The global average air and ocean temperatures have been gradually increasing from 1880-2010 (Fig.1) This increase in temperature melted a massive amount of ice and snow globally which ultimately, caused the global average sea level to also rise (Fig.2). The Earth’s average surface temperature rose by 0.74 ± 0.18°C over the period 1906–2005. Global average sea level rose at an average rate of about 3.1 mm per year from 1993 to 2003(Fig 2).
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The potential total sea level rise over the period of the end of the 21st century relative to the end of the 20th century is estimated to be an increase in the sea level between 18 and 59 cm. The main consequence of the rising of sea levels is the impact on deltas and small island states that undergo coastal inundations. The reason for this is because deltas and small island states are vulnerable to sea level rise because of their size and height above sea level. The flooding of coasts that are highly populated will cause a major impact in the economic and social dilemmas of the future.
Furthermore, this report will discuss the future of Bangladesh since Bangladesh is the most vulnerable to sea-level rise. Bangladesh is very low-lying and sits on the northern part of the sea that is close to the Bay of Bengal. In addition to being one of the world’s poorest nations, this country has also experienced many catastrophic events in the past such as severe storm surges.
Global Warming and Sea Level Rise
Water temperature tends to lag the general surface temperature because of the phenomenon of latent heat which is the heat absorbed or released by a chemical substance. Energy must be taken from the environment for water or ice to move to a less ordered state. Thus, energy in the form of heat from the air will be transferred to the liquid water or solid ice. This is called the absorption of latent heat which causes the temperature of water to lag the general surface temperature. There are two major reasons for relationship between global warming and increased sea level: thermal expansion and the contribution of land-based ice.
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Thermal Expansion of Water
Thermal expansion of water occurs when the temperature of water increases which causes the water molecules to expand and change in volume. When water is heated, the atoms in the water molecules begin to gain kinetic energy and move at a greater rate. Water is a very unique substance. It will expand when heated but in the range of about 0-4°C, water will undergo negative thermal expansion rather than thermal contraction.
The concern for ice melting is for land-based ice because a larger amount of water flows into the sea on the rising of the sea level to be more evident. As stated earlier, as sea water freezes, it undergoes negative thermal expansion. The liquid water will expand into floating ice and displace water. Thus, the melting of ice does not have a large impact on the change of sea level. The main storage of water on land is located in ice sheets and glaciers. Partial melting of the Greenland ice sheet and possibly the West Antarctic ice sheet, could contribute 4-6 metres or more to sea level rise projections.
The IPCC 2001 Third Assessment has been replaced (Fig. 3) by the IPCC 2007 Fourth Assessment due to false predictions which underestimates the true rise of the sea level. SRES emissions scenarios are projections that estimate future development of the economy. Across the six SRES marker scenarios, sea level was projected to rise by 18 to 59 cm for the time period 2090-99. The Rignot projection suggests that the sea level rise would reach 32 centimetres by 2050 due to the melting of the Antarctic and Greenland ice sheets. Projections after 2100 include the continuation of the melting of the Greenland and Antarctic ice sheet. The estimated timescale for partial melting would contribute 4 to 6 metres or more to sea level rise over this period. In general, the cause of most of these future projections of sea level rise is due to rapid change of the glaciers and ice sheets.
Bangladesh is located in South Asia and is bordered by India on the majority of the sides except for a tiny part where it is bordered with Myanmar to the southeast towards the southern Bay of Bengal (Fig. 4).
The population of Bangladesh is approximately 142.3 million and it is the most densely populated large country in the world. Bangladesh is also one of the poorest nations and relies heavily on foreign aid. The main source of income for the majority of the people of Bangladesh involves working in agriculture and also producing other goods such as leather, textiles, ceramics and fish farming. Thus, natural disasters such as flood and cyclonic storms have large impacts on both the economy and the lives of many people.
Bangladesh is a very flat country that has a very close proximity to the current sea level and lies in the Ganges-Brahmaputra or Ganges Delta (Fig. 5). The majority of Bangladesh is less than 12m above sea level and many parts of Bangladesh that are indicated in red are near or close to the river systems (Fig. 4). Thus, the likelihood of floods in Bangladesh is high due to the low height above sea level. Bangladesh also holds the most trans-boundary rivers with a large number of 57, resulting in many complicated water issues. 8 Unfortunately, these fertile plains are vulnerable to flood and drought.
The rivers of Bangladesh play a major role in the physical geography of Bangladesh. The larger rivers of Bangladesh serve as transportations for material goods. They are also the main source to raise crops, thus flooding of rivers is extremely devastating and quite frequent. On the contrary, the fresh deposits of the rich silt provide a great source of nutrient to the soil. Therefore, the great river system is both beneficial and hazardous to the country.
