Global Warming Effects Around the World

Kolkata, West Bengal, India

Top Impact

Ecosystems (Salt water)

Other Impacts

Oceans (Sea level)

Freshwater (Extreme wet)

Loss of mangroves puts cities like Kolkata, India -- where this man walks -- at risk from tropical storm  flooding

The mangrove forest of the Sundarbans provides important protection against storms and flooding for cities including Kolkata (Calcutta), India, on the Bay of Bengal. But global warming is simultaneously inundating already unstable mangroves and intensifying storms, with potentially devastating consequences for millions of people.1

Key Facts

The Sundarbans of India and Bangladesh is the world's largest mangrove forest.2 Accelerating sea-level rise due to global warming16,22,23 is likely to submerge the Sundarbans.20 This would eliminate the protection they provide against the region's increasingly intense tropical storms.5

  • By absorbing some of the force of wind and waves and serving as a flood barrier, mangroves can lessen the damage caused by cyclones and other storms.6,7,8
  • In early 2010, a disputed Sundarbans island disappeared under the rising waters of the Bay of Bengal.19 Scientists project that under a high emissions scenario, relative sea-level rise is likely to inundate most of the Sundarbans by mid-century, and could wipe them out by the end of the century.20,24
  • Without the mangroves of the Sundarbans to serve as a buffer, more frequent and intense storms are likely to pose a growing danger to the residents of Ganges basin including cities like Kolkata (Calcutta).3

Details

The people of the Ganges-Brahmaputra Delta—including the metropolis of Kolkata (Calcutta), India—depend on the mangroves of the Sundarbans for protection against storms and floods. As climate change destroys mangroves and worsens storms in the region, it puts lives and livelihoods at risk.

The Sundarbans is the world's largest mangrove forest. Designated as a United Nations World Heritage site in both India and Bangladesh,2 it covers nearly 4,000 square miles (10,000 square kilometers). The forest provides habitat for the Bengal tiger, as well as numerous other rare and endangered species of birds, reptiles, and aquatic mammals.2

Mangroves play a vital role in coastal ecosystems and food chains, by supporting communities of fish and shellfish.3,4 Mangroves are salt-tolerant trees and shrubs that help protect coastal areas from increasingly intense tropical storms, waves, and erosion.5,6,7,8 By serving as a flood barrier, they can reduce the damage caused by storms such as cyclones.6,7,8 Damage and erosion to Mangroves leave the coast increasingly exposed and therefore more vulnerable to storms. More than a quarter of a million people—60 percent of them in Bangladesh—died in tropical cyclones in the last two decades of the twentieth century.3

Densely populated coastal areas like the Ganges-Brahmaputra Delta are the most vulnerable to deadly storms.3,9 Scientists determined that intact and healthy mangroves in the Indian state of Orissa saved many lives in a 1999 cyclone originating from the Bay of Bengal.10 Another study found that the 2004 Indian Ocean tsunami caused significantly less damage in areas of southeastern India protected by mangroves and other forests, and simulations show that a dense belt of mangroves can dramatically reduce the peak pressure of a tsunami wave.11

However, in the last half of the twentieth century a substantial portion of the mangroves in South and South-East Asia were lost. This was largely due to human activities—including deforestation and large-scale conversion of mangroves to shrimp farming.3,5,12,13

Global warming compounds the dangers to the Sundarbans. These low-lying mangrove forests are highly susceptible to the effects of sea-level rise—including inundation of coastal areas, increased exposure to storm surges, increased coastal erosion, and rising salinity in ground and surface waters.3,5,14,15

During the twentieth century, global mean sea level rose at an average of 0.07 inches (1.8 millimeters) per year,16 but between 1993 and 2003, the average rate of sea-level rise nearly doubled to increase around 0.12 inches (3.1 millimeters) per year.17

Local sea-level rise of as much as 1 inch (25 millimeters) per year has been recorded in parts of the Ganges-Brahmaputra Delta.14,18 In early 2010, the rising waters of the Bay of Bengal claimed the Sundarbans island of New Moore/South Talpatti, the subject of a long territorial dispute between India and Bangladesh.19

