According to Warrick (1993), global warming is the main contributor to the rise in global sea level since the Industrial Revolution. Human activities such as burning coal and oil and cutting down tropical forests increase atmospheric concentrations of heat-trapping gases. The three main factors leading to the rise in sea levels include: thermal expansion, ice loss from Greenland and West Antarctica and melting of glaciers and polar ice caps.
Land ice, that is, glaciers, ice caps, and ice sheets, stores nearly two-thirds of the world’s freshwater and is shrinking in response to higher temperatures. Glaciers partially melt each summer and grow again each winter. However, as temperatures rise, ice growth in winter is often less than ice melt in summer. The result is that nearly all the world’s surveyed glaciers, ice caps, and the Greenland ice sheet are losing ice, adding water to the oceans and causing global sea level to rise. Indeed, the pace of ice loss from both small glaciers and large ice sheets has accelerated. The rising air temperatures, thermal expansion, are also warming ocean waters making them expand as the temperatures continue to increase (Wigley 1993). Indeed, the oceans have absorbed eighty five percent of the excess heat trapped by the atmosphere since 1880, as ocean water warms, it expands. This thermal expansion was the main driver of global sea level rise for seventy five to a hundred years after the start of the Industrial Revolution. Melting land-based ice is common in areas such as Greenland or the West Antarctica ice sheet. There is growing evidence that ice flow around Greenland and Antarctica is accelerating. These areas represent the largest potential sources of additional sea level rise.
According to the National Geographic Society, the average sea level around the whole world has been increasing for many years. The graph below shows the likely range of sea level- shown by the shaded band- which depends on the number of measurements and the methods used at different times.
Sea level rise causes the salt content of aquifers and estuaries to migrate landward. In coastal aquifers, a layer of freshwater floats on top of the heavier saltwater. If the sea level increases and the outcome is a landward movement of the shoreline, the boundary between fresh and saltwater will move inland as well. This is because the water table level is itself determined by sea level, thus, when sea level rises; it causes the freshwater or saltwater boundary to ascend. The upward and landward shift of this boundary implies that certain freshwater wells may become salty. Over pumping of coastal aquifers also has resulted in salt intrusion, however, and in many instances this problem dwarfs the possible outcomes of a rise in the sea level (Barrow 1993).
A rise in sea level also would increase the salinity of rivers and estuaries. A drop in the flow of a river or an increase in the water’s volume allows salt to migrate upstream. When the sea level increases for only thirteen centimeters, that is, five inches, it could result in salt concentrations in the Delaware River migrating two to four kilometers, one to two miles, upstream (Milliman 1996). A rise of one meter could cause salt concentrations to migrate over twenty kilometers because some rivers recharge aquifers; a few aquifers may become salty as well.
Like the physical effects, how an increase in the sea level will impact the environment falls into the categories of salt intrusion, shoreline retreat and increased flooding. Possibly the most serious environmental outcome would be the inundation and erosion of thousands and thousands of square miles of marshes and other wetlands. According to Barrow (1993), there are two kinds of inundation that will be caused by sea level rise: an episodic inundation and a permanent inundation. The impact that permanent inundation will have on areas is dependent on the local gradient. At a coast, places that have low gradients include: Chenier plains, beach ridges, deltas, estuaries, mudflats, bays and lagoons. Storm surges cause episodic inundations. As sea level rises, an episodic inundation will be more frequent for these low-lying areas.
People living in states with low-lying coastlines have been subjected to severe flooding and damage from coastal storms in recent years. Although all coastal states are vulnerable, Florida, Louisiana, New York, and California have the most residents living on land less than three point three feet above high tide. The global average sea level could rise to the three point three foot mark within this century depending on our future emissions and the resulting ocean warming and land ice loss. Sea level rise is changing the dynamics at play along the coasts and with them the coastal communities, ecosystems and economies.
These dynamics include: Amplified storm surge. Coastal storms often cause storm surge, where high winds push water inland. With rising seas, storm surge occurs on top of an elevated water level. That means a storm today could create more extensive flooding than an identical storm in 1900, with sometimes catastrophic damage to our homes and critical infrastructure—as recent events have shown. In the future, with even higher sea levels, storm surges could reach even further inland. More than five million people along the Eastern Seaboard live in areas at risk of coastal flooding, and population density in coastal counties is projected to grow at more than three times the national pace over the next ten years. Coastal communities have seen severe damage from storms—most recently from Hurricane Sandy (Warrick 1993).
Wetlands, that is, areas that are flooded by tides at least once every fifteen days, are critical to the reproductive cycles of many marine species mainly because marsh vegetation can collect sediment and build upon itself, marshes can grow with small rises in sea level. Its resulting deterioration may significantly erode land previously held together only by the marsh vegetation. Relative sea level rise of one meter per century is eroding over one hundred square kilometers, fifty square miles, per year of marshland in Louisiana.
Salt intrusion is a threat to marine animals as well as vegetation. Many species must swim into fresher water during reproduction. In response to the increase in sea level, fish might swim farther upstream, but water pollution could prevent such an adaptation from prevailing. Several species, on the other hand, need salty water, for example, the oyster drill and other predators of oysters.
There are over one thousand active hazardous waste facilities in the United States located in hundred-year floodplains (Warrick 1993) and perhaps as many inactive sites. The rise in the Sea level could increase the risk of flooding in these hazardous waste sites. For example, if a hazardous waste facility is subjected to over wash by strong waves or simply to flooding that weakens the facility's cap, the wastes can be spread to nearby surrounding; hence exposing the inhabitants to possibly contaminated surface water. Moreover, by interfering with clay soils, which are frequently used as liners for hazardous waste disposal, saltwater can increase leaking of wastes.
Sea level rise also has a profound effect on the rate of sedimentation for different parts of the coastal gradient. Peak rates of sedimentation occur at higher elevations on the march and less sedimentation occurs on the lower elevations. Varying of sedimentation rates result in changing vegetation zones and succession on marshes. In addition, storm surges would force large quantities of shore face sediments through inlets and create tidal deltas on which barriers would later transgress (Milliman 1996).
References
Milliman, J. (1996). Sea Level Rise and Coastal Subsidence: Causes, Consequences, and
Strategies. Netherlands: Kluwer Academic Publishers.
National Geographic Society. (2013). Sea Level Rise. Retrieved on Nov 4 from
http://ocean.nationalgeographic.com/ocean/critical-issues-sea-level-rise/
Warrick, A, Barrow, M, & Wigley, L. (1993). Climate and Sea Level Change: Observations,
Projections, and Implications. Britain: Cambridge University Press.
