4.1 Decadal maximum temperature change trend
Figure 2 shows the maximum temperature trend analysis. The study demonstrates that in 1981’s decade Bangladesh's western region, with a maximum average temperature of 31.25°C to 31.54°C, had the greatest temperature. The lowest temperature, between 29.21°C and 29.50°C, was recorded in the eastern mountainous regions. The variation in mean maximum temperature over the entire research area is 2.33°C. The lowest value between 1981 and 1990 was also recorded in the coastal south-east regions.
The results for the next ten years (1991–2000) indicate no discernible change. Eastern mountain regions have the lowest value while western region has the highest. Coastal regions were getting warmer than the previous decade. But there is a 2.36°C difference in the maximum mean temperature.
A noticeable change is noted from 2001 to 2010. The increase in the mean maximum temperature was 0.63°C. The northern region had the lowest value. The eastern hilly regions of Bangladesh can clearly notice an increase in the maximum temperature. There is a 2.53°C difference across all weather stations. The maximum mean temperature in the coastal region rises by 0.321°C.
Analyzing data from the last 12 years (2011–2022). It demonstrates that the difference of maximum mean temperature was 2.011°C throughout all station. A significant portion of the study area is warmer than 31°C. Bangladesh's western parts are becoming warmer. Although the highest temperature reduced 0.119°C from previous decade. The lowest maximum temperature is only present in the northern portion.
4.2 Decadal minimum temperature change trend
Figure 3 illustrates the minimum temperature trend analysis maps. The minimum average temperature follows a distinct trend than the maximum average temperature. Studying the analyzed data from 1981 to 1990, the highest minimum mean temperature ever recorded was between 21.92°C to 22.27°C in coastal areas and Dhaka. Lowest minimum mean temperature was recorded in the northern and eastern hilly regions (Sylhet, Srimongal), which ranged from 19.43°C to 19.79°C. Across all stations, the decadal mean minimum temperature difference is 2.84°C.
The mean lowest temperature increased 0.2°C over the following ten years, from 1991 to 2000. The area near the shore was relatively warmer, and Bangladesh's northern region experienced low mean minimum temperatures. However, there was a 3.04°C temperature difference across the entire station.
The minimum mean temperature situation was comparable to the previous one in the decade from 2001 to 2010. However, the highest mean value 0.336°C was raised. Although the minimum average temperature was 19.43°C in 1981, it increased to 20.08°C during this decade. The temperature variation is comparable to the past ten years.
The mean lowest temperature climbed 0.07°C from the previous decade and 0.41°C from 1981 to 1990 in the ten-year period from 2011 to 2022. The minimum temperature trend is consistent with the preceding ten years. The cities along the coast and Dhaka, the capital, have the highest average low temperatures. Srimongal, Saidpur, and Dinajpur have the lowest value. From the study, the mean minimum temperature is rising as each decade passes.
4.3 The change of temperature in 42 years period
The highest mean maximum temperature climbed 4.62%. Around Kutubdia, Sandwip station the temperature change is distinct during the course of the past 42 years, which is the evidence of the study area's changing climate (Fig. 4). Almost every research area's maximum mean temperature is rising at a different rate except Chuadanga, Satkhira and Madaripur has decreasing rate. This alteration becomes noticeable after 42 years. The rate of growth in hilly (Chittagong, Sitakundo), coastal (Cox’s Bazar, Patuakhali) and northern part (Rangpur, Saidpur) areas is substantially higher than in neighboring areas (Fig. 5). At the northern region (Rangpur, Saidpur), the value is increased by 3.75%.
The study shows a difference between the minimum temperature average during the previous 42 years and last year (Fig. 6). The mean minimum temperature of Dhaka, Dinajpur, and Chittagong is rising and ranges from 3.46–5.28%. The city with the highest minimum temperature increase is Dhaka. Areas close to the coast are trending downward (Patuakhali, Mongla, Hatiya, Kutubdiya) (Fig. 7). In summary, city areas fluctuating rate is higher than other locations. Around the northern region (Rangpur, Saidpur), the minimum temperature also inclined 4.5%.