(Ref: pixabay.com
and flickr.com)
Global warming in response to
accumulation of human-induced greenhouse gases inside the atmosphere has
already caused several visible consequences, among them 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 level 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. Next
address should be the issue of the sea level rise impacts. The question is whether
there is alreadyenough 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 preliminary assessment of regional impacts of sea level
rise.

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2.
In the 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 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 rise 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, 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.
3.
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 relatively to the ground, hence
monitor ground motions also. In active 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 crust and
mantle to last DE glaciation (also called glacial isotactic adjustment, GIA) is
another process that gives rise to 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

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4. 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 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).
5. 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: since more than a decade, regular extent
of the sea level time series gives a nearly constant rate value in the range of
3.1–3.3 mm/yr.
6.
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 de- glaciation (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.
7.
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.
8.
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.
9.
In India,WadiaInstitute ofHimalayan 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:- reason being deserts were seas/oceans only-million
years ago), Deptt. of Space [ dedicated Ocean Satellites (already there ) ;
dedicated climate transponders on board 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).
10.
Oceanography is still in “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 National Institute  of Oceanography  needs expansion). A right &suitabletechnology
is one which has the ability to twist economy either for better / or
neutrality.We need to come up from the stage of “Desalination plant& Marine
Aquaria”.
11.
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 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 those could be dragged
out of ocean may be made the basic focus.

T.K.Choudhury