Chapter -3
SEA LEVEL RISE (SLR)
3.1 Concept
Mean sea level can be measured at both a global and local scale. Local ‘mean sea level’ (LMSL) is defined as “the height of the sea with respect to a benchmark, averaged over a period of time, such as a month or a year, long enough that fluctuations caused by waves and tides are largely removed”(Wikipedia Website 2006). LMSL takes into account the tectonic movements of the earth’s crust, atmospheric pressure, ocean currents and local ocean temperature changes that may result in different mean sea levels between localities.
The other and more commonly used measure of sea level at a global scale is known as global mean sea level (GMSL) and is influenced by ‘eustatic changes’. Eustatic change or eustasy is defined as “the world wide sea level regime and its fluctuations, caused by absolute changes in the quantity of sea water” (Warrick et al 1993:107)1. The key factors that influence GMSL are thermal expansion and glaciations which are discussed below in further detail.
Global mean sea level (GMSL) is commonly used by academics as a broad measurement of sea level rise to highlight the issue on a world-wide scale. However, LMSL provides a more accurate measurement of sea level at a local scale and therefore allows a greater understanding of specific impacts region and localities. There are a number of ways that sea level can be measured. Measurement of sea level in the earth’s history has been based on scientific research into sediment core samples (in wetlands), ice sheets and geological surveys (Gehrels et al 2005). The natural environment has many indicators that can be used to ascertain historic sea levels2. For example, contours on ice sheets can be read to determine the age of that ice (and therefore calculate sea level) the same way tree rings on a tree stump can be read to determine the age of a tree (Gore 2006).
Recent fluctuations of both LMSL and GMSL have been measured with the use of tidal gauges. Tidal gauges have been instrumental in determining sea level fluctuations in the recent past but this system is not without its flaws. In particular, the location of the tidal gauges results in an inaccurate depiction of sea level rise in oceans around the globe. This is due to the uneven distribution of the 229 tidal gauges in 21 locations around the world, with only 6 located in the southern hemisphere (Bird 1993). As a result of this, tide gauges are generally more accurate at determining local fluctuations in sea level. Tidal gauges are now generally regarded as back-up indicators to determine sea level fluctuations as satellite imagery covers more and more of the globe and reveals greater depths of data.
Predictions of future sea level fluctuations are generally based on satellite imaging and provided through computer climate modeling, normally Geographic Climate Modeling’s the level of the ocean's surface. Sea level at a particular location changes regularly with the tides and irregularly due to conditions such as wind and currents. Other factors that contribute to such fluctuation include water temperature and salinity, air pressure, seasonal changes, the amount of stream runoff, and the amount of water that is stored as ice or snow. The reference point used as a standard for determining terrestrial and atmospheric elevation or ocean depths is called themean sea leveland is calculated as the average of hourly tide levels measured by mechanical tide gauges over extended periods of time.The so-called greenhouse effect or global warming may cause a Sea Level Rise, which will have a great impact on the long-term coastal morphology. The possible and gradual Sea Level Rise will cause a general shoreline retreat and an increased flooding risk and has to be handled according to the local conditions3.
Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused bywavesandtidesare smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as sea level changes. Some land movements occur because ofisostatic adjustment of themantleto the melting ofice sheetsat the end of the lastice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away therebounds. Atmospheric, oceanand local ocean temperature changes also can affect LMSL “Eustatic” change (as opposed to local change) results in an alteration to the global sea levels, such as changes in the volume of water in the world oceans or changes in the volume of anocean basin.
Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The two primary influences are temperature (because the density of water depends on temperature), and the mass of water locked up on land and sea as fresh water in rivers, lakes, glaciers,ice caps, andsea ice. Over much longergeological timescales, changes in the shape of oceanic basins and in land–sea distribution affect sea level. Observational and modeling studies ofmass loss from glaciers and ice capsindicate a contribution to sea-level rise of 0.2–0.4mm/yr, averaged over the 20th century. Sedimentarydeposits follow cyclic patterns. Prevailing theories hold that this cyclic primarily represents the response of depositional processes to the rise and fall of sea level. The rock record indicates that in earlier eras, sea level was both much lower than today and much higher than today. Such anomalies often appear worldwide. For instance, during the depths of the lastice age18,000 years ago when hundreds of thousands of cubic miles of ice were stacked up on the continents as glaciers, sea level was 120 metres (390ft) lower, locations that today support coral reefs were left high and dry, and coastlines were miles farther outward4. During this time of very low sea level there was a dry land connection between Asia and Alaska over which humans are believed to have migrated to North America (Bering Land Bridge).
