which are particularly vulnerable to flooding.
Over the 20th century as a whole, sea levels rose by an average of around 1.7mm a year with evidence that in recent years the rate of rise has increased. Latest satellite data put the rise at around 3mm a year. The most recent IPCC assessment based on the most gloomy scenario puts predictions of 21st century sea level rise at between 26 and 59cm (10-23 inches). There are many variables - including how much the expected increases in precipitation will add to snow packs and, most importantly, our greenhouse gas emissions over the next decades. What we can expect is that even a small amount of sea level rise will have profound and largely negative effects.
What we can expect
With a fossil fuel intensive future, a global average sea level rise of 26-59cm (10-23 inches) is expected over the next hundred years, thanks to the greenhouse gases we have emitted to date and likely future emissions. This will be around two thirds due to thermal expansion of the oceans (water expands as it heats up) and one third due to melting. Even with more optimistic projections based on greenhouse gas emissions being controlled, sea level could rise by between 10-24cm (4-9 inches). Over several centuries, therefore, sea level could rise by several metres.
Even the comparatively modest projected sea level rise will wreak havoc. Coastal flooding and storm damage, eroding shorelines, salt water contamination of fresh water supplies, flooding of coastal wetlands and barrier islands, and an increase in the salinity of estuaries are all realities of even a small amount of sea level rise. Some low lying coastal cities and villages will also be affected. Resources critical to island and coastal populations such as beaches, freshwater, fisheries, coral reefs and atolls, and wildlife habitat are also at risk.
It's worth keeping in mind, however, that changes in sea level do not occur uniformly around the globe. There is actually a fair amount of difference in sea level rise in different parts of the world due to ocean circulation and wind pressure patterns. Accordingly, the effects of storm surges and of spring tides also need to be kept in mind when evaluating sea level rise impacts.
The disappearing Greenland ice sheet
After stabilizing 2000-3000 years ago following the last ice age 20,000 years ago, sea levels remained stable until the late 19th century when then they began to rise once more. The rate of rise seems to be accelerating.
One area of critical concern are the ice sheets of Greenland and Antarctica which if fully melted would raise sea level by 64m. In the case of Greenland, loss of between 50 and 100 billion tons of ice has taken place annually over the period 1993-2003 with evidence of higher rates more recently. Recent research has suggested that the ice sheet could melt completely over the very long term (millennia) if global temperatures rise by somewhere between 1.9 and 4.6 degrees and they have already risen by an estimated 0.8 degrees. Complete melting would lead to a sea level rise of around 6-7m.
Larsen B collapse
Top: 31 January 2002 - Bottom: 05 March 2002
Photos courtesy of the US National Snow and Ice Data Center (NSIDC).
The Antarctic ice sheets
In 2002, the 500 billion tonne Larsen B ice shelf, which covered an area twice the size of greater London, disintegrated in less than a month. This did not directly add to sea level rise since the ice shelf was already floating, but it was a dramatic reminder of the effects of warming in the area. It's also thought that the ice shelf helped to hold some of the area's land-locked ice in place, and now that it's gone more land ice will fall into the sea over time. The Larsen A ice shelf, two thirds as big as Larsen A, collapsed in 1995
In 2005, the British Antarctic Survey released findings that 87 percent of the glaciers on the Antarctic Peninsula have retreated over the past 50 years in common with many of the glaciers being monitored around the world. Between 2000-2005, the Antarctic Peninsula glaciers lost an average of 50 metres (164 feet) per year.
The most recent IPCC assessment suggests that overall, the Antarctic ice sheets could already be losing many billions of tonnes a year of ice, although the precise figures are highly uncertain due to the quality of the data and methodologies used for the estimates. Potentially, according the IPCC, the West Antarctic ice sheet (WAIS) alone could contribute an additional six metres (20 feet) to sea level rise over several thousand years, although more recent estimates suggest a lower figure of 3.3 metres may be more plausible. The differences in these figures are evidence of the huge uncertainties that exist in the predictions of sea level rise, and the huge gamble being taken if greenhouse gas emissions are not quickly stabilized and reduced.
While melting of the Greenland ice sheet and the Western Antarctic ice sheet could ultimately raise sea levels by anything up to 13 metres or so (43 foot) if we do not drastically curb our greenhouse gas emissions, even the small fraction of this predicted by 2100 would be an economic and humanitarian disaster. A few possible consequences of rising sea levels:
- Billions spent on adaptation - if you can afford it. The US, for example, has roughly 20,000 km (12,400 miles) of coastline and more than 32,000km (19,900 miles) of coastal wetlands. A recent study estimated the costs of adapting to even a one metre sea level rise in the US would amount to US$156 billion (3 percent of GNP). More recent estimates of the impact of a 66 cm sea level rise by 2100 suggest costs could be in the region of US$ 236 billion, with nearly 9000 square kilometers of land having to be abandoned and 13,000 km of coastal defences needing to be constructed. Needless to say many poorer countries do not have these resources.
- With a one metre sea level rise some island nations, such as the Maldives, would be submerged. Already, two of the islands that make up Kiribati (a Pacific island nation) have gone under the waves, and in early 2005 others were inundated by a high spring tide that washed away farmland, contaminated wells with salt water, and flooded homes and a hospital.
- If current warming trends are allowed to continue, London, Bangkok and New York, Shanghai and Mumbai will be among a number of cities which will eventually end up below sea level - displacing millions and causing massive economic damage. There come a point at which, building higher and higher sea defences becomes impractical, and even the wealthiest nations will see cities flood.
- Rising oceans will contaminate both surface and underground fresh water supplies - worsening the world's existing fresh water shortage.
- Rural populations will be displaced and farmland (especially rice) on some coasts will be lost to the seas. For example, according to the UK Royal Society a one metre sea level rise could flood 17 percent of Bangladesh, one of the world's poorest countries, displacing tens of millions of people and reducing its rice-farming land by 50 percent.
There is some good news, though. If we act rapidly to reduce emissions we can still prevent the worst effects of climate change. Switching to renewable energy sources, if we do it fast enough, is our only hope to avoid disastrous sea level rise.