BANGLADESH is a flat deltaic country where 80% of the land is less than 12 metres above sea level. Coastal southern Bangladesh is mostly at sea level. Because of the geographical setting and physical characteristics, the country is regularly inundated by riverine and coastal flooding.
Climate change will make the country highly vulnerable to sea level rise, intense cyclones and storm surge flooding. A recent special report entitled "Bangladesh is set to disappear under the waves by the end of the century" by Johann Hari, published in the British daily Independent, has drawn significant attention around the world. It has sent a shockwave through the people, scientists and policy-makers, in Bangladesh.
However, will Bangladesh completely disappear under water by 2100 as claimed in the Independent, which cited National Aeronautics and Space Administration (Nasa)? This issue deserves discussion in the context of the findings of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) that was released in 2007, and the scientific developments that have taken place since then.
Causes of sea level rise
Sea level varies from temporal to spatial scales. For the inhabitants of coastal areas, relative sea level -- the level of the sea surface in relation to land -- is important. Relative sea level can change through vertical movement of the land or through a change in ocean level.
Vertical movement can occur due to tectonic activities and balance between deltaic subsidence caused by massive weight of sediments, and due to the accretion of land as additional sediments are deposited in the coastal areas. Changes in sea surface topography can occur in the shortest time-scales due to tidal and meteorological phenomena.
Sea level changes are recorded by tide gauges. The relative sea level at a gauge may show long-term changes due to the vertical motion of the gauge, circulation of the ocean, or change in the volume of the ocean, which is caused by melting of ice masses, and warming of the ocean and its thermal expansion. As the ocean warms, the density of water decreases and volume increases. This is called "oceanic thermal expansion." There are three variables in ascertaining the rate of thermal expansion. They are: changes in the heating of the climate system, the sensitivity of climate, and the rate of heat uptake by the oceans.
Sea level changes in the recent past
According to the IPCC, recorded sea level changes show evidence of onset of sea level rise during the 19th century. Estimates for the 20th century show that average sea level rise was at a rate of about 1.7 mm per year. Satellite observations available since the early 1990s provide more accurate sea level data with nearly global coverage.
This decade-long satellite data set shows that, since 1993, sea level has been rising at around 3 mm per year. However, sea level is not rising uniformly around the world. In some regions, the rates are up to several times the global mean rise, while in others sea level is falling.
For the past decade, sea level rise has been the highest in the western Pacific and eastern Indian oceans. Sea level rise in some tidal stations in the Bangladesh coast are: Hiron Point: 4 mm per year; Char Changa: 6 mm per year and Cox's Bazar: 7.8 mm per year, as reported by the Saarc Meteorological Centre in Dhaka. Regional variability of the rates of rise is due mostly to non-uniform changes in temperature and salinity, and is related to changes in ocean circulation.
What factors contributed to the observed sea level rise? As per IPCC's 4th Assessment Report, among the measurable factors, glaciers and ice caps were found to be largest contributors, for example, from 1961-2003, there contribution was estimated to be 28%, followed by thermal expansion (23%). But for the decade 1993-2003, contribution of thermal expansion was much larger (52%).
Future sea level projections of the IPCC
In its 4th Assessment Report, the IPCC projected that global sea level rise would be in the range of 18 cm to 59 cm by 2100, depending on a range of greenhouse gas emission scenarios. This projection is relative to 1980-1999, and excludes carbon-cycle feedback and future rapid dynamical change in ice flow because of lack of published literature. This is an emerging science. However, Nasa scientist Dr. James Hansen disagrees with the IPCC findings and says that it had addressed "a portion of the problem."
2100: Doomsday for Bangladesh?
The Independent article is partly based on two recent publications of Dr. Hansen, where he discussed the limitations of the IPCC's business as usual (BAU) projection of sea level rise. According to him, the most important component left out was the disintegration of ice sheets in Greenland and West Antarctica. But the IPCC considered 10-20 cm additional sea level rise because of ice sheet melting.
However, this has not been integrated in its sea level rise projections. Dr Hansen's concerns have been addressed differently by the IPCC. It states: "Larger values cannot be excluded, but understanding of these effects is too limited to assess their likelihood, or provide a best estimate or an upper bound for sea level rise."
According to Dr. Hansen, the warming by 0.7oC has already produced summer melting in Greenland and West Antarctica. He says: "Global warming of several more degrees, with its polar amplification, would have both Greenland and West Antarctica bathed in summer melt for extended melt seasons." Dr. Hansen further says that until the past few years, contribution from ice sheet disintegration was insignificant, but it has doubled in the past one decade (1995-2005) and is close to 1 mm per year. So if 10 mm contribution from the ice sheets for the decade 2005-2015 doubles in every decade, by 2100 sea level rise only from the melting of ice sheets would be 5 metres.
This estimate is based only on an assumption and there is no concrete reasoning to back it up. In this regard, Dr. Hansen says: "Of course I cannot prove that my choice of a ten-year doubling time for non-linear response is accurate, but I am confident that provides a far better estimate than a linear response for the ice sheet component of sea level rise under BAU scenario." So, in order to verify Dr. Hansen's "ten-year doubling" assumption, we need to wait a couple of more decades.
The scary part of the Independent article was 25 metres sea level rise and complete disappearance of Bangladesh. Johann Hari wrote: "[A]nd found that many climatologists think the IPCC is way too optimistic about Bangladesh. I turned to Professor James Hansen, the director of Nasa's Goddard Institute for Space Studies, whose climate calculations have proved to be more accurate than anybody else's. He believes the melting of the Greenland ice cap being picked up his satellite today, now, suggests we are facing a 25-metre rise in sea levels this century -- which would drown Bangladesh entirely."
Note that the IPCC report has not considered Bangladesh exclusively, although it has appeared in many instances because of its special geo-physical characteristics and its future vulnerability to climate change and sea level rise.
Sea level rise: Implications for Bangladesh
Because of the flatness of the country, for any given magnitude of future sea level rise, the impacts could be devastating. IPCC's Third Assessment Report published in 2001 projected 11% inundation for a 45 cm sea level rise. However, the inundated area may be doubled for a 1 metre rise (Figure 1). Another study conducted by the Institute for water Modeling (IWM), Dhaka, shows intrusion of seawater up to Chandpur, about 80 km upstream from the estuary. With a 32 cm sea level rise, 84% of the Sundarbans (a Unesco Heritage Site) would be mostly inundated by 2050, and the entire Sundarbans may be lost in a one-metre rise. In Bangladesh, the impact of sea level rise on land and water, crops, livestock, human health and livelihood would be significant. It is, therefore, necessary to formulate and implement appropriate adaptation measures under a long-term plan.