For the past 6,000 years, the world's sea level gradually approached the current level. During the previous interglacial about 120,000 years ago, sea level was for a short time about 6 metres (20ft) higher than today, as evidenced by wave-cut notches along cliffs in theBahamas. There are also Pleistocenecoral reefsleft stranded about 3 metres above today's sea level along the southwestern coastline ofWest CaicosIsland in the West Indies. These once-submerged reefs and nearby paleo-beach deposits indicate that sea level spent enough time at that higher level to allow reefs to grow (exactly where this extra sea water came from—Antarctica or Greenland—has not yet been determined). Similar evidence of geologically recent sea level positions is abundant around the world.
Causes of Sea Level Rise
There are two ways in which global warming is causing sea levels to rise are: (a) thermal expansion and (b) the melting of glaciers, ice caps etc. Global warming or increases in temperatures (due to increase in the concentrations of greenhouse gases) cause the oceans to warm and expand in volume inducing a rise in the sea levels. Furthermore, warmer climate facilitates melting of glaciers, ice caps and ice sheets causing further addition of water to the oceans. In fact, the major cause of SLR is the thermal expansion of the oceans which contributes substantially in recent time (1993-2003).
Figure 3.1 Causes to sea level change/rise
Source:
The main cause for rising sea levels is the expansion of water due to an increase in water temperature and is thus a mere physical phenomenon. Additional factors are the melting of mountain glaciers and the ice crust in Greenland, caused by an increase in temperature of the earth’s atmosphere. Yet, an increase in rainfalls and the subsequently growing Antarctic ice cover can also cause the sea levels to fall. The influence of the Antarctic, however, is small in relation to other factors, resulting in an overall rise of the sea level. Sea levels do not rise identically in every geographical region. Therefore, in some regions sea levels are expected to rise slightly more than in others, as the increase in temperature within the different (vertical) layers of water takes place in different stages. Independent of global warming, changes in regional sea levels can also result from continental drifts. For example, land in some river deltas subside by several millimeters per year because sediments collapse. In these cases, a rising sea level intensifies the existing regional effects. In other regions, a rise in sea level remains unnoticed because the land is rising to the same extent or even more than the sea level itself. In the past, the rise of sea level was measured solely by fixed measuring positions ashore. As measuring positions did and do not exist at every point along the coast, the web of data collected was rather wide-meshed. Since the 1980s, satellite technology has facilitated the collection of more comprehensive data6.
Sea level, throughout the earth’s history has fluctuated in accordance with temperature changes in the atmosphere. It is through natural process of climate change in the past that current mean sea levels have been determined. Natural sea level fluctuations have been predominately influenced by two main natural factors: glaciations and thermal expansion of the ocean. Both concepts are defined and discussed below Glaciations refer to the process of the accumulation of ice on land to form glaciers (Strahler and Strahler 1999). Glaciers are created by a buildup of snow which, when on land, freezes into ice and accumulates. Glaciers at any scale are frozen water reservoirs storing water that would have otherwise run-off land and flown into rivers and the ocean. The state of a glacier is determined by the surrounding temperature of the earth’s atmosphere. For example, if temperatures are high then glaciers will be melting, and if temperatures are low then glaciers will be accumulating. This is a simplistic example provides a general understanding of the intimate relationship between glaciers and the temperature of the earth’s atmosphere.
The accumulation and ablation of glaciers, as a result of temperature changes throughout the earth’s history has influenced global sea level fluctuations with sea level falling in periods of accumulation (i.e. with less water running off into the ocean) and rising in periods of ablation (i.e. through an increase of water being released into the ocean). In periods of low temperature glaciers have dominated the earth’s surface. These periods are known as Ice Ages. Glaciations is an important process in influencing sea levels around the world however its influence is minor in comparison to another process; thermal expansion of the oceans. Walsh et al states that “thermal expansion is the most important component of global sea level rise” (2004:588)7. Thermal expansion refers to the heating of the ocean’s water as a result of increases of temperature in the atmosphere. When the water heats it expands, increasing the overall volume of the ocean and therefore raising global sea level. The massive size of the ocean and the volume of water contained within means that sea level fluctuations from thermal expansion will be experienced at a delay from temperature changes in the atmosphere. This delay is known as a ‘thermal lag’ and is said to be in the order of around 30 years2 (Walsh et al 2004, Flannery 2005). Both glaciations and thermal expansion have been important in determining sea fluctuations throughout the earth’s history. Each process and therefore the level of sea level resulting from it, is critically dependent on the temperature of the earth’s atmosphere. The intimate relationship between the earth’s atmosphere and the sea levels has been evident throughout time “with ocean levels always fluctuating with changes in global temperatures”8
This timeframe (30 years) refers to the absorption of heat from the atmosphere into the ocean, not the entire warming of the ocean which takes about 1000 years or more. This period of 30 years is when serious impacts of sea level rise due to temperature changes will start to be experienced (Flannery 2005). In addition to glaciations (ablution and accumulation) and thermal expansion there are other more minor factors that have been responsible for fluctuations in sea level. These additional factors for sea level fluctuation include the Greenland and Antarctic Ice Sheets (melting and accumulation of ground ice), surface and ground water storage, and global tectonic effects (Walsh et al 2004). The input of these factors is considered relatively minor in comparison to the process of thermal expansion and glaciations .Fluctuations in sea level are intimately linked to fluctuations in global temperature, with changes in temperature impacting on glaciers and the thermal expansion of the ocean.
