Although sea levels have been rising since the end of the last glaciation (nearly 11,000 years), the rate of rise has increased over the past 200 years as average global temperatures have increased. The rise is due to two factors, the freshwater being added to the oceans from ice melt in the cryosphere , and the thermal expansion of the oceans due to rises in sea temperature.

The contribution from Antarctica melt water is uncertain, and there is a distinct possibility of surprises from this southern region. The floating ice shelves, notably the Wordie and Larsen A and B shelves, broke up very rapidly during the 1990s, after rapid regional warming. Climate, like other complex systems do not always vary in a smooth fashion, and sudden changes can occur over wide areas. Critical levels, or thresholds may be reached in a system whereupon drastic, and perhaps disastrous results occur.

Threshold events in this case include the complete or partial shutdown of the ocean thermohaline circulatory system, disintegration and melting of Antarctica and Greenland Ice Sheets (the polar caps) , and major changes in the carbon cycle, due to biospheric effects (see the Snowball Earth scenario).

The IPCC Report

The IPCC 4th Report shows there is strong evidence that global levels gradually rose in the 20th century and is currently rising at an increased rate, after a period of little change between AD 0 and AD 1900.

Levels are projected to rise at an even greater rate in this century. The two major causes for the rise are thermal expansion of the oceans (water expands as it warms) and the loss of land-based ice due to increased melting.

How Much Is The Sea Rising?

Estimates for the 20th century show that global average sea level rose at a rate of about 1.7 mm per year. Satellite observations available since the early 1990s provide more accurate data with nearly global coverage. This decade-long satellite altimetry data set shows that since 1993, and shows rising at a rate of around 3 mm per year, significantly higher than the average during the previous half century. Global levels are projected to rise during the 21st century at a greater rate than during 1961 to 2003.

Thermal expansion is projected to contribute more than half of the average rise, but land ice will lose mass increasingly rapidly as the century progresses. An important uncertainty relates to whether discharge of ice from the ice sheets will continue to increase as a consequence of accelerated ice flow, as has been observed in recent years. In particular, the Arctic is warming at a higher than global average, resulting in increasing surface melt from the Greenland Ice Sheet.

More detail on how the oceans are measured is available here.

Figure Above: Time series of global mean levels (deviation from the 1980-1999 mean) in the past and as projected for the future. For the period before 1870, global measurements of sea level are not available. The grey shading shows the uncertainty in the estimated long-term rate of change. The red line is a reconstruction of global mean sea level from tide gauges and the red shading denotes the range of variations from a smooth curve. The green line shows global mean sea level observed from satellite altimetry. The blue shading represents the range of model projections for the 21st century, relative to the 1980 to 1999 mean, and has been calculated independently from the observations. Beyond 2100, the projections are increasingly dependent on the emissions scenario.


Impacts of Sea Level Rise

Rapid urbanisation in low-lying coastal areas of both the developing and developed world is increasing population densities and the value of human-made assets exposed to coastal climatic extremes such as tropical cyclones. IPCC model based projections of the average annual number of people who would be flooded by coastal storm surges is estimated to increase several fold, creating 200 million climate refugees.

This is based on what is called a ‘mid-range scenario’ of a 40-cm sea-level rise by 2080, which is pretty conservative. The U.S. Environmental Protection Agency (EPA) notes that the sea level has risen 15 to 20 cm (6 to 8 inches) in the past 100 years, and it is predicted to continue another 50 centimeters (20 inches) over the next century (with some estimates as high as 90 centimeters, or 3 feet).

The sea level is definitely rising, and it is jeopardizing rapidly growing coastal communities. Official decisions on evacuation of whole populations from some atolls in the Pacific Ocean have been taken or are being considered. For example, 980 people, the entire population of the Carteret Atoll, will need to be evacuated by 2015, and the island is destined to become history. A similar fate awaits the small nation of Tuvalu and Majuro in the Marshall Islands.

The potential of damage to infrastructure in coastal areas from sea-level rise will be tens of billions US$ for individual countries like; Egypt, Poland, and Vietnam.

The first image above on the far left was taken on 12 August 1997 of a house at Floralton Beach, Florida. When Hurricane Frances came through on 8 September 2004 all vegetation and dune lines were wiped out (middle image). As a result, the house was directly exposed and completely destroyed when coastal surges from Hurricane Jeanne hit on 29 September 2004.

It is interesting to note that seventy one percent of annual United States disaster losses are the result of coastal storms. It is estimated that within 60 years, one out of every four of those structures will be destroyed and insurance costs will sky rocket. This is not suprising given that the narrow fringe comprising less than one fifth of the contiguous United States land area, accounts for over one half of the nation’s population and housing supply.

Below you can watch a video on the impacts of sea level rise on coastal communities in Orissa, India. The poor village literally got sucked into the rising ocean. The sea used to be a half day walk away, now it has claimed their village.

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