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| Climate Refugees: A Rejoinder Hugh Brammer gradually westward from none at the Haringhata river mouth in Bangladesh to a linear maximum of three-four km at the Hooghly river mouth. Unpublished measurements by the author show that erosion |
The assertions regarding the impact of sea-level rise in India and Bangladesh (“Climate Refugees”, 6 June 2009) are not defensible. Sea levels have not risen in such magnitudes and therefore this is unlikely to be the reason for vast land loss. Indisputably, the potential impact of sea-level rise in the 21st century on people living in such regions is serious, and the subject needs determined deliberation.
Hugh Brammer (h.brammer@btinternet. com) is an independent researcher, formerly with the Food and Agriculture Organisation, Bangladesh.
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The sea level has not risen by 3.14 cm a year in the Indian Sunderbans. Unnikrishnan and Shankar (2007), using data from tidal stations with 40 years or more of data, estimated an average rise of 1.29 mm per year in the Indian Ocean up to 2004, a figure that is consistent with global estimates of 1-2 mm/year reported by the Inter national Panel on Climate Change.
The land loss of 80 km2 in the Indian Sunderbans in the past 30 years to which the editorial refers is therefore unlikely to be the result of sea-level rise. One reason for land loss could be subsidence in embanked areas, where natural subsidence is no longer compensated by tidal sediment accre tion as it is in the unembanked Sunderbans. Unikrishnan and Shankar reported a net average rise in sea-level of 4.87 mm/year at Diamond Harbour, Kolkata, between 1948 and 2004, and attributed the difference between that figure and the ave rage of 1.29 mm/year in the Indian Ocean to land subsidence at the gauge site. Stanley and Hait (2000) estimated subsidence rates up to about 5 mm/year in the Indian Sunderbans. Goodbred and Kuehl (2003) and Alam et al (2003) reported similar rates for the Ganges-Brahmaputra-Meghna (GBM) delta in Bangladesh. Subsi dence is compensated by accretion of tidal sediments in unembanked areas: Allison et al (2003) reported accretion rates of four to seven mm/ year on land near the coast, decreasing to negligible amounts 70 km inland.
A more probable cause for the land loss reported in the Indian Sunderbans, depending on location, is coastal erosion. However, that erosion is not a recent phenomenon that can be attributed to global warming. Allison (1998) measured a net land loss of
1.9 km2/year along the western part of the GBM delta front between 1792 and 1984. Between 1840 and 1984, erosion increased
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along this coastal section continued in a similar manner between 1984 and 2007.
Similarly, the displacement of 5,00,000 inhabitants of Bhola island in Bangladesh in 2005 to which you refer cannot be attributed to a gradual sea-level rise of 1.29 mm/year (and it is unlikely to have occu rred in a single year). Bhola island is subject to bank erosion by channels in the Meghna estuary. Again, this is a long-sustained process, as is shown by the map produced by Allison (1998). Erosion on the eastern side of Bhola has been more than compensated by land acc retion elsewhere in the Meghna estuary, including on the southern margin of Bhola island itself. Allison calculated a net gain of land in the Meghna estuary ave raging 4.4 km2/year between 1840 and 1984. Measurements by the author show that the net positive balance between accretion and erosion in this estuary continued at an increased rate between 1984 and 2007 (data awaiting publication).
Undoubtedly, the potential impact of sealevel rise in the 21st century on people living in the GBM delta is serious, and the subject must be considered seriously. But it is no help to those people if wrong impacts and causes are identified. That is likely to lead to inappropriate interventions, which could waste funds without benefiting potential victims.
References
Alam, M, M M Alam, J R Curray, M L R Chowdhury and M R Gani (2003): “An Overview of the Sedimentary Geology of the Bengal Basin in Relation to the Regional Tectonic Framework and Basin-Fill History”, Sedimentary Geology, 155: 179-208.
Allison, M A (1998): “Historical Changes in the Ganges-Brahmaputra Delta”, The Journal of Coastal Research, 1, 14.4: 1269-75.
Allison, M A, S R Khan, S L Goodbred and S A Kuehl (2003): “Stratigraphic Evolution of the Late Holocene Ganges-Brahmaputra Lower Delta Plain”, Sedimentary Geology, 15: 317-42
Goodbred, S L and S A Kuehl (2000): “The Significance of Large Sediment Supply, Active Tectonism and Eustasy on Margin Sequence Development: Late Quaternary Stratigraphy and Evolution of the Ganges-Brahmaputra Delta”, Sedimentary Geo logy, 133: 227-48.
Stanley, D J and A K Hait (2000): “Holocene Depositional Pattern, Neotectonics and Sundarban Mangroves in the Western Ganges-Brahmaputra Delta”, The Journal of Coastal Research,16.1: 26-39.
Unnikrishnan, A S and D Shankar (2007): “Are Sealevel Trends Along the Coasts of the North Indian Ocean Consistent with Global Estimates”? Global and Planetary Change, 57, 3-4, 301-07.
