|A recent study sounds a warning about the impact of sea level rise on coastal populations owing to global warming.|
Rising sea levels and coastal erosion threaten vast populations in Asia. Here, the ravaged structure of a police station on Sagar island in West Bengal.
SEA level rise (SLR) is a consequence of global warming. Populations living along the coasts are, therefore, particularly vulnerable to the ingress of water caused by rising seas. An equally significant threat for coastal settlements comes from the severe storm surges associated with the more intense hurricanes and tropical cyclones that are likely to occur as a result of climate change. From this perspective, a systematic assessment of the populations at risk from SLR, stronger storms and other coastal hazards, and their distribution globally, is of paramount importance.
The first ever such study, by Gordon McGranahan of the United Kingdom-based International Institute for Environment and Development, Deborah Balk of the City University of New York and Bridget Anderson of Columbia University (also in New York), will be published in the April 14 issue of the journal Environment and Urbanization. Since low elevation coastal populations are at the greatest risk, the study has mapped settlement patterns worldwide in low elevation coastal zones (LECZs), defined in the work as contiguous areas along the coast that are less than 10 metres above the current mean sea level.
The study essentially integrates three one-kilometre-resolution spatial databases - that of global population distribution, urban extents and elevation data - to arrive at countrywise estimates of urban land area and population in the LECZs. The LECZs were delineated using the elevation data set from the National Geospatial Intelligence Agency and the National Aeronautics and Space Administration's (NASA) joint Shuttle Radar Topography Mission (of February 2000). According to the authors of the study, in some places, mostly the mouths of major rivers such as the Amazon in Brazil and the Yenisey in Russia, the LECZ extends well beyond 100 km inland, although for most of its extent, it is much less. In Bangladesh (and in southern parts of West Bengal), where the terrain is made up of delta regions, the situation is much more serious because the swath of the LECZ is much wider and extends right across the mouth of Bangladesh and significant parts of West Bengal. In Bangladesh, in fact, a significant part goes right across in the north-south direction as well.
The SLR is not expected to reach anything like 10 m even in the worst-case scenario, at least, not in the foreseeable future. Even with storm surges, there will be a large margin of safety from direct flooding within an LECZ. However, as the authors point out in their paper, SLR and storm surges can certainly have an adverse impact on people living in such areas, for instance, through saline water intrusion of groundwater. But the principal reason, according to them, for choosing 10 m as the cut-off point was the fact that "estimates based on elevations below 10 m could not be considered globally reliable, particularly in some types of coastal areas such as those characterised by mountainous bays".
For delineating global urban sprawl, the scientists used "the only globally consistent urban footprint data set", prepared by Columbia University's Centre for International Earth Science Information Network (CIESIN) through a project called GRUMP (Global Rural-Urban Mapping Project)). The GRUMP data set is based largely on the United States's National Oceanic & Atmospheric Administration's (NOAA) 1994/95 satellite data of night "city" lights coupled with settlement information. Because of the "overglow" that would be characteristic of an urban footprint based on lights, the GRUMP data set is probably an overestimation of the real urban extent and includes the surrounding suburban and peri-urban extensions, the authors point out. The third data set pertains to countrywise population grid and land area associated with each grid. For this too, the authors made use of the 1999/2000 gridded estimates made as a part of GRUMP.
The recently released Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) gives an idea of the magnitudes of the coastal hazards likely to arise as a result of climate change in the short and long terms. There is enough evidence already to suggest that SLR accelerated in the 20th century as a result of the warming caused by human activities. "There is high confidence," the IPCC Report has said, "that the rate of observed sea level rise increased from the 19th to the 20th century." (see Frontline, March 9).
The 20th century rate, too, increased significantly towards the end of the century, an indication of the consequence of heightened human activities that contributed to global warming. As compared with the rate of rise of the global average sea level in 1961-2003 of 1.8 millimetres/year, the rate in 1993-2003 was 3.1 mm/year, which is 70 per cent faster. According to the AR4, new data suggest that losses from the ice sheets of Greenland and Antarctica have very likely contributed to the SLR in 1993-2003. The total rise in the 20th century is estimated to be 0.17 m. On the basis of model projections, the global average SLR at the end of the 21st century (2090-2099) relative to the average level in 1980-1999 is expected to be in the 0.18-0.59 m range, depending on different greenhouse gas (GHG) emission scenarios.
What is, however, clear from the IPCC Report is that, irrespective of the actual SLR in the 21st century, it will represent the beginning of a longer-lasting and larger SLR in the long term. "Anthropogenic warming and SLR would continue for centuries due to timescales associated with climate processes and feedbacks, even if GHG concentrations were to be stabilised," the report has warned. According to the AR4, contraction of the Greenland ice sheet is expected to continue to contribute to SLR even after 2100. Further, dynamical processes related to ice floes not included in current models, but suggested by recent observations, could increase the vulnerability of the ice sheets to warming, increasing future SLR. However, its magnitude is uncertain as there is no complete understanding of these processes.