What the Future Holds

Unless we make deep and swift cuts in our heat-trapping emissions, most of the Sundarbans may disappear underwater, and those that remain could be threatened by saltwater incursion.20

If we continue along a high heat-trapping emissions trajectory,21 global sea level is projected to increase as much as 23 inches (59 centimeters) over recent average levels by the end of this century.22 If, on the other hand, we make significant efforts to reduce emissions,21 sea level rise between now and the end of the century could be limited to around 15 inches (38 centimeters).22

Taking into account recent evidence of accelerating global sea-level rise16,23 as well as regional variations, scientists project that relative sea-level rise is likely to inundate most of the Sundarbans by mid-century, and could wipe them out by the end of the century.20,24

Sea-level rise and loss of the Sundarbans could have a devastating impact on the 500 million people of the Ganges basin.5 Tens of millions of people in low-lying areas of South Asia could be flooded annually.5,25 India and Bangladesh are particularly susceptible to increasing salinity of water resources, especially along the coast.5,14

For residents of cities like Kolkata, the greatest danger is likely to come from higher tides and more intense storms—with storm surges unchecked by the disappearing mangroves of the Sundarbans.3 As sea levels rise and storm patterns shift in the Bay of Bengal, scientists project increases in extreme water levels near Kolkata.3,26

The choices we make today could determine whether the ecologically rich Sundarbans stay on the map—affecting the very survival of people in Kolkata and throughout the Ganges basin.

