Research & Reviews: Journal of Ecology
ISSN: 2278-2230
Volume 3, Issue 1
www.stmjournals.com
Warning Bell of Climate Change in the Lower
Gangetic Delta
Sufia Zaman1, Abhijit Mitra *
Department of Marine Science, University of Calcutta, 35, B. C. Road, Kolkata, India
Abstract
Climate change and subsequent temperature rise, sea level rise and acidification are the
major threats to mangrove ecosystem of the world. Several researchers observed through
short term and long term experiments, that climate change induced fluctuations of
temperature, salinity and pH pose a serious threat to the coastal ecosystem. In this
article, three basic indicators of climate change have been selected in the frame of Indian
Sundarbans namely surface water temperature, salinity and pH of the aquatic sub-system
to scan the pulse of climate change in this mangrove dominated delta in the lower
Gangetic region. The ecosystem receives the tidal water from Bay of Bengal and thus
serves as a natural experimental ground to observe the trend of the selected indicators
over a period of more than two decades. Our investigation strongly confirms the foot
prints of climate change in this delta region.
Keywords: Climate change, Deltaic complex, Salinity, Fishery
*Author for Correspondence E-mail: abhijit_mitra@hotmail.com
THE STUDY SITE: NON UNIFOR MITY WITHIN THE SAME SYSTEM
The Gangotri Glacier in the Himalayan range
(about 7010 m above the mean sea level) is the
origin of the mighty River Ganga. The river
flows through several states of India and
finally meets Bay of Bengal covering a
distance of 2525 km. At the confluence of
River Ganga and the Bay of Bengal a deltaic
complex has been formed which is dominated
by some 34 species of true mangrove flora and
62 species of mangrove associate species [1].
This complex is referred to as Indian
Sundarbans (between 21040′N to 22040′N
latitude and 88003′E to 89007′E longitude) and
is the home ground of Royal Bengal Tiger
(Panthera tigris tigris).
The deltaic complex sustains 102 islands, out
of which 48 are inhabited and 54 are
uninhabited. The flow of the River Ganga
through Hooghly estuary in the western sector
of Indian Sundarbans to end up at Bay of
Bengal has made the geographical situation
totally different from the central sector, where
the major river Matla has lost its connection
with Ganga due to heavy siltation and solid
waste disposal from the adjacent cities and
towns. The water chemistry has changed
accordingly. It is on this basic matrix, we
attempted to understand with our data bank
since 1980 (for surface water temperature) and
1990 (for surface water salinity and pH) the
pulse of climate change in the region.
CLIMATE CHANGE: AN ATTEMPT
TO LINK EVENTS AND ECOSYSTEMS
We are travelling through a most unusual
episode in the climatic history of the planet
Earth. Recent events like flood, hurricanes,
tornadoes, extreme cold in the northern
hemisphere and forest fires have emphatically
demonstrated our growing vulnerability to
climate change. Climate change impacts
encompass several sectors like agriculture –
further endangering food security, sea level
rise and the accelerated erosion of coastal
zones, increasing intensity of natural hazards,
species extinction and the spread of vector
borne diseases. The impact of climate change
on the aquatic ecosystem is an interlinked
event between the melting of polar ice or
glaciers feeding the rivers and the alteration of
salinity in the riverine and estuarine waters of
the tropics, temperate and sub-temperate
zones. Thus, climate change is not one story,
RRJE (2014) 41-45 © STM Journals 2014. All Rights Reserved
Page 41
Warning Bell of Climate Change in the Lower Gangetic delta
but many parallel stories of several ecosystems
- only sporadically connected so far without
very concrete data base. The study of climate
change in coastal and deltaic complex of
Indian Sundarbans needs to be addressed
through an integrated approach as the region is
the final receptacle of all waters from the
upstream zone, river discharges and run-off,
and is also submerged twice by the saline
water from Bay of Bengal. The salinity of the
aquatic phase is regulated by several physical,
geographical, geological and even man-made
factors (like dam construction in the upstream
region). Hence all these parameters need long
term assessment to critically visualize the
variations in hydrological parameters. Here we
focus on three primary signals associated
directly with climate change episode and try to
evaluate the strength of these signals in two
sectors (western and central) of the region.
