Global Warming assumption of climate change attributed to greenhouse effects. It is one of the major environmental issues facing the world today. Its importance lies in the fact that the chemical composition of the earth's atmosphere is already quite different from what it was a century ago and the fact that these changes are not reversible in the foreseeable future with the present level of technical know-how. Average global temperature is now about 0.6°C higher than it was 100 years ago. Computer models of climatic change are predicting that by the end of the 21st century the global temperature may rise by an additional 2.5° to 5.5°C. As a result, there may be a substantial change in the sea level caused by thermal expansion of the earth's surface water, and from a rapid melting of alpine and polar glaciers and ice caps.
Two million years ago, the earth's temperature was substantially higher. Since then the earth had experienced alternative glacial-interglacial cycles. The oscillations between glacial-interglacial cycles during the Pleistocene period were due to variations in the earth's orbit around the sun as well as due to changes in the earth's tilt in relation to the sun. This swing from glacial to interglacial cycles occurr approximately every 100,000 years. During the glacial periods the earth's temperature was 4° to 5°C less than the interglacial period. About 10,000 years ago, the latest ice age ended and the continental glaciers retreated. During the last 10,000 years, global temperature fluctuated by not more than 2°C.
Recent global warming has been attributed to the increased presence of certain greenhouse gases in the atmosphere. These atmospheric gases allow the shortwave solar radiation to reach the earth but prevent the long wave infrared radiation from the earth to escape into outer space, thus warming up the earth's surface. The increased amounts of gases, which have induced global warming are carbon dioxide, nitrous oxide, methane, chlorofluorocarbon and water vapour in the atmosphere. Industrialisation, agricultural extension, specially wet rice cultivation, growth in the number of individual automobiles and deforestation have all made significant contributions to the emission of first four gases.
Three major impact of global warming on Bangladesh may be categorised as: climate change, sea level change, and changes in the pattern of distribution and density of population.
Climatic change Using general circulation models (GCMs) of the climate system, scientists had predicted a global change in temperature in the range of 1.5°C to 4.5°C with the doubling of carbon dioxide in the atmosphere.
Over the past 100 years, temperature has increased by 0.5°C, but in the next 50 years, that is, by 2050 AD the temperature in Bangladesh is expected to rise by 1.5°-2.0°C. As temperature increases on a global scale, the hydrologic cycle will intensify and the rate of evaporation is expected to increase by 12%. This, in turn, will increase the level of precipitation globally. There may be regional variations in the amount of precipitation, but Bangladesh will experience an increase in rainfall. Climate models developed by IPCC indicate that Bangladesh may experience 10 to 15 percent more rainfall by 2030 AD. Bangladesh is visited by devastating floods quite frequently. In the past two decades, extensive flooding had occurred in 1987, 1988 and 1998 leading to colossal damages to infrastructures and the destruction of standing crops. An increase in rainfall will only worsen the situation.
Scientists also predict that global warming will increase the frequency and intensity of tropical storms. A high surface temperature of over 27°C of the sea helps in the formation of cyclones. Global warming will raise the sea surface temperature more than what it is now, thus increasing the possibility of cyclone occurrences. Bangladesh has already been experiencing catastrophic cyclones. The one that hit Bangladesh in 1991 claimed 150,000 lives and caused staggering losses to livestock and property. A warmer climate, by increasing the frequency of natural hazards such as floods and cyclones, will further aggravate such situations and negate the development efforts of the country.
Sea level change The sea level changes constantly in every locality due to changes in tides, atmospheric pressure and wind velocity. But long-term sea level changes occur only due to changes in the global climate. Global warming will cause a thermal expansion of the seawater. A rise in temperature will also melt the polar and alpine ice sheets. Since Bangladesh is deltaic, vast areas of the country will be submerged with the rise of the sea level. How much of the country will be lost to the sea will depend on the relative change in the sea level. As Bangladesh is part of an active delta a huge load of sediments is brought down and deposited by the great river systems of the ganges, brahmaputra and meghna. This leads to elevation of land. However, the weight of the huge load of sediment results in compaction of the soil and land subsidence. The sea floor is also rising because of deposition of sediments brought down by the rivers. subsidence and uplift of land may also occur from tectonic processes. A Taskforce report predicts that for one metre rise in the sea level will lead to loss of about 22,889 sq km of land, which is about 15.8% of the total area of Bangladesh.
Being a low-lying and densely populated country Bangladesh would be worst hit by any rise in the sea level. Coastal areas will experience erosion and inundation due to intensification of tidal action. A rise in seawater will enable saline water to intrude further inland during high tides. Destruction of agricultural land and loss of sweet water fauna and flora could also occur. The shoreline will retreat inland, causing changes in the coastal boundary and coastal configuration. The process will also shrink the land area of Bangladesh.
It is predicted that the surface temperature of the earth could rise by about 1.8° to 6.3°C by 2100. This could lead to a rise in sea levels by 0.5m and could threaten with inundation heavily populated coastal and deltaic areas from Mississippi to Bangladesh. Climatic disasters such as hurricanes, cyclones and droughts could become more frequent and severe. Tropical diseases such as malaria and cholera may move north and south of the equatorial region due to global warming.
To sum up, for Bangladesh, the climate changes caused by global warming may lead to more rainfall, massive floods, great cyclones and droughts. However, conservation of the environment and sustainable development strategies will definitely protect the earth and its environment and will particularly save countries like Bangladesh from' the catastrophe which can be induced by global warming. [Masud Hasan Chowdhury]