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Growing threat of rising sea levels

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Mumbai is among the cities cited in a survey as likely to be affected.
Mumbai is among the cities cited in a survey as likely to be affected.

N. Gopal Raj

Asia, with its high population densities, is particularly at risk.

MILLIONS OF people around the world, including in fast-growing Asian cities such as Shanghai and Mumbai, are likely to be affected by rising sea levels and stronger storm surges caused by global warming, according to a study that has just been released.

The study found that over 630 million people, accounting for a tenth of the world's population, live within 10 metres above the current sea level. The research was carried out by Gordon McGranahan of the International Institute for Environment and Development, a London-based non-governmental policy research body, Deborah Balk at the City University of New York, and Bridget Anderson of Columbia University in New York.

In order to identify places that would be affected by climate change-induced rises in sea level and bigger storm surges, the researchers combined terrain height information derived from radar observations during a Space Shuttle mission in February 2000 with databases giving the global spatial distribution of population and urban areas. This made it possible to identify vulnerable human settlements and estimate the number of people in the "low elevation coastal zone (LECZ)," which the scientists defined as coastal areas within an elevation of 10 metres.

The study, to be published in April in the journal Environment and Urbanization, found that Asia, with its high population densities and countries having large populations, was particularly at risk from increases in sea level. Almost three-quarters of the people currently living in the low elevation coastal zone are Asian. China with an estimated 144 million people, India with more than 63 million, and Bangladesh with more than 62 million lead the ranking of countries with the largest populations in the zone.

About 13 per cent of urban dwellers are in the zone compared to 10 per cent of the global population. Once again, Asia accounts for two-thirds of the 360 million city residents in the zone. Globally, 65 per cent of the cities with populations of over 5 million are at least partially in the zone and in Asia that proportion rises to 70 per cent. "The world average share of the population of cities over 5 million in the zone is a very high 21 per cent, but this average is highly influenced by the coastal location of large Asian cities, in which 32 per cent of the population of cities over 5 million resides in the LECZ," according to the journal paper. The study estimates that three per cent of India's land area would fall in the vulnerable zone. Among the cities, the study suggests that, apart from Mumbai, parts of Kolkata and Chennai too could be affected.

The study also found that a higher proportion of people in the world's poorest countries were living in the vulnerable zone compared to the number in the prosperous nations.

Double disadvantage

"From an environmental perspective, there is a double disadvantage to excessive [and potentially rapid] coastal settlement," the scientists pointed out in the paper. "First, uncontrolled coastal development is likely to damage sensitive and important ecosystems and other resources. Second, coastal settlement, particularly in the lowlands, is likely to expose residents to seaward hazards such as sea-level rise and tropical storms, both of which are likely to become more serious with climate change. Unfortunately, such environmental considerations do not have the influence on settlement patterns that they deserve."

"It is too late to rely solely on a reduction in greenhouse gas emissions to mitigate climate, although this is clearly an imperative," said Dr. McGranahan in a press statement. "Migration away from the zone at risk will be necessary but costly and hard to implement, so coastal settlements will also need to be modified to protect residents."

In the paper, Dr. McGranahan and fellow researchers noted that "measures to support previously disfavoured inland urban settlements, away from the large cities on the coast, could not only reduce risks from climate change but also support a more balanced and equitable pattern of urban development."

"In cities like Mumbai, locally-driven efforts will be needed to evaluate and respond to the changing risks," Dr. McGranahan told this correspondent in an email. One of the key ingredients of successful adaptation will be efficient and equitable negotiation between local authorities and the groups of, often low-income, residents living in vulnerable locations, he added.

However, as the paper itself points out, sea-level rise was not expected to reach anything like 10 metres in the foreseeable future. The summary report issued by the Intergovernmental Panel on Climate Change in February had estimated the sea level rise that was likely by the end of this century was 0.59 metres. But other experts have indicated that sea level rises of over one metre could be expected by the end of the century for strong global warming scenarios.

The 10-metre level was chosen because adequate estimates could not be prepared for lower elevations, explains Dr. McGranahan. Besides, storm surges and tides would increase any flooding that occurred, many river delta areas were subject to subsidence and saline intrusions could impact areas further inland. In addition, disruptions to the local economy and related social problems would affect people living in areas where a significant share of the population faced increased flooding.

"Overall, the 10-metre figure identifies a zone where issues of sea level rise and storm surges need to be taken seriously, not where flooding is expected," he said in his email.


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