Bangladesh climate is tropical with a mild winter from October to March, a hot, humid summer from March to June. 8 Bangladesh experiences much heavy rain falls and has high average precipitation. The majority of the rain falls occur during the monsoon season which is a result of the difference between high and low air pressures caused by differential heating of land and water. The climate of Bangladesh contributes to much of the natural disasters including tropical cyclones and floodings.
Storms of the Bay of Bengal
The Bay of Bengal is situated in the north-eastern part of the Indian Ocean and is the largest bay in the world. The climate of the Bay of Bengal consists of a lot of heavy monsoons. The continental pressure system changes during the winter and summer from high to low which produce northeast and southwest monsoons respectively. Also, in spring and fall seasons, powerful cyclones arise and produce very strong winds and subsequently, devastating floods.
Storms in the Bay
The 1991 Bangladesh cyclone occurred on April 29th, 1991 and is one of the deadliest tropical cyclones killing at least 138, 000. A very strong tropical cyclone carried winds at 250 km/h. With the speed of the wind, the low pressure and high tide, the storm surge was extremely catastrophic. This triggered a 6 metre storm surge that carried inwards over an extensive area and causing many deaths. The storm surge was able to travel very far inland. The storm initiated about 6,000 km away and the amount of time to reach the coast was around 20 days. The growth of the storm was favoured by the temperature of the warm sea and also the weather conditions which enabled the storm grow to a diameter larger than the entire country itself.
Storm Mala was another incredibly strong cyclone and was titled the strongest cyclone in the Bay of Bengal in 2007 before Cyclone Sidr. Mala hit the coast of Burma with winds of 185 km/h causing $6.7 million in damages and at least 22 deaths. The southern Bay of Bengal consisted of an area of convection that initially had weak currents in mid- to late-April. The Cyclonic Storm of Mala began to intensify as favourable conditions such as low pressure continued.
The strongest cyclone in the Bay of Bengal was Cyclone Sidr with wind speeds of 260 km/h. The storm triggered large-scale evacuations and an estimated number of deaths to be up to 10,000 people. Throughout the country, heavy precipitation were experienced initially. The cyclone struck offshore islands in the Bay of Bengal and slowly downgraded to a Category 2 storm while it moved north into the central Bangladesh.
The worst natural disaster to be recorded in Burma would be Cyclone Nargis. It powerful tropical cyclone triggered a landfall killing at least 138, 000 people and causing major destruction on May 2, 2008. Cyclone Nargis initially developed in central Bay of Bengal and then encountered favourable conditions which caused the storm to intensify to wind speeds to at least 165 km/h. The intensity of the storm finally decreased near the border of Burma and Thailand.
An increase in water level on an offshore that is affiliated with a low pressure weather system is known as a storm surge (Fig.6). A storm surge typically involves strong winds that force great pressure on the ocean’s surface. This causes a rise in sea level from the accumulation of water being piled up by the high winds. Another factor that causes a rise in sea level is the low pressure at the center of the weather system. In addition to Bangladesh’s geography and region, the effects of both strong winds over the shallow water in Bangladesh and low pressure causes extremely harmful storm surge flooding problems in Bangladesh.
Global Warming and the Future of Bangladesh
Since the global average air and ocean temperatures are gradually increasing, this affects the future of Bangladesh to be at a higher risk of more disastrous events. Global warming will trigger a chain of unfortunate events. Initially, global warming will cause the sea level to rise due to thermal expansion and the contribution of land-based ice. In addition to Bangladesh being highly populated with a poor economy and a low-lying land, the combination of these factors will place Bangladesh at risk to a very disastrous situation in the future.
Although there is still uncertainty to whether the temperature of the sea water affects cyclonic storms, the present study does show a good relationship between both ocean and atmospheric variables and severe cyclonic storms. The study shows that an increase in the sea surface temperature may be responsible for extreme weather. Scientific research suggests that warmer water dissipates more readily into vapour, making it easier for small ocean storms to become larger and more powerful. The effects of higher sea temperatures will also form more frequent tropical storms and cyclones. Due to Bangladesh’s low elevation, each year around 18% of the country is flooded (about 26, 000 km2) and during severe floods, the percentage may exceed 75% of the country’s area which is about 95% of the total annual flow.
As of March 2011, there are 6.5 million people in Bangladesh that are climate refugees and the Bangladesh Government is emphasizing on relocating these refugees in foreign countries. Association for Climate Refugees has already begun to relocate them within the country borders and also other civil society groups are also in the progress of creating a framework to protect and rehabilitate climate refugees.