Global warming in response to the accumulation of human-induced greenhouse gases inside the atmosphere has already caused several visible consequences, among them an increase of the Earth’s mean temperature and ocean heat content, melting of glaciers, and loss of ice from the Greenland and Antarctica ice sheets. Ocean warming and land ice melt in turn are causing sea levels to rise.
Sea level rise and its impacts on coastal zones have become a question of growing interest in the scientific community, as well as in the media and public. we need to summarize the most up-to-date knowledge about sea-level rise and its causes, highlighting the regional variability that superimposes the global mean rise. We require updating of sea-level projections for the 21st century under different warming scenarios.
The next address should be the issue of the sea level rise impacts. The question is whether there is already enough observational evidence of coastal impacts of sea-level rise and results differ from one location to another. This indicates that the response of coastal systems to sea level rise is highly dependent on local natural and human settings. We need to ascertain that finally in spite of remaining uncertainties about future sea levels and related impacts, it becomes possible to provide a preliminary assessment of regional impacts of sea-level rise.
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In recent years, sea-level rise induced by global warming and its impacts on coastal zones has become a question of growing interest in the scientific community, as well as the media and public. It is now well established that the Earth’s climate is warming and that the main cause is the accumulation of greenhouse gases (GHGs) inside the atmosphere, produced by anthropogenic fossil fuel combustion and change in land use (mostly deforestation); Global warming has already given rise to several visible consequences, in the particular increase of the Earth’s mean surface temperature and of ocean heat content melting of sea ice and glaciers; and loss of ice mass from the Greenland and Antarctica ice sheets. Ocean warming causes thermal expansion of sea waters, hence sea-level rise. Similarly, water from land ice melt ultimately reaches the oceans, thus also causes sea level rise. Direct sea level observations available since the mid-to-late nineteenth century from in situ tide gauges and since the early 1990s from high-precision altimeter satellites indeed show that sea level is rising observations also show that the rate of rising displays strong regional variations Modeling of future climate change under different radiative forcing scenarios indicates that sea level will continue to rise during the next decades and even centuries Adverse effects of sea-level rise in coastal areas are generally considered as a major threat of climate change if we consider that 10% of the world population is living in coastal areas less than 10 m above sea level. Twentieth-century observations report shoreline erosion in many areas of the world coastlines but it remains unclear whether this is due to climate-related sea-level rise or to more local no climatic factors such as ground subsidence (causing relative sea-level rise), coastal management, land use and land-use changes, waves and currents, the deficit in sediment supply, etc., or to the combination of all factors Nevertheless, it is virtually certain that in the coming decades, the expected acceleration of sea-level rise in response to continuing global warming will exacerbate the vulnerability of many low-lying, densely populated coastal regions of the world, and very likely will become a major threat in the near future for a significant fraction of human beings.
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The very first step should be summarizing the most up-to-date observations about sea-level change and variability at global and regional scales, focusing on the twentieth century and last two decades. We need to discuss the various climatic and no climatic factors responsible for the sea-level variations at global and regional scales. And for this we have to have present global mean and regional sea-level projections for the 21st century. At a much wider level, we need to discuss the implications of recent and future sea-level rise, placing them in the context of the numerous natural and anthropogenic factors affecting coastal zones, such as coastal erosion and marine submersion.
