Rising Sea Levels
Trying to find out more about rising sea levels?
Sea level changes can be local (i.e. isostatic) or global (eustatic).
Isostatic changes in the sea level can be measured by the local mean sea level (LMSL). Changes in the height of the sea relative to a particular land surface are being observed over time (eg. a month), and fluctuations as a result of waves and tides are factored out mathematically. Localized vertical movements of the land (e.g. when the tectonic plates adjust to pressure release when glaciers melt) can affect the calculations of LMSL. In addition, localized changes in ocean currents and atmospheric pressures can also affect LMSL values.
On the other hand, eustatic changes in sea level measures global changes in the volume of the world’s oceans. When we talk about how global warming causes glaciers to melt and subsequently bring about rising sea levels, we are usually referring to eustatic increases in sea levels.
Causes of rising sea levels
Global sea levels have been rising for the past century. According to the Australian government Department of Sustainability, Environment, Water, Population and Communities, Australian Antarctic Division, research combining satellite altimetry and in situ data showed a rise in global average sea level by about 20cm (an average of 1.7mm per year) from 1870 to 2004.
And this rising sea levels has been largely attributed to global warming. In fact, the National Snow and Ice Data Center considered rising sea levels as one of the most serious consequences of global warming. Cited by NASA, the Intergovernmental Panel on Climate Change (IPCC) report in 2001 documented that global temperature increases of 1.4° to 5.8°C could increase global sea level by 0.09-0.88m (or 4 inches to 2.9 feet) by 2100.
According to the 2007 report “Observations: Oceanic Climate Change and Sea Level", in “The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (iPCC)”, rises in the global average sea level today are a result of two main processes related to global warming – thermal expansion of water and melting of glaciers.
The extreme levels of carbon emissions released by human activities have reached all time high, and due to this, excessive heat is getting trapped in our atmosphere – the green house effect. As the earth's temperature continues to rise, the temperature of the oceanic waters also increases. This temperature increase then brings about an increase in the volume of the oceanic waters (relative to the volume of the oceanic basins holding the water). This process is known as the thermal expansion of water.
With global warming, the melting of glaciers and ice sheets and caps in the polar (e.g. Arctic) and non-polar (e.g. Greenland) regions in the world is also being accelerated. As the large volumes of freshwater released from the melted glaciers flow into the rivers, seas and oceans, they add to the volume of water in the world’s oceans, increasing the global average sea levels. With the melting glaciers, the large ice sheets that function like reflectors of the sun's rays are being reduced, enhancing the green house effect brought about by the green house gasses, in turn melting the glaciers at an even faster rate. A vicious cycle has been created.
Other factors such as vertical land movements arising from glacial isostatic adjustment (GIA), plate tectonics or sedimentation can also cause rising sea levels by altering the volume of the ocean basins containing the sea water (which remains constant).
Factors such as changes in ocean currents and atmospheric pressures affect local mean sea levels but usually do not affect global average sea level.
In 2007, the International Panel on Climate Change (IPCC) estimated that the global average sea level will rise between 0.6 and 2 feet (0.18 to 0.59 meters) in the next century. This rise would not be uniform around the world. In fact, depending on the location, the LMSL might be several times higher, or lower, than the global average. The estimated range was rather large because of uncertainty when it comes to predicting global temperatures and the rate of glacier melting. Moreover, current sea level changes might also be affected by climate change events taking place centuries (or even longer) ago.
Effects of rising sea levels
Rising sea levels is a disaster for the environment and wildlife, and is just as devastating for humans.
According to GreenPeace, over 70% of the world's population is settled on coastal plains, and 11 of the world's 15 largest cities are actually found along the coast. With the rate that the global average sea level is rising today, many of these communities would be displaced in no time. Cities, villages and farmlands would need to be relocated, or risk being wiped out by floods. For small island countries like Maldives with average height of just 1.5 m above sea level, a rise of just 1m in the sea level could submerge a large portion of the island underwater.
The rise in sea level can lead to flooding over low lying coastal regions. The overflow of sea water inland can threaten the supplies of fresh drinking water found on lower inland grounds, through contaminating the surface and underground freshwater sources. The contamination of already limited freshwater supplies on this planet will only serve to worsen the world’s freshwater crisis. According to GreenPeace, countries like Thailand, Vietnam, Israel and some island countries are already experiencing saltwater contamination of their underground water supplies.
Other than contaminating freshwater supplies, flooding that arise from rising sea levels can disrupt the containment functions of landfills and hazardous-waste sites, threatening to disperse the polluting materials found on such sites to surrounding populated areas or natural habitats.
Rising sea levels not only mean loss of land for humans. The animal kingdom is also affected.
Do you know that the polar bear is becoming an endangered species because its habitat is affected? As glaciers melt and sea level rises, the polar bear’s habitat shrinks. Similarly, its hunting and breeding grounds are also affected.
Other than polar bears, marine fishes and shellfishes are also affected by rising sea levels. As sea level rises, former tidal wetlands that serve as nurseries to these marine populations would be fully submerged, all the time. The population of these marine creatures would inevitably be affected, as they lose their sites for bringing up their young.
Rising sea levels subject coastal areas to greater vulnerabilities in the event of hurricanes or storms. This is because the surge storm that arise from hurricanes or storms near the coast is built on top of a higher depth of water. In the report “The Critical Decade”, the Australian government appointed Climate Commission noted that even modest sea-level increase of 0.5m can bring about unexpectedly large increases in the frequency of extreme high sea-level events. According to the U.S. Environmental Protection agency which draws its sources from a report by the Federal Emergency Management Agency (FEMA) to Congress, it is estimated that a 1 foot increase in sea level could potentially bring about a 36% to 58% increase in damages to existing developments in US’s coast in the event of a surge storm.
The Intergovernmental Panel on Climate Change (IPCC) believes that sea levels will continue to rise during the 21st century. If nothing is done to stop the global-warming triggered changes in sea levels soon, there could be major consequences to even cities like London, Bangkok and New York – according to GreenPeace, these cities could end up below sea level!
We need to act fast. One of the most critical things we need to do now is to reduce our carbon emissions. By switching to greener, renewable energy sources and spreading awareness about the need to stop global warming, we can hope to avoid worldwide disastrous events brought about by rising sea levels.
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