The 25-metre sea level rise is inappropriately cited in the UK’s Independent newspaper in the name of NASA and certainly entire Bangladesh is not going under water by the end of this century,

writes Dr M Monirul Qader Mirza

The New Age – July 5, 2008

BANGLADESH is a flat deltaic country where 80 per cent of the elevations are less than 12 metres above sea level. Terrain of the coastal southern Bangladesh is mostly at sea level. Because of the geographical setting and physical characteristics, the country is regularly inundated by riverine to coastal flooding. Under the future climate change regime, the country will be highly vulnerable to sea level rise, intense cyclones and storm surge flooding. A recent special report by Johann Hari – titled Bangladesh is set to disappear under the waves by the end of the century and published in the British daily Independent, has drawn significant attention around the world. It has particularly sent a shockwave among the people, scientists and policymakers in Bangladesh and overseas. However, will Bangladesh completely disappear under water by 2100, as claimed in the Independent citing the National Aeronautics and Space Administration of the United States? This issue deserves discussion in the context of the findings of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, which was released in 2007, and the scientific developments that have taken place since the release of the IPCC report.

Causes of sea level rise

Sea level varies from temporal to spatial scales. For the inhabitants of the coastal area, relative sea level – the level of the sea surface in relation to land – is important. Relative sea level can change by vertical movement of the land or changes in the level of ocean surface itself. Vertical movement can occur due to tectonic activities and balance between deltaic subsidence caused by massive weight of sediments, and the accretion of land as additional sediments are deposited in the coastal areas. Changes in sea surface topography can occur at the very 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 changes in global volume of the ocean which is caused by melting of land ice masses and warming of the ocean and its thermal expansion. In the context of greenhouse effect, the ocean, as well as land is warming up. As the ocean warms, the density of water would decrease and its volume would increase. This is termed ‘oceanic thermal expansion’. There are three uncertainties to ascertain 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, the instrumental record of modern sea level changes shows evidence for onset of sea level rise during the 19th century. Estimates for the 20th century show that global average sea level rose at a rate of about 1.7mm per year. Satellite observations available since the early 1990s provide more accurate sea level data with nearly global coverage. This decade-long satellite altimetry dataset shows, since 1993 sea level has been rising at a rate of around 3mm per year, significantly higher than the previous half century. However, sea level is not rising uniformly around the world. In some regions, rates are up to several times the global mean rise, while in other regions sea level is falling. For the past decade, sea level rise shows the highest magnitude in the western Pacific and eastern Indian oceans. Sea level rise in some tidal stations in the Bangladesh coasts are: Hiron Point – 4mm per year; Char Changa – 6mm 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 sea level is due mostly to non-uniform changes in temperature and salinity and related to changes in ocean circulation.

What factors contributed to the observed sea level rise? As per the IPCC’s Fourth Assessment Report, among the measurable factors, melting glaciers and ice caps were found to be the largest contributor, for example, from 1961-2003, their contribution was estimated to be 28 per cent followed by thermal expansion (23 per cent). But for the decade 1993-2003, contribution of thermal expansion was much larger (52 per cent).

Future sea level projections of the IPCC

In its Fourth Assessment Report, the IPCC projected that global sea level rise by 2100 would be in the range of 18cm to 59cm depending on a range of greenhouse gas emission scenarios. This full range of projection is relative to 1980-1999 and excluded of carbon-cycle feedback and future rapid dynamical change in ice flow because of lack of published literature. This is an emerging science. However, the NASA scientist Dr James Hansen (http ://www.columbia.edu/~jeh1/) disagrees with the IPCC findings and said it had addressed ‘a portion of the problem’.

2100: the 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 projection of sea level rise. According to him, the most important left out component of sea level rise was contributions from the disintegration of ice sheets in Greenland and West Antarctica. But the IPCC in its Fourth Assessment Report considered 0.1 to 0.2 metre additional sea level rise for the ice sheet melting. However, this has not been explicitly integrated in its sea level rise projections. Dr Hansen’s concerns have been addressed differently by the IPCC as 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 past warming of 0.7oC already produces large amount of summer melt on Greenland and West Antarctica. He iterates, ‘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 the ice sheet disintegration was insignificant, but it has doubled in the past one decade (1995-2005) and close to 1mm per year. So if 10mm or 1cm 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 only based 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 [business as usual] scenario.’ We need at least two more decades of observational data from Greenland and West Antarctica to verify Dr Hansen’s ‘ten-year doubling’ hypothesis.

The scary part of the Independent article was 25-meter sea level rise and complete disappearance of Bangladesh from the world map. Mr Hari wrote: ‘…and 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 in its report has not considered Bangladesh exclusively although it has appeared in many instances because of special geophysical characteristics of the country and its future vulnerability to climate change and sea level rise.

In my long association with the IPCC, I have not come across any literature that has particularly projected a 25-metre sea level rise by 2100. Therefore, I decided to verify it with Dr Hansen and sent him an email on June 26 and he was very kind to write back a day later. He replied: ‘I have made no such projection, although the long-term response to 2-3oC warming would probably be a sea level rise of that order – it is hard to say how much would occur by 2100 – it could be a few metres.’ This long-term timeline is debatable, may be thousands of years. So the 25-metre sea level rise is inappropriately cited in the Independent in the name of NASA and certainly entire Bangladesh is not going under water by the end of this century.

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. The IPCC’s Third Assessment Report published in 2001 projected 11 per cent inundation for a 45cm sea level rise. However, the inundated area may be doubled for a 1-metre rise. Another study conducted by the Institute for Water Modelling, Dhaka shows intrusion of seawater up to Chandpur, about 80km upstream from the estuary. With a 32cm sea level rise, 84 per cent of Sundarban, a UNESCO world heritage site, would be deeply inundated by 2050 and the entire Sundarban may be lost for about one-metre rise. In Bangladesh, impacts 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 perspective plan.

Dr M Monirul Qader Mirza is currently with the Adaptation and Impacts Research Division, Environment Canada and the Department of Physical and Environmental Sciences, University of Toronto. He acted as coordinating lead author of the Intergovernmental Panel on Climate Change of the United Nations, winner of the Nobel Peace Prize in 2007. Views presented are those of the writer’s.

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