Warning Bell of Climate Change in the Lower Gangetic Delta

Transcription

1 ISSN: Volume 3, Issue 1 Warning Bell of Climate Change in the Lower Gangetic Delta Sufia Zaman 1, 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 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 N to N latitude and E to 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) STM Journals All Rights Reserved Page 41

2 Warning Bell of Climate Change in the Lower Gangetic delta Zaman and Mitra 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 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: 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 ( F) relative to pre-industrial levels [4] Year Fig. 1: Increasing Trend of Surface Water Temperature (in 0 C) in Western (red) and Central (black) Indian Sundarbans (Source: Mitra et al., 2009). RRJE (2014) STM Journals All Rights Reserved Page 42

3 Volume 3, Issue 1, ISSN: 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 15 th 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) STM Journals All Rights Reserved Page 43

4 Warning Bell of Climate Change in the Lower Gangetic delta Zaman and Mitra 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 greater scale. The increase of atmospheric carbon dioxide in the lower Gangetic delta region has touched almost 51% since 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) STM Journals All Rights Reserved Page 44

5 Volume 3, Issue 1, ISSN: CLUTCH OF CLIMATE CHANGE: WHO ARE THE WORST SUFFERERS? 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 25 th 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 percent of the urban population in developing countries already live in impoverished slums, with little or no access to water and sanitation. ACKNOWLEDGEMENT The authors express their gratitude to Society for Heritage and Ecological Researches [SHER], Kolkata , 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 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): p. 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 RRJE (2014) STM Journals All Rights Reserved Page 45