Our knowledge of past century sea-level change comes from tide gauge measurements located along continental coastlines and islands. The largest tide gauge database of monthly and annual mean sea level records is the Permanent Service for Mean Sea Level which contains data for the twentieth century from ∼2000 sites. However, only ∼10% of this data set is useable for historical sea-level studies because of data gaps and limited tide gauge distribution in the past. Tide gauges measure sea level relative to the ground, hence
monitor ground motions also. Inactive tectonic and volcanic regions, or in areas subject to strong ground subsidence due to natural causes (e.g., sediment loading in river deltas) or human activities (groundwater and oil/gas extraction), tide gauge data are directly affected by corresponding ground motions. Post-glacial rebound, the viscoelastic response of the Earth’s crust and mantle to last DE glaciation (also called a glacial isotactic adjustment, GIA) is another process that gives rise to the vertical land movement, e.g., crustal uplift
in high latitudes of the Northern Hemisphere.If one is interested in the climate-related components of sea-level rise, vertical land
motions need to be removed. On the other hand, for studying coastal impacts of sea-level rise, it is the relative (i.e., including vertical land motion as measured by tide gauges) sea-level rise that is of interest. Most recent analyses of long, good-quality tide gauge records (corrected for GIA and when possible for other vertical land motions by the Global Positioning System, GPS) indicate a mean rate of sea-level rise of 1.6–1.8 mm/yr over the twentieth century
The advancement:-. Since the early 1990s, sea level is routinely measured with quasi-global coverage and a few days/weeks revisit time (called “orbital cycle”) by high-precision altimeter satellites such as Topex/Poseidon and its successors Jason-1 and Jason-2 as well as Envisat, Cryosat. Compared to tide gauges which provide sea level relative to the ground, satellite altimetry measures “absolute” sea-level variations in a geocentric reference frame. The concept of the satellite altimetry measurement is simple; the onboard radar altimeter transmits microwave radiation toward the sea surface which partly reflects back to the satellite. Measurement of the round-trip travel time of the electromagnetic signal provides the height of the satellite above the instantaneous sea surface (called “range”). The sea surface
height (SSH) above a fixed reference surface (typically a conventional reference ellipsoid) is then simply computed from the difference between the altitude of the satellite above the reference (deduced from precise orbit computation) and the range measurement. The SSH measurement needs to be corrected for various factors due to ionospheric and tropospheric delays, instrumental biases and drifts, and effects of the electromagnetic scattering of the radar signal at the air-sea interface. Other corrections due to solid earth, pole and ocean tides and atmospheric loading are also applied. The precision of an individual SSH measurement has now reached the 2–3 cm level. Further averaging over the oceanic domain during an orbital cycle leads to a precision of ∼0.4 mm for a single global mean sea
level measurement. In terms of a multiyear linear trend, error budget analyses of all sources of errors affecting the altimetry system, as well as comparisons with tide gauge-based sea-level measurements, suggest errors in the order of 0.4 mm/yr). Associated uncertainty is based on the dispersion of individual time series around the mean. A small correction of −0.3 mm/yr is applied to account for the GIA effect on the global mean absolute sea level. The altimetry-based sea-level curve shows an almost linear increase since 1993, except for some temporary anomalies associated with ENSO (El Niño-Southern Oscillation) events Over last 20-year-long time span, the rate of global mean sea-level rise amounts to 3.2 ± 0.1 mm/yr (the 0.1 mm/yr(the 0.1mm/yr uncertainty is based on individual point errors; as mentioned above, a more realistic value accounting for systematic errors is closer to 0.4 mm/yr).
Present Scene: –The twentieth-century sea level curve is not purely linear. Sea level rate appears to increase with time. This led a number of investigators to estimate the acceleration at the multidecadal to century time scale. Results are highly scattered (in the range of 0–0.019 mm/yr/yr), with the computed acceleration being highly dependent on the record length and the considered data set. Nevertheless, most studies conclude to a slight acceleration of the global mean sea level in the course of the twentieth century. The rate of sea-level rise of the last two decades is double the mean rise of the twentieth century. It has been suggested that this higher rate cannot be attributed to decadal variations but rather reflects a recent acceleration of the global mean rise (since the early 1990s) However, this can be questioned by other studies stating that because of low-frequency, multidecadal sea-level fluctuations, any recent acceleration is hard to detect. It has been established that this is well possible considering the still short length of the altimetry record, it is worth mentioning that the altimetry-based rate of sea level rise is remarkably stable: for more than a decade, the regular extent of the sea level time series gives a nearly constant rate value in the range of 3.1–3.3 mm/yr.
