Climate Change in Sundarbans After Cyclone Sidr and Aila: an Interpretation of Meteorological Data

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Proceedings of the 5th International Conference on Natural Sciences and Technology (ICNST’18) 31 March 30 - 31, 2018, Asian University for Women, Chittagong, Bangladesh

Climate change in Sundarbans after Cyclone Sidr and Aila: An interpretation of Meteorological Data

Shuvo Saurav Dey

University of Dhaka, Department of Meteorology. Shabagh, Dhaka – 1000, Bangladesh. E-mail: [email protected]

Abstract The Sundarbans, situated at the south-western part of Bangladesh, is the world’s single largest block of mangrove forest; which is significant in maintaining the ecological stability of both Bangladesh and the World. This study shows the changes in temperature, rainfall and wind speed around Sundarbans regions through a period of 14 years (2000 to 2013); especially on the aftermath of two tropical cyclones – Sidr (November, 2007) and Aila (May, 2009). Devastation of forest was more severe during cyclone Sidr than Aila – NDVI and LULCC analysis of LANDSAT ETM+ images for 2007, 2008, 2009 and 2010 proves this fact. The change in vegetation coverage and the corresponding meteorological data illustrates a change in climatic pattern in this region. Average temperature of the Long-term Average Maximum Temperature has an increase 0.57˚C for Summer (March – May) and 0.28˚C for Monsoon (June – October), while that for Long-term Average Minimum Temperature has an increase of 0.16˚C for Summer and 0.18˚C for Monsoon. The decrease in Long- term Average Rainfall is highly conspicuous for Monsoon season (1411.6 mm). The change in Long-term Average Windspeed is more in Summer than Monsoon and Winter (November – February); being 1.66 ms-1, 1.14 ms-1 and 0.975 ms-1 respectively.

Keywords: Sundarbans, cyclone, temperature, rainfall, windspeed.

1. Introduction Sundarbans, situated at the south-western part of Bangladesh, is the largest forest cover of the country. It is also world’s single largest block of mangrove forest. This forest is vital for maintaining the ecological balance of both the country and also the world. [1] However, due to the country’s geographical location, Sundarbans is regularly being hit by several cyclones formed around the Bay of Bengal and also the Indian Ocean. After the turn of this century, two major cyclones, namely – Sidr (in 2007) and Aila (in 2009) caused catastrophe around this region. [5] Significant loss of vegetation cover induced a change in the typical climate pattern there. [6] This study focuses on the change of characteristic climate pattern around the Sundarbans region. The climate change scenario is being studied for a period of 33 years (1980 to 2013). However, the intensity of this change has a sharp increase in the last 14 years (2000 to 2013) since 2000. Therefore, the major portion of the discussion is mostly confined to this time period of 14 years. It is to be mentioned that meteorological data (along with satellite image analysis) provided the base of the analysis.

2. Methodology The total analysis is carried out in three segments: ! Satellite image analysis: LANDSAT 5 and LANDSAT 7 images of Sundarbans for the years of 2000, 2007, 2008, 2009 and 2013 are being used as primary source of data. Analyses of NDVI (Normalized Difference Vegetation Index) and LULCC (Land-Uses and Land-Cover Changes) are used for further explanation. Both of them are highly reliable for such kinds of analysis, especially on vegetation cover. [2] ! Representation of data through GIS: GIS (Geographic Information System) is being used for representing the spatial data. The results of satellite image analyses are mostly presented by using the GIS. Also, the areal coverage of the forest are being calculated through GIS. ! Statistical analysis of meteorological data: The data of temperature, rainfall and wind speed are used for this particular study. These data are collected for three climate stations of BARC; namely – Khulna, Mongla and Satkhira. Of the three recognized meteorological seasons of Bangladesh [4], summer

Proceedings of the 5th International Conference on Natural Sciences and Technology (ICNST’18) 32 March 30 - 31, 2018, Asian University for Women, Chittagong, Bangladesh

(March to May) and monsoon (June to October) are being considered more in the analysis than winter (November to February). The huge amounts of meteorological data are interpreted through necessary statistical analysis.

