PREDICTING CHANGE OF FUTURE CLIMATIC EXTREMES OVER BANGLADESH IN HIGH RESOLUTION CLIMATE CHANGE SCENARIOS

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1 PREDICTING CHANGE OF FUTURE CLIMATIC EXTREMES OVER BANGLADESH IN HIGH RESOLUTION CLIMATE CHANGE SCENARIOS Mohammad Alfi Hasan 1 *, A. K. M. Saiful Islam 2 and Bhaski Bhaskaran 3 1 Institute of Water and Flood Management, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh, mdalfihasan19@gmail.com 2 Institute of Water and Flood Management, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh, akmsaifulislam@iwfm.buet.ac.bd 3 Met Office, Hadley Center, United Kingdom, b.bhaskaran@metoffice.gov.uk ABSTRACT As one of the most vulurable countries to climate change, it is important to know the possible changes of extreme climate in future. In this context, a study has been carried out based on regional climate modeling results. A total of 17 simulations of Perturbed Physics Ensemble of the Hadley Centre Coupled Model (HadCM3) for the Quantifying Uncertainty in Model Predictions (QUMP) project have conducted using regional climate model, PRECIS. The SRES A1B emission scenario has used to run all these ensemble simulations for continuous period from 1961 to Extreme indices have been derived from the results of these simulations by a software called RClimdex (version 1.1). A total of 14 core indices recommended by the CCl/CLIVAR Expert Team for Climate Change Detection Monitoring and Indices (ETCCDMI) have been computed using the software. It has found that mean future climate extremes over Bangladesh will have some significant changes at the end of the century. Bangladesh will experience less number of continuous rainy days but will have a higher intensity of rainfall during 2080 s ( ). Total rainfall in each rainy day of a year (PRECTOT) will be more from present time during 2080 s but duration of consecutive rainfall will be lesser at same time. Both the 95th percentile rainfall (R95P) and the 99th percentile rainfall (R99P) show more intense rainfall for shorter time period for the future years. On the other hand, maximum daily rainfall (RX1) will increase in future and exhibits steeper trend than the maximum 5-day rainfall (RX5) over the country. In a single rainy day, the changes of temperature will be higher than present conditions which will affect ecological and fishery productions. The increase of night temperature will more prominent than day time temperature. Probabilities of heat wave will also increase in future. As climatic extreme indices are good interpreter of climate change information, this high-resolution climate change information would advocate future policies of the nation. Keywords: Bangladesh, climate change, extreme indices, PRECIS, rainfall, rclimdex 1. INTRODUCTION Climate change is one of the vital environmental issues for humanity at the twenty-first century. It is expected that surface temperature will continue to increase globally and major changes are likely to be occurred in the global hydrological and energy cycles (McCarthy et al., 2001).The greatest threat to humans (and other components of terrestrial ecosystems) will be manifested locally via changes in regional extreme weather and climate events. Future projections from regional climate models and recent observations show that Bangladesh will experience in change 583

2 of temperature and rainfall in the upcoming years.(islam 2008, 2012) Temperature rise associate with increasing frequency of cyclonic storm surge have made Bangladesh particularly a vulnerable country to climate change (Karim and Mimura, 2006). As country s economy is largely depends on agriculture, climate change will have severe impacts on crop production during future years. While changes in average climate are important, changes in extreme climatic events are also important. These are likely to have greater impacts than changes in average climate over nature and society (Trenberth et al., 2007). Since the second Inter governmental Panel on Climate Change report highlighted the trends and variability in daily climate and climate extremes (Nicholls et al. 1996), many studies on extreme climatic events have been conducted at different parts of the globe(frichetal, 2002; Arblaster and Alexander, 2005; Alexander et al., 2006). Some study of temperature and rainfall extremes have also conducted over Bangladesh at regional scale recently (Islam and Hasan, 2012). However, none of these studies have analyzed with high resolution 25km scenarios over Bangladesh to capture the changes of temperature and rainfall extremes. PRECIS is a regional climate model developed by Met Office, UK that has been used to generate high resolution (25km) climate change information using the boundary condition data from Global Circulation Model (GCM). Bangladesh University of engineering and technology and Met office, UK, conducted a collaborative research to generate high resolution climate change information for future years to help in planning and decision making up to district level. 17 ensembles of A1B scenario have been simulated using PRECIS over whole Indian Sub-continent. This paper tries to analyze the change of future rainfall and temperature extremes over Bangladesh using all 17 ensembles simulations. 2. METHODOLOGY PRECIS has been run at 25km horizontal grid resolution in a large domain covers E and 0-41 N using GCM model of Hadley Centre Coupled Model made by Met Office, UK. Previously, experiment has been conducted at Institute of Water and Flood Management (IWFM) for a smaller domain with 50km horizontal grid resolution over Bangladesh using A1B scenario from special report on emissions scenarios (SRES) of the Intergovernmental Panel on Climate Change (IPCC). To produce high-resolution fine scale information, a domain of 25km resolution is used under a collaborative research project between Institute of Water and Flood Management, BUET and Met Office, UK. Seventeen ensemble experiments using lateral boundary conditions (LBCs) of the 138year ( ) is conducted at Met office using regional climate model, PRECIS. These simulations are based on the 17-member perturbed physics ensemble (PPE) produced by the HadCM3 under the QUMP project of the Hadley Centre, Met Office, UK. Availability of data of continuous 138-year period provides an opportunity to analyze the future climate over Bangladesh for implications of future policy and development works. To analyze future extremes, software called RClimdex has been used in the study. The joint Expert Team (ET) on Climate Change Detection and Indices (ETCCDI) has recognized a suite of 27 core climate change indices which derived from daily rainfall and temperature data using userfriendly software package RClimdex ( From that list, eight extreme rainfall-related indices and six temperature related indices are used in this study as shown in Table 1. Analysis has been conducted for the periods of 2080 s (2071 to 2100) considering baseline period