Recent studies based on Paleo sea level data (coral reef cores, geological and archeological data, etc.) have shown that since 2–3 millennia, the mean sea level has remained quasi-stable. Using biomarkers of ancient sea levels in salt marsh environments has shown that the rate of sea-level change did not exceed 0.5 mm/yr during the last 2000 years; a conclusion confirmed by other studies showing that no acceleration occurred until the mid-to-late nineteenth century or even later. Thus, compared to the late Holocene period, the twentieth century and last two decades’ rates of sea-level rise are unusually high. However, the mean rate of rise during the last deglaciation (between −20 000 years and early Holocene) amounted 12 mm/yr; a value significantly higher than today. Moreover, during short periods of only 300 years, the rate of sea level rise reached 40 mm/yr. Although involved land ice volume was much greater than nowadays, Paleo-observations indicated s that very high sea level rates are not impossible.
The main factors causing current global mean sea level rise are thermal expansion of sea waters, land ice loss, and fresh water mass exchange between oceans and land water reservoirs.. These contributions vary in response to natural climate variability and global climate change induced by anthropogenic GHG emissions.
Although considerable progress has been realized during the past one to two decades in measuring sea level change globally and regionally, and in understanding the climate-related causes of observed changes, we are still faced to new challenges in terms of observations, modeling, and impact studies. Continuity of space-based and in situ observing systems of sea level variations and components as well as of coastal changes is clearly a major need. Besides, high priority should be given to the development of integrated,
multidisciplinary studies of present-day sea-level changes (global and regional), accounting for the various factors (climate change, ocean/atmosphere forcing, land hydrology change—both natural and anthropogenic, solid Earth processes, etc.) that act on a large variety of spatiotemporal scales. Sea level projections from climate models need to include all factors causing regional sea level changes. In addition, as local (relative) sea-level rise is among the major threats of future global warming, it is of primary importance to develop multidisciplinary studies to understand and discriminate causes of current sea level changes in some key coastal regions, integrating the various factors that are important at local scales (climate component, oceanographic processes, sediment supply, ground subsidence, anthropogenic forcing, etc.). Ultimately, such studies would be useful for coastal scientists and stakeholders concerned by relative sea level rise, at it can be felt at the coast.
In India, the WadiaInstitute of Himalayan Studies, Tata Energy Research Institute, Indian Institute of Tropical Meteorology, Defense’ Institute on Glacier Studies, Naval Oceanographic Research, National Institute of Oceanography (CSIR) Central Arid Zone Research Institute(CAZRI, Deptt. of Environment:- the reason being deserts were seas/oceans only-million years ago), Deptt. of Space [ dedicated Ocean Satellites (already there ) ; dedicated climate transponders onboard INSATs (having networking with INMARSAT) ]- need to draw out a detailed plan of action bringing in a holistic approach towards unpredictable acts and vulnerability of oceans while on contrary comparatively stable behavior of tropical climate and their effect on human settlements particularly in & around the coast (EEZ inclusive).
Oceanography is still in the “Science” stage in India. We need to not only study Oceans but exploit & extract. We require to upgrade oceanography to the level of economic & usable technologies (not only efforts for metallic extractions, which is too complicated.-role of the National Institute of Oceanography needs expansion). A right &suitabletechnology is one that has the ability to twist the economy either for better / or neutrality.We need to come up from the stage of “Desalination plant& Marine Aquaria”.
For,a middle-class country like India, schemes like- “Antarctic expedition” through decades without any tangibility /visibility, general Oceanography research or even the sophisticated “Climate change” etc. continue to be of academic interests and to be true in many a cases a good luxury; is a fact –we need to admit it and act accordingly. Environmentally viable technologies that could be dragged out of ocean may be made the basic focus.
Written by T.K.Choudhury