On the basis of a range of models, the IPCC Report has concluded that future tropical cyclones (typhoons and hurricanes) will be more intense, with larger peak wind speeds and heavier precipitation associated with ongoing increases of tropical sea-surface temperatures. This implies that the associated storm surges will be much stronger and, therefore, that the associated hazards (massive waves and flooding) will be much greater. For instance, storm surges as high as 9 m apparently resulted from Hurricane Katrina. The hazard will be much greater along the coasts of the Bay of Bengal, which is generally prone to severe storm surges.
So, what is the population size that will be at risk from hazards from the seas due to climate change, and what is its global distribution pattern?
On the basis of estimates made in 2000, the study of McGranahan and others finds that the LECZs include a total area of 27,00,000 sq km, which is about 2 per cent of the world's land, and 634 million inhabitants, who constitute about 10 per cent of the world population. The top ranking in terms of distribution of population in LECZs, as would be expected, corresponds to the most populated regions of the world. While China, with an LECZ population of 143.9 million, ranks first, India and Bangladesh, with the respective LECZ populations of 63.2 million and 62.5 million, rank second and third.
But, given Bangladesh's geography and the nature of its terrain, 46 per cent of its population lives in LECZs as compared with 6 and 11 per cent for India and China respectively. Bangladesh's greater vulnerability is also apparent from the fact that in terms of the share of total population it ranks sixth in the world. The same is true of Vietnam, which ranks fourth in terms of the number (43 million) as well as the share of the total population (55 per cent) in LECZs. Globally, about 21 nations have more than half their populations in the zone, 16 of which are small island states, the most vulnerable group of nations.
The study has also found that the extent of urbanisation is more in the LECZs than in the world as a whole. The urban population in the LECZs is 360 million - an urbanisation level of about 60 per cent, which is higher than the worldwide level of 50 per cent. The zone has a 10 per cent share of the total population of the world, but urban dwellers in LECZs account for 13 per cent of the total urban population. Significantly, with a total LECZ area of 881,000 sq km, Asia accounts for about one-third of the world's total area of LECZs, but because it has a higher population density, it accounts for two-thirds of the world's LECZ urban population (and almost three-fourths of the total LECZ population of the world).
An interesting finding of the research is that not only do the LECZs account for a greater share of the world's urban population, but they also contain a greater share of large urban settlements. Higher population densities coupled with higher concentrations of commerce and industry in coastal regions are clearly the reasons for such large coastal urban agglomerations. The study also found that compared with more prosperous nations a higher proportion of people in the poorest countries were living in the vulnerable LECZs.
While only 13 per cent of settlements with populations under 100,000 are in the LECZs, the proportion becomes 65 per cent when only larger settlements of five million and more are considered. This is particularly borne out by Asian data where the two fractions are respectively 12 per cent and 70 per cent, an aspect that Asia shares with North America whose figures are 9 and 80 per cent respectively. The world average share of the cities with populations more than five million is a high 21 per cent, but, as the authors point out, "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 live in the LECZ". Among Indian cities, Mumbai, Kolkata and Chennai fall into this category.
Among Asian countries, China and Bangladesh, the former with rapid urban growth along the coast and the latter with the most populous delta regions of the world, raise serious concerns about the need for adaptation to climate change to start at the earliest, say the scientists. "Even as the seaward risks associated with climate change are increasing," point out the researchers, "the areas most at risk are experiencing particularly high population growth." According to the study, for both Bangladesh and China, the population in the LECZ grew at almost twice the national population growth rate between 1990 and 2000.
The urban populations in both countries, in fact, grew more rapidly; for China, for instance, the urban population growth was three times the national rate. The study points out that in China the movement towards the coast is driven by trade-oriented economic strategies and policies, dating back to the 1980s, that favour urban development along the coast. "The geographical advantage of coastal development has been enhanced by the creation of special economic zones (SEZs) in coastal locations," the study points out. The scientists have also expressed concern that continued urbanisation will draw still greater populations into the LECZs. Perhaps, there is a lesson here for India, particularly in the context of the current policy of encouraging SEZs across the country.
"From an environmental perspective, there is a double disadvantage to excessive (and potentially rapid) coastal development," the study warns. "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 SLR 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."
People being rescued from a rooftop in a flooded New Orleans neighbourhood after Hurricane Katrina struck, on August 29, 2005.
"The risks of human settlements could be reduced if people and enterprises could be encouraged to move away from the coast, or at least from the most risk-prone coastal locations... [but] the current population movements are in the opposite direction," the paper points out. "Given the character of urban development, and the factors driving coastward movement are still poorly understood, turning these flows around is likely to be slow, costly or both. In particular, there is the danger that ill-considered or politically short-sighted measures... fail to provide the basis for viable alternatives inland or in more appropriate coastal locations. More appropriate measures are sorely needed, and the earlier the better," the paper adds.
"In order to support efficient and equitable means for moving the most vulnerable urban settlements, a better understanding is needed of why urban settlements in coastal areas are growing more rapidly than inland [areas]. Avoiding policies that favour coastal development (such as SEZs in China), and imposing more effective costal zone management, could make a difference in the longer term," says the study. Here, too, there could be a lesson for India, which is in the process of reformulating its coastal management plan on the basis of vulnerability criteria.