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Long-term sea level changes Sea level rose above or fell below the present sea level many times due to • The climatic conditions, • Plate tectonic processes or volcanic activity,. • Melting of glacial ice caps in the polar regions. • warming of climate T ime (million years ) before pres ent 0 5 10 20 15 +150 About 90 million years ago, the sea level was about 300-400 m higher than today. Sea level rose by about 120 m during the last 18000 years, mostly because of the melting of glacial ice caps in the Polar Region S ea Level (cm) +100 +50 C urrent S ea Level 0 -50 -100 The global average rate of rise of sea level is 1-2 mm/year. This is partly caused by continued melting of glaciers and ice caps as a result of warming of the globe, and partly by the expansion of near-surface water caused by the rise in average temperature. -150 Sea level fluctuations over last 20 million years. Storms in the Bay of Bengal 0 20 40 60 80 100 120 140 160 180 200 220 240 0 20 40 60 80 100 120 140 160 30°N 20°N 10°N 0° 10°S 30°E 50°E 70°E 90°E 110°E 30°E 50°E 70°E 90°E 110°E Cumulative rainfall (colour bar, cm) and storm tracks during June-September and October-January. Data for 19901997 have been used. The light blue (cyan) tracks are for depressions, the red tracks for cyclones (storms), and the black tracks for severe cyclones. Note the large number of depressions that form in the northern Bay of Bengal during June-September, when cyclones are fewer. These depressions move northwestward into the Gangetic plains and bring rain to much of northern India. During October-January, there are fewer depressions, but the number of cyclones is larger. • The Bay of Bengal is one of the major centres of the world for breeding of tropical storms (known as cyclones over the Indian Ocean, and southwestern Pacific, as typhoons over the northwestern Pacific, and as hurricanes over the Atlantic). • Cyclones over the Bay of Bengal usually move westward, northwestward, or northward and cross the east coast of India or Bangladesh. It brings strong winds and high rainfall to the coastal region, causing damage to property and loss of life. • Storm surge is primarily piling up of water due to effect of winds of the storm. It raises the mean water level in the coastal zone. The magnitude of rise being dependent on strength of the winds. For major cyclones rise may be 1-2 meters. • Rather flat topography of this country makes it particularly vulnerable to storm surges.