Credits

Endnotes

  1. Photograph: Public Domain. Cyclone Aila.
  2. United Nations Educational, Scientific and Cultural Organization (UNESCO). World Heritage Sites: Sundarbans National Park, India and The Sundarbans, Bangladesh. Online at http://whc.unesco.org/ en/ list/ 452 and http://whc.unesco.org/ en/ list/ 798. Accessed May 18, 2010.
  3. Nicholls, R.J., P.P. Wong, V.R. Burkett, J.O. Codignotto, J.E. Hay, R.F. McLean, S. Ragoonaden, and C.D. Woodroffe. 2007. Coastal systems and low-lying areas. In: Climate change 2007: Impacts, adaptation and vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden, and C.E. Hanson. Cambridge University Press, 315-356.
  4. Jennerjahn, T.C., andV. Ittekkot. 2002. Relevance of mangroves for the production and deposition of organic matter along tropical continental margins. Naturwissenschaften 89, 23-30.
  5. Cruz, R.V., H. Harasawa, M. Lal, S. Wu, Y. Anokhin, B. Punsalmaa, Y. Honda, M. Jafari, C. Li and N. Huu Ninh, 2007: Asia. In: Climate change 2007: Impacts, adaptation and vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden, and C.E. Hanson. Cambridge University Press, 469-506.
  6. Mazda Y., M. Magi, M. Kogo and P.N. Hong. 1997. Mangroves as a coastal protection from waves in the Tong King Delta, Vietnam. Mangroves and Salt Marshes 1, 127-135.
  7. Mazda Y., M. Magi, Y. Ikeda, T. Kurokawa and T. Asano. 2006. Wave reduction in a mangrove forest dominated by Sonneratia sp. Wetlands Ecology and Management 14, 365-378.
  8. Vermaat, J. and U. Thampanya. 2006. Mangroves mitigate tsunami damage: a further response. Estuarine, Coastal and Shelf Science 69, 1-3.
  9. United Nations Development Program. 2004. Reducing disaster risk: A challenge for development, M. Pelling, A. Maskrey, P. Ruiz and L. Hall (eds.), United Nations, New York, NY, Bureau for Crisis Prevention and Recovery, 161 pp. Available at www.undp.org/ cpr/ disred/ rdr.htm
  10. Das, S., and J.R. Vincent. 2009. Mangroves protected villages and reduced death toll during Indian super cyclone. PNAS 106 (18) 7357-7360.
  11. Danielsen, F., M.K. Sørensen, M.F. Olwig, V. Selvam, F. Parish, N.D. Burgess, T. Hiraishi, V.M. Karunagaran, M.S. Rasmussen, L.B. Hansen, A. Quarto, N. Suryadiputra. 2005. The Asian tsunami: A protective role for coastal vegetation. Science 310: 5748, 643.
  12. Zweig, R. 1998. Sustainable Aquaculture: Seizing Opportunities to Meet Global Demand. Agriculture Technology Notes 22.World Bank, Washington, District of Columbia.
  13. Zafar, A., 2005: Training and capacity building for managing our mangroves resources—UNU's role to meet regional challenges. Environment and Sustainable Development Programme, Tokyo, Japan, Technical Notes of United Nations University, 5 pp. http://www.inweh.unu.edu/ Coastal/ Mangroves/ Report_Train&CapBuild.pdf.
  14. Ericson, J.P., C.J. Vorosmarty, S.L. Dingman, L.G. Ward, and M. Meybeck. 2005. Effective sea-level rise and deltas: Causes of change and human dimension implications. Global Planetary Change 50:63-82.
  15. Han, M., M.H. Zhao, D.G. Li and X.Y. Cao. 1999. Relationship between ancient channel and seawater intrusion in the south coastal plain of the Laizhou Bay. Journal of Natural Disasters, 8,73-80.
  16. Bindoff, N.L., J. Willebrand, V. Artale, A. Cazenave, J. Gregory, S. Gulev, K. Hanawa, C. Le Quéré, S. Levitus, Y. Nojiri, C.K. Shum, L.D. Talley, and A. Unnikrishnan. 2007. Observations: Oceanic climate change and sea level. In: Climate change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller. Cambridge University Press.
  17. Douglas, B.C. 1997. Global sea rise: A redetermination. Surveys in Geophysics 18: 279-292. doi:10.1023/A:1006544227856.
  18. Alam, M., 1996. Subsidence of the Ganges-Brahamaputra Delta of Bangladesh and associated drainage, sedimentation and salinity problems. In: Milliman, J.D., Haq, B.U. (Eds.), Sea-Level Rise and Coastal Subsidence. Kluwer Academic Publishers, pp. 169-187.
  19. "Disputed Bay of Bengal island 'vanishes' say scientists." 2010. BBC News. Online at http://news.bbc.co.uk/ 2/ hi/ south_asia/ 8584665.stm. Accessed May 18, 2010.
  20. Mohal, N., Z.H. Khan, N. Rahman. 2006. Impact of sea level rise on coastal rivers of Bangladesh. Coast, Port and Estuary Division, Institute of Water Modelling (IWM), Bangladesh. Online at: www.riversymposium.com/ 2006/ index.php? element=06MOHALNasreen. Accessed May 17, 2010.
  21. The Intergovernmental Panel on Climate Change emissions scenarios referred to in this hotspot are the high emissions path known as A1FI and the low emissions path known as B1.
  22. Solomon, S., D. Qin, M. Manning, R.B. Alley, T. Berntsen, N.L. Bindoff, Z. Chen, A. Chidthaisong, J.M. Gregory, G.C. Hegerl, M. Heimann, B. Hewitson, B.J. Hoskins, F. Joos, J. Jouzel, V. Kattsov, U. Lohmann, T. Matsuno, M. Molina, N. Nicholls, J. Overpeck, G. Raga, V. Ramaswamy, J. Ren, M. Rusticucci, R. Somerville, T.F. Stocker, P. Whetton, R.A. Wood, and D. Wratt. 2007. Technical Summary. In: Climate change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H. L. Miller. Cambridge University Press, 20-91.
  23. Rahmstorf, S., A. Cazenave, J.A. Church, J.E. Hansen, R.F. Keeling, D.E. Parker, and R.C.J. Somerville, 2007: Recent climate observations compared to projections. Science 316(5825), 709.
  24. Colette, A. 2007. Chapter 2, world heritage marine biodiversity. In Case Studies on Climate change and World Heritage. UNESCO World Heritage Centre ISBN : 978-92-3-104125-9. Online at http://whc.unesco.org/ uploads/ activities/ documents/ activity-473-1.pdf. Accessed May 18, 2010.
  25. Stern, N. 2007. Stern Review on the Economics of Climate Change. Cambridge University Press. Cambridge, 692 pp.
  26. Mitchell, J.F.B., J. Lowe, R.A.Wood and M. Vellinga. 2006. Extreme events due to human-induced climate change. Philos. T. Roy. Soc. A 364, 2117-2133.
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