Signal 1: Increasing Trend of Surface
Water Temperature: A Strong Evidence
The Third Assessment Report of the
Intergovernmental Panel on Climate Change
stated that “there is new and stronger evidence
that most of the warming observed over the
last 50 years is attributable to human
activities”. Detecting the role of human on
climate change is difficult because any climate
change “signal” is superimposed by the
background “noise” of natural climate
variability. Nevertheless, there is now good
Zaman and Mitra
evidence that the climate is changing. The
global average land and sea surface
temperature increased by 0.6 ± 0.2°Cover the
20th century. Nearly all of this increase
occurred in two periods: 1910–1945 and more
specifically since 1976 [2]. At the regional
scale, warming has been observed in all
continents, with the greatest temperature
changes occurring at middle and high latitudes
in the Northern Hemisphere [3]. The
mangrove dominated deltaic complex of
Indian Sundarbans is no exception to this. The
surface water temperatures in both the sectors
of Indian Sundarbans have shown a rising
trend since 1980 (Figure 1).
Thus a very strong signature of climate change
in terms of surface water temperature is felt in
the entire deltaic complex in a uniform manner
(6.14 and 6.12% increase in western and
central sectors, respectively since 1980).
Changes in aquatic temperature could have
implications for all types of uses. For example,
higher temperatures could affect recreational
use of coastal waters, productivity and fishery
potential of the area. The Intergovernmental
Panel on Climate Change reviewed relevant
published studies of biological systems and
concluded that 20 to 30 percent of species
assessed may be at risk of extinction from
climate change impacts within this century if
global mean temperatures exceed 2-3°C (3.6–
5.4°F) relative to pre-industrial levels [4].
33
32.5
32
31.5
31
30.5
1975
1980
1985
1990
1995
2000
2005
2010
Year
Fig. 1: Increasing Trend of Surface Water Temperature (in 0C) in Western (red) and Central (black)
Indian Sundarbans (Source: Mitra et al., 2009).
RRJE (2014) 41-45 © STM Journals 2014. All Rights Reserved
Page 42
Research & Reviews: Journal of Ecology
Volume 3, Issue 1, ISSN: 2278-2230
Signal 2: Differential Footprints of Surface
Water Salinity: Noise in Signal
Surface water salinity is one of the most
important indicators of climate change induced
sea level rise. Because of the rising sea level,
there is considerable intrusion of saline water
from the oceans and seas into the coastal
region, bays and estuaries. The lower Gangetic
delta region is no exception to this rule.
However, the interesting feature is that the
salinity alteration pattern is significantly
different in the western and central Indian
Sundarbans. It is observed that salinity
gradually exhibits a decreasing trend in the
western sector, whereas in the central sector
the trend is totally different. The reason behind
this is gradual siltation of the Bidyadhari River
since the 15th century, which has stopped the
fresh water flow in the Matla River (located in
the central Indian Sundarbans). The Hooghly
estuary in the western sector of Indian
Sundarbans, on contrary, is receiving the
snowmelt water from the Himalayan Range,
flowing through several states and regulated
by a number of barrages and dams on the way.
Farakka is one of the major barrages
regulating the dilution factor of the Hooghly
estuary and because of such flow the salinity
has not hiked up to the level unlike the Matla
River in the central sector (Figure 2).
Fig. 2 (a): Decreasing Trend of Salinity in the Western Indian Sundarbans: Fresh Water Discharge
from the Upstream Region has overpowered the Sea Water Intrusion.
Fig. 2 (b): Increasing Trend of Salinity in the Central Indian Sundarbans: Blockage of Fresh Water
Discharge from the Upstream Region has increased the Salinity.