This relationship has been witnessed throughout the earth’s history. An example of this relationship is shown in the period that preceded the last Ice Age; approximately 120,000 years ago the global average temperature was slightly warmer than that of today. This resulted in a global sea level five to six metres higher than it is today (Houghton 2004). This is in contrast to 18,000 years ago where the world was in an Ice Age (hence temperatures where substantially colder than today) resulting in a sea level of approximately 120 metres lower than the current level9. Sea level fluctuations have been a key factor in the formation and separation of continents and islands over the earth’s history. This was illustrated clearly between 18,000 - 12,000 years ago when sea level rise was so significant that it separated
This dramatic rise in sea level was a direct result of a periodical increase in temperature by 50C. This temperature rise is known as “the fastest rise recorded in recent earth history” (Flannery 2005).After the dramatic sea level rise commencing at the end of the last Ice Age (18,000 years ago), sea level rise has been relatively consistent. For the past 6,000 years sea level has consistently risen 5 to 10 metres to reach current global sea levels (Aubrey & Emery 1993). The average rate of sea level rise has been between 0.14–0.1cm/year. This period of consistent sea level rise has been a result of the earth’s atmospheric temperature remaining relatively stable in this period. In the last century, the sea level has risen 10 to 25 centimeters (i.e. an average of 0.1- 0.25cm/year) (Titus 1990). An estimate of the contribution that each factor of sea level fluctuation (discussed above) has on current global sea level in the past century is shown in Figure. These figures reinforce the dominance of both thermal expansion and glaciers as key factors in the fluctuation of sea level. In particular, these figures show that thermal expansion has been the highest contributor to sea level rise, with glaciers and ice caps also having significant contributions. Together, thermal expansion and glaciers and ice caps resulted in a positive increase in sea level of 4 centimeters by the year 199010. This is in contrast to the contribution of the Greenland ice sheet which is relatively minor, and the contribution of the Antarctic ice sheet which has reduced sea level and somewhat offset rises from other sources as shown in both figures. Scientific information is inconclusive at the moment to whether sea level rises over the last century have been as a result of natural or human induced climate change (i.e. brought about by global warming). For example, studies were undertaken by Woodworth, Gornitz, Solow and Douglas separately to determine whether sea level rise has been a result of natural or human induced influences. From these studies” no author found conclusive evidence of a global acceleration of sea level, especially compared to what is predicted to accompany future global warming” (Douglas 2001:61)
Sea level rise Indian and world scenario
Many scientists consider global warming-forced climatic change as the most serious environmental threat facing the world today (IPCC 2007). Global warming has the potential to affect many humans dramatically and adversely as a consequence of both natural and anthropogenic changes to temperature, precipitation, sea level, storms, air quality, and other climatic conditions.
Figure3.2 : IPCC estimated contributions to sea level rise over twentieth century (in cm)
Source: IPCC 2001:2006
Sea-level rise (SLR) poses a particularly ominous threat because 10% of the world’s population (634 million people) lives in low-lying coastal regions within 10 m elevation of sea level (McGrananhan et al.2007). Much of this population resides in portions of 17 of the world’s 30 largest cities, including Bombay, India; Shanghai, China; Jakarta, Indonesia; Bangkok, Thailand, London and New York. The population of many of the Asian cities will likely continue to increase as ports and work forces expand to keep pace with economic globalization and increasing shipping traffic (McGrananhan et al, 2007). Between 1980 and 2003, the population of 672 coastal counties in United States increased from 120 million to 153 million people (to 53% of the total population) and this number is expected to rise to 160 million people by 2008 (Crossett et al. 2004). In 2003, coastal counties in the United States accounted for 23 of the 25 most densely populated counties. In addition to inundating low-lying coastal areas, rising sea level increases the vulnerability of coastal regions to flooding caused by storm surges, tsunamis, and extreme astronomic tides. As sea level rises, storms of a given magnitude reach higher elevations and produce more extensive areas of inundation11.