It is not only small island states that need to worry about sea level rise. More than 70 percent of the world's population lives on coastal plains, and 11 of the world's 15 largest cities are on the coast or estuaries.
Over the 20th century sea levels rose between 4-8 inches. The IPCC puts predictions of 21st century sea level rise at 9 to 88 cm. There are many variables — including how much the expected increase in precipitation will add to snow packs and, most importantly, our greenhouse gas emissions over the next decades. What we do know is that even a small amount of sea level rise will have profound negative effects.
What we can expect
A global average sea level rise of 3.5-34.6 inches is expected over the next hundred years, thanks to the greenhouse gasses we have emitted to date and likely future emissions. This will come in roughly equal measure from melting ice and from thermal expansion of the oceans (water expands as it heats up).
Even this 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 costal 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 is also at risk.
It's worth keeping in mind 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. The effects of storm surges and spring tides need to also be kept in mind when evaluating sea level rise impacts.
The disappearing Greenland ice sheet
Over the last hundred years, sea levels rose ten times faster then their 2000-year average, but in the next hundred years the rate of sea level rise could increase dramatically. One frighteningly real possibility is the melting of Greenland's ice sheet. According to the IPCC:
"Climate models indicate that the local warming over Greenland is likely to be one to three times the global average. Ice sheet models project that a local warming of larger than 5.4°F, if sustained for millennia, would lead to virtually a complete melting of the Greenland ice sheet with a resulting sea-level rise of about 23 ft."
— IPCC 3rd Assessment, Synthesis Report, Summary for Policy Makers
The amount of global warming predicted by the IPCC over the next hundred years is 2.5-10.4°F, and warming around Greenland is likely to be one to three times the global average. The Greenland ice sheet is already shrinking and melting.
The West Antarctic ice sheet
Only four years ago, it was commonly accepted that the West Antarctic ice sheet was stable, but unexpected melting in the region is causing scientists to re-think this assumption.
In 2002, the 500 billion ton 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 collapsed in 1995 and was two-thirds as big as Larsen B.
Then 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 the past five years, the retreating glaciers have lost an average of 164 feet per year.
Potentially, the West Antarctic ice sheet (WAIS) could contribute an additional 20 feet to sea level rise. Although the chances of this are considered "low" in the IPCC's Third Assessment report, recent research indicates new evidence of massive ice discharge from the WAIS.
The entire Antarctic ice sheet holds enough water to raise global sea levels by 203 feet.
Between the Greenland ice sheet and the Western Antarctic ice sheet the world could well be facing a 43 foot rise in sea level if we do not drastically curb our greenhouse gas emissions. Even a small fraction of this much sea level rise 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 has roughly 12,400 miles of coastline and more than 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 $156 billion (3 percent of GNP). Most countries don't have this kind of money to spend.
- With only 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 saltwater, and flooded homes and a hospital.
- If current warming trends continue, cities like London, Bangkok and New York will end up below sea level — displacing millions and causing massive economic damage. Alexandria, Egypt, is one of the many cities that could be inundated by a one meter sea level rise. At some point, building higher and higher sea walls 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. Underground water sources in Thailand, Israel, China, Vietnam and some island states are already experiencing salt water contamination.
- Rural populations and farmland (especially rice) on some coasts will be wiped out. 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.
UNFCCC (2005) climate change, small island developing states.