RRJE (2014) 41-45 © STM Journals 2014. All Rights Reserved
Page 43
Warning Bell of Climate Change in the Lower Gangetic delta
Change in salinity has got substantial impact
on the biotic community. The gradual
extinction of the fresh water loving mangrove
floral species, Heritiera fomes (local name
Sundari) from the Matla estuarine stretch is a
litmus test for the rise of salinity in this region.
Signal 3: Acidification: A real Witness of
Climate Change
The foot prints of climate change have been
magnified through the lens of aquatic pH at a
Zaman and Mitra
greater scale. The increase of atmospheric
carbon dioxide in the lower Gangetic delta
region has touched almost 51% since 1980.
The gradual increase of carbon dioxide
coupled with unplanned expansion of shrimp
culture, urbanization and industrialization in
the western sector has lowered the aquatic pH
considerably, compared to the central sector
(Figure 3), where the human activities are
relatively less.
Fig. 3 (a): Decreasing Trend of Surface Water pH in the Western Indian Sundarbans.
Fig. 3 (b): Decreasing Trend of Surface Water pH in the Central Indian Sundarbans.
RRJE (2014) 41-45 © STM Journals 2014. All Rights Reserved
Page 44
Research & Reviews: Journal of Ecology
Volume 3, Issue 1, ISSN: 2278-2230
CLUTCH OF CLIMATE CHANGE:
WHO ARE THE WORST
SUFFERERS?
countries already live in impoverished slums,
with little or no access to water and sanitation.
People all over the world are facing the
adverse impact of climate variability. More
than 5,30,000 people died as a direct result of
almost 15,000 extreme weather events, and
losses of more than USD 2.5 trillion (in PPP)
occurred from 1993 to 2012 globally. The
deltaic complex of Sundarbans, at the apex of
Bay of Bengal is a tiny subset in this mega set,
but the impacts of frequent super cyclone (like
Aila on 25th May, 2009), tidal surges, salt
water intrusion and erosion of river banks have
crunched the domain of livelihood. About 4.2
million people live in Indian Sundarbans,
whose main occupations are agriculture and
fishery. A large fraction of these populations
thrive below poverty line and serve as landless
labourers in agricultural fields. While change
in
hydrology
along
with
localized
anthropogenic factors has every possibility to
hinder economic profile of the deltaic complex
by shrinking the livelihood options, the
impacts are likely to be hardest felt by the
people living below poverty line. Pressure on
livelihoods may force poor landless island
dwellers of the central and eastern sectors of
Indian Sundarbans to migrate to the western
part or adjacent cities like Kolkata, Howrah
and the newly developing Haldia industrial
complex. This will definitely amplify the
magnitude of vulnerability as some 25–40
percent of the urban population in developing
ACKNOWLEDGEMENT
The authors express their gratitude to Society
for Heritage and Ecological Researches
[SHER], Kolkata 700 006, India for providing
the team members the infrastructural facilities
to collect the secondary data from different
sources. The authors also collected
information on the demographic profile of
Indian Sundarbans from the members of
SHER, which have served to formulate the
present manuscript.
REFERENCES
1. Mitra A, Pal S. The Oscillating Mangrove
Ecosystem and the Indian Sundarbans.
First Edition, WWF-India, WBSO. 2000.
2. Mitra A, Banerjee K, Sengupta K, et al.
Pulse of Climate Change in Indian
Sundarbans: A Myth or Reality? Natl.
Acad. Sci. Lett. 2009, 32(1 & 2): 19–25p.
3. Church, John A. How fast are Sea Levels
Rising? Science 2001;
294(5543):
802–803p.
4. IPCC. Climate Change: The Physical
Science Basis. The Report of Working
Group I, Assesses the Current Scientific
Knowledge of the Natural and Human
Drivers of Climate Change, Observed
Changes in Climate, The Ability of
Science to Attribute Changes to Different
Causes, and Projections for Future Climate
Change. 2007.
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