3. Results and Discussion 3.1 Change in vegetation cover The LULCC and NDVI analyses of Sundarbans area reveal that, around 4226.35 sq. km of vegetation cover were there in 2000. But on the aftermath of cyclone Sidr, the areal coverage was about 4031.58 sq. km. Then in February 2009, it further reduced to 3099.9 sq. km areas. In December 2009, after cyclone Aila, it became only 2614.86 sq. km areas. There is an increase of vegetation covers in that area lately, as in December 2013, 3698.53 sq. km areas of vegetation cover were found around Sundarbans.

3.2 Long-term average maximum temperature The long-term average maximum temperature is a valuable indicator for understanding the climatic condition of any area, particularly for cities. Therefore, the changes for Khulna should be considered more seriously than the other two areas. [3] Khulna had a long-term maximum temperature of 34.1° Celsius during 2000 to 2007. However, it rose to 34.73° Celsius during the 2010 to 2013. If considered from 1980 to 2013, the increase is 0.33° Celsius. Mongla, on the other hand, have an increase of 0.57° Celsius during 2000 to 2013. The increase is just 0.3° Celsius for Satkhira, probably due to the fact that - it is located farthest of the three stations. But, an extensive analysis will bring the real factor behind it. Overall, the average increase in 0.5° Celsius for the three stations (for 2000 to 2013). This analysis is for the summer period. Now, the change during the monsoon period is also easily discernable. Overall, the increase of temperature is 0.34° Celsius for all the three stations during 2000 to 2013. Individually, the increase is 0.42° Celsius for Khulna, 0.28° Celsius for Mongla and 0.32° Celsius for Satkhira.

Fig. 1. The change of vegetation covers in Sundarbans.

3.3 Long-term average minimum temperature Long-term average minimum temperature is another important indicator for understanding the overall climate situation of any area. The difference between the maximum and minimum temperature must also be taken seriously, for it is often responsible for atmospheric phenomena (e.g. cyclone, storms etc.). The increase

Proceedings of the 5th International Conference on Natural Sciences and Technology (ICNST’18) 33 March 30 - 31, 2018, Asian University for Women, Chittagong, Bangladesh of summer-time temperature is 0.5° Celsius for Khulna, 0.16° Celsius for Mongla and 0.43° Celsius for Satkhira from 2000 to 2013. It is to be noted, that Mongla is located nearer to Bay of Bengal than the other two stations. So the change of temperature is less in Mongla than Khulna and Satkhira. During the monsoon, the increase is 0.32° Celsius for Khulna, 0.18° Celsius for Mongla and 0.34° Celsius for Satkhira. The overall change is an increase of 0.37° Celsius during the summer and 0.28° Celsius during the monsoon.

3.4 Long-term average rainfall Rainfall is a very important atmospheric phenomena. It is a very good indicator of the overall environmental condition of any country or any particular locality. During the period of 2000 to 2013, a decrease of 1411.6 mm rainfall is recorded for Mongla in the monsoon season. The decrease is 1400.8 mm for Khulna. The overall decrease is 1416.74 mm for all the three stations during the monsoon season.

3.5 Long-term average windspeed An increase in windspeed has been found across the three stations. May be this is somewhat linked to the devastation of forest areas in Sundarbans. More in-depth analysis will be beneficial for this matter. However, average windspeed has increased to 1.23 ms-1 and 0.86 ms-1 for Khulna during the summer and monsoon seasons respectively. For Mongla, the increase is 1.66 ms-1 and 1.14 ms-1 respectively.

4. Conclusion Although it is a very small-scale study, it is easily seen that the characteristic climatic pattern around the Sundarbans is changing gradually. The change has propelled after the two tropical cyclones, which caused many severe catastrophe around that area. More rigorous analysis is mandatory in order to stop this deterioration and protect the Sundarbans.

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