3 Table 1: Rainfall and temperature indices calculated by RClimdex Index Descriptive Name Definition Units PRCPTOT wet day rainfall annual total rainfall from wet days mm SDII simple daily intensity average rainfall on the wet days mm/day index CDD consecutive dry days maximum number of consecutive dry days days CWD consecutive wet days maximum number of consecutive wet days days R99p extremely wet day annual total rainfall when daily rainfall rate > mm rainfall 99 th percentile of daily rainfall R95p very wet day rainfall annual total rainfall when daily rainfall rate > mm 95 th percentile of daily rainfall RX1day maximum 1-day annual maximum 1-day rainfall mm rainfall RX5day maximum 5-day annual maximum consecutive 5-day rainfall mm rainfall SU25 hot days Annual count when daily maximum temperature days > 25 ºC SU38 extreme hot days Annual count when daily maximum temperature days > 38 ºC TN10p cool night frequency Percentage of days when daily minimum % temperature < 10 th percentile of base period TX10p cool day frequency Percentage of days when daily maximum % temperature < 10 th percentile of base period TN90p hot night frequency Percentage of days when daily minimum % temperature > 90 th percentile of base period TX90p hot day frequency Percentage of days when daily maximum temperature > 90 th percentile of base period % 3. RESULTS 3.1. Rainfall Indices Bangladesh is one the most vulnerable countries of the world in context of climate change. Climate change has already intensified the extreme climate phenomenon. Extreme rainfall is responsible for disaster like flooding or land slide and creates serious water logging problems in the cities. Change of consecutive wet days and dry days that are showed in Figure 1, suggest that future continuous rainy day will decrease. But it has been found from 17 ensemble simulations that most of QUMP runs show increase rate of rainfall. This give an indication that future rainfall duration will be lesser but magnitude will be higher. Majority of the ensemble simulations suggest that around 4 days decrease of consecutive wet days (CWD) will happen at the end of the century. For estimation of changes of consecutive dry days in future, large uncertainty still prevails among various ensemble members. No prominent change of future has found from CWD index as there is both possibility of rise and fall of consecutive dry days during the 2080 s. Annual total and average rainfall changes at the end of the century from current climate are shown in Figure 2. It is found that majority of the ensemble simulations show that wet day rainfall will increase about ~200mm annually which might reach up to 400mm for some ensemble simulations. There is an exception in QUMP 3 run that shows no increase of total annual wet day rainfall (PRCPTOT). Average rainfall per rainy day (SDII) will increase about 0.7 to 1.0mm at 2080 s time slice which 585

4 will eventually increase the drainage problems in the city areas. It will also increase possibility of flash flood in the north-eastern part of the country. Figure 3 shows change of heavy wet days (95 percentile of annual rainfall considers major wet day) and extreme wet days (99 percentile of annual rainfall considers extreme wet day) that clarify the vulnerability of the country. Analysis shows around 200mm increase of rainfall per year on annual major wet days of the country in the majority of the ensembles runs during 2080 s time period. In some extent, it will reach up to 400mm increase per year. Around 50% ensemble simulations suggested that extreme rainfall will be more extreme up to 100mm/year. Figure 4 shows maximum one day rainfall and maximum five days rainfall change over Bangladesh. Both parameters show that at the end of the century, daily rainfall will be more intense than long duration rainfall, which will eventually create serious problem in urban areas Temperature Indices Some studies have suggested that temperature extremes will intensify cyclone frequency and will have an adverse effect on agricultural sector of the country. Figure 5 shows change of extreme hot days and relative hot days at the end of the century for the 17 ensembles members. Number of extreme hot days will increase at least 1 month per year which might cause a problem in crop productions at the end of the century. Most of the productive crops of the country require below 38ºC temperature to survive. So temperature increase will cause major reduction of crop yield. On the other hand, 60 days will increase on days with 25 ºC or above in a year during 2071 to 2100 time period. This will cause serious reduction of cold days, and reduce the production of cold loving crops. Figure 6 shows the change of the frequency of cold days and cold nights. Both indices show decrease per year at the end of the century. However, decrease of the cold days is more prominent than that of cold nights. Finally, the state of change of hot days and hot nights are shown in the Figure 7. During the 2080 s, frequency of hot nights will increase ranging from 1 to 6% per year. Change of the frequency of hot days will be lesser than night though it will increase from 1 to 5% per year. However, few ensemble simulations show reduction of the both indices.. Figure 1: Change of Consecutive wet days (CWD) in days (left) and consecutive dry days (CDD) (right) in days for 17 Mean Ensemble Runs for the Period from the Baseline Period

