The rise 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 rise in sea level will wreak havoc, including:
- 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.
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 are also at risk.
"The Maldives is one of the small states. We are not in a position to change the course of events in the world. But what you do or do not do here will greatly influence the fate of my people. It can also change the course of world history." -- Statement by H.E. Maumoon Abdul Gayoom (Maldives) Kyoto, Japan, 3rd Conference of the Parties of the UNFCCC
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. So, it isn’t only small island states that need to worry about sea level rise.
Over the 20th century sea levels rose between 10 and 20 centimetres (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, the level of greenhouse gas emissions over the next decades. But even a small amount of sea level rise will have profound negative effects.
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 10 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:
"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 3°C [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 7 m [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 1.4-5.8° Celsius (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. The entire Antarctic ice sheet holds enough water to raise global sea levels by 62 metres (203 feet).
In 2002, the 500 billion tonne Larsen B ice shelf, covering an area twice greater London, disintegrated in less than a month. This did not directly add to a rise in sea level 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 50 metres (164 feet) per year.
Potentially, the West Antarctic ice sheet could contribute an additional six metres (20 feet) to the rise in sea level. Although the chances of this are considered "low" in the Intergovernmental Panel on Climate Change’s Third Assessment report, recent research indicates new evidence of massive ice discharge from this ice sheet.
The world could face a 13 metre (43 foot) rise in seal levels from the melting of the Greenland ice sheet and the Western Antarctic ice sheets, unless global greenhouse gas emissions are cut significantly. Even a small fraction of this much sea level rise would be an economic and humanitarian disaster. Some possible consequences:
- Billions spent on adaptation. The US has roughly 20,000 km (12,400 miles) of coastline and more than 32,000 km (19,900 miles) of coastal wetlands. A study estimated that adapting to even a one metre sea level rise in the US would cost US$156 billion. Most countries don't have this kind of money to spend.
- A one metre sea level rise would submerge some small island nations, such as the Maldives.
- Two of the Pacific islands of Kiribati have gone under the waves.
- In early 2005 other Kiribati islands 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 existing shortage of the world's fresh water.
- Underground water sources in Thailand, Israel, China, Vietnam and some island states are already experiencing salt water contamination.
- Rural populations and farmland (especially rice farms) on some coasts will be wiped out. The UK Royal Society says 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.
The disaster that will come with a rise in sea levels can be prevented, if greenhouse gas emissions are reduced. Switching to renewable energy sources quickly can help avoid a disastrous rise in sea levels.