5 Figure 2: Change of wet day rainfall (prcptot) (left) and simple daily intensity index (sdii) (right) for 17 mean ensemble runs for the period from the baseline period Figure 3: Change of wet day rainfall (r95p) (Left) And extremely wet day rainfall (r99p) (Right) for 17 mean ensemble runs for the period from the baseline period

6 Figure 4: Change of maximum 1-day rainfall (RX1day) (left) and maximum 1-day rainfall (RX5day) (right) for 17 mean ensemble runs for the period of from the baseline period Figure 5: Shows change of extremely hot days (SU38) (left) and hot days (SU25) (right) for 17 mean ensemble runs at the time period of 2071 to 2100 from the baseline period of 1971 to

7 Figure 6: Change of cool day frequency (TX10p) (left) and cool night frequency (TN10p) (right) for 17 mean ensemble runs for the period of 2071 to 2100 from the baseline period of 1971 To 2000 Figure 7: Hot night frequency (TN90p) (left) and hot day frequency (TX90p) (right) from the mean of 17 ensemble runs for the period from the baseline period CONCLUSIONS Seventeen ensemble simulations have been used in this study to capture all possible future uncertainties. Extreme temperature analysis has made with all the ensembles to quantify changes of future extreme events at the end of the century. Duration of storms will be reduced in future but magnitude will be higher which will have profound impacts on the development of the country especially in agriculture and economic sectors. On the other hand, extreme temperature rise will 589

8 create adverse environment for the ecosystem and crop (e.g. rice, wheat or potato etc.) productions. Possibility of heat wave and health hazard might be enhanced due to climate change. Therefore, to address these consequences, the impact of climate change needs to be considered while formulating policies for every development sectors. ACKNOWLEDGEMENTS The PRECIS simulations made at Met Office, UK were facilitated under a collaborative research project entitled High-resolution Regional Climate Change Information for Bangladesh to inform Impacts Assessments, Vulnerability indicators and Adaptation Policies of BUET with Hadley Centre, Met office, UK funded by DFID. REFERENCES Alexander, L.V., Zhang, X., Peterson, T., and Cetal. (2006) Global observed changes in daily climate extremes of temperature and rainfall. Journal of Geo-Physics, Vol. 5. Arblaster, J., and Alexander, L.V. (2005). Extreme change: an analysis of past, present and future changes in global temperature and rainfall indices. Bulletin of Australia Metrological Ocean Society, Vol. 18, Frich, P., Alexander, L.V., Della-Marta, P., Gleason, B., Haylock, M., Klein, T., and Peterson, T. (2002). Observed coherent changes in climatic extremes during the second half of the twentieth century. ClimRes, Vol.19, Islam, M.N., Rafiuddin, M., Ahmed A.U., and Kolli, R.K. (2008). Calibration of PRECIS in employing future scenarios in Bangladesh, International Journal of Climatology. RMetS, Vol. 28, Islam, A.K.M., and Hasan, M.A. (2012), Climate induced changes of rainfall extremes over Bangladesh Proceeding of 3 rd International Conference on Environmental Aspect of Bangladesh. Japan, ID 110, Jones, R.G., Noguer, M., Hassell, D.C., Hudson, D., Wilson, S.S., Jekins, G.J., and Mitchell, J.F.B. (2004). Generating High Resolution Climate Change Scenarios Using PRECIS. Met Office Hadley Center, Exeter, UK. Karim Z, Hussain Sk.G., Ahmed, A.U. (1998). Climate change vulnerability of crop agriculture. In Vulnerability and Adaptation to Climate Change for Bangladesh. Kluwer Academic Publishers, Dordrecht, Karim, M.F. and Mimura, N. (2006). Impact of climate change and sea level rise on cyclonic storm surge floods in Bangladesh. Global Environmental Change, Vol.18, Kumar, K. K., Patwaedhan, S. K., Kulkarni, A., Kamala, K., Koteswara, K. R. and Jones, R.(2011). Simulated projections for summer monsoon climate over India by a highresolution regional climate model (PRECIS), Current Science, Vol. 101, No. 3., McCarthy, J.J., Canziani, O.F., Leary, N.A., Dokken, D.J., White, K.S., (2001). Climate Change 2001: Impacts, Adaptation and Vulnerability. Inter-Governmental Panel on Climate Change(IPCC), Work Group II Input to thethird Assessment Report Cambridge University Press: Cambridge. 590