Rahman 2015
Rahman 2015
DOI 10.1007/s00704-015-1688-3
ORIGINAL PAPER
Abstract In this paper, temperature and rainfall data series            temperature exhibited an increase of 0.018 °C per year in
were analysed from 34 meteorological stations distributed               2011–2020, and if this trend continues, this would lead to
throughout Bangladesh over a 40-year period (1971 to 2010)              approximately 1.0 °C warmer temperatures in Bangladesh
in order to evaluate the magnitude of these changes statistical-        by 2020, compared to that of 1971. A greater rise is projected
ly and spatially. Linear regression, coefficient of variation,          for the mean minimum (0.20 °C) than the mean maximum
inverse distance weighted interpolation techniques and geo-             (0.16 °C) temperature. Annual rainfall is projected to decline
graphical information systems were performed to analyse the             153 mm from 2011 to 2020, and a drying condition will per-
trends, variability and spatial patterns of temperature and rain-       sist in the northwestern, western and southwestern parts of the
fall. Autoregressive integrated moving average time series              country during the pre- and post-monsoonal seasons.
model was used to simulate the temperature and rainfall data.
The results confirm a particularly strong and recent climate            Keywords Climate change . Temperature . Rainfall . Trends .
change in Bangladesh with a 0.20 °C per decade upward trend             Variability . GIS . ARIMA . Spatial analysis
of mean temperature. The highest upward trend in minimum
temperature (range of 0.80–2.4 °C) was observed in the north-
ern, northwestern, northeastern, central and central southern           1 Introduction
parts while greatest warming in the maximum temperature
(range of 1.20–2.48 °C) was found in the southern, southeast-           Climate change is the greatest threat that our Earth faces today.
ern and northeastern parts during 1971–2010. An upward                  A warming planet impacts various aspects, which includes the
trend of annual rainfall (+7.13 mm per year) and downward               weather system, hydrology, ecology and environment.Climate
pre-monsoon (−0.75 mm per year) and post-monsoon rainfall               change refers to a change in the state of the climate that can be
(−0.55 mm per year) trends were observed during this period.            identified (e.g., using statistical tests) by changes in the mean
Rainfall was erratic in pre-monsoon season and even more so             or the variability of its properties, which persists for an ex-
during the post-monsoon season (variability of 44.84 and                tended period, typically decades or longer (IPCC 2007). The
85.25 % per year, respectively). The mean forecasted                    classical period for averaging these variables is 30 years, as
                                                                        defined by the World Meteorological Organization (WMO). It
                                                                        was observed that, in general, developing countries are more
* Md. Rejaur Rahman                                                     vulnerable to the effects of climate change than developed
  rejaur2001@yahoo.com                                                  countries, mainly because of the low capacity to adapt to cli-
* Habibah Lateh                                                         mate change in the developing world. According to the fifth
  habibah@usm.my                                                        Intergovernmental Panel on Climate Change (IPCC) report,
                                                                        significant trends in temperature and precipitation were ob-
1
     School of Distance Education, Universiti Sains Malaysia (USM),     served around the world but with different magnitudes
     Gelugor, Malaysia                                                  (IPCC 2014). Globally, surface temperatures are rising, but
2
     Department of Geography and Environmental Studies, University of   warming is not uniform all over the globe (Jones 2001;
     Rajshahi, Rajshahi, Bangladesh                                     Jones and Moberg 2003; Solomon et al. 2007; Kerr 2009;
                                                                                                               Rahman M.R., Lateh H.
Nick et al. 2009; Lorentzen 2014). In the fifth assessment of       impacts will therefore ultimately depend on the availability of
IPCC, it was stated that the average temperature of the Earth       accurate and cost-effective forecasts.
has risen approximately 0.85 °C from 1880 to 2012 and tem-              Several studies have been carried out over different parts of
perature of the world would increase between 0.3 and 4.8 °C         the world with regard to the changing temperature and rainfall
by the end of the twenty-first century (Hartmann et al. 2013).      patterns (Bloomfield 1992; McCarthy et al. 2001; IPCC 2001;
Furthermore, coupled with global warming, there is strong           Solomon et al. 2007; IPCC 2007; Reiter et al. 2012; Mair et al.
evidence that changes in rainfall patterns have already taken       2013; IPCC 2014). In recent years, in some parts of the globe,
place on both global (Hulme et al. 1998) and regional scales        investigations of climatic parameters were also assessed with
(Yu and Neil 1993; Rodrı′guez-Puebla et al. 1998; Trenberth         weather station data to detect temperature and precipitation
et al. 2007). Variations to regional and local climate depend on    trends (Domonkos and Tar 2003; Matulla et al. 2003; Franke
regional and local features; therefore, climate change at re-       et al. 2004; Founda et al. 2004; Begert et al. 2005; Feidas et al.
gional and local scales often does not match those on a global      2007; Good et al. 2008; Zhang et al. 2008; Brázdil et al. 2009;
scale. In this regard, the assessment of climate variability and    Schaefer and Domroes 2009; Tabari and Hosseinzadeh 2011;
change on a smaller scale (country level) is a key issue and this   Pérez and Jury 2013). Moreover, recently, the extent of future
will help to improve our understanding of long-term climate         temperature and rainfall change has also been estimated by
variability and change and as well as the associated mecha-         using different simulation techniques (Hansen et al. 2006;
nisms of forcing of change at country level or local scales.        Rahmstorf et al. 2007; Diomede et al. 2008; Lin et al. 2009;
    Like global climate change, the climate in Bangladesh is        Marengo et al. 2009; Karmalkar et al. 2011; Afshin et al. 2011;
also changing since it is becoming more unpredictable in re-        IPCC 2007, Esfahani and Friedel 2014). In these studies, re-
cent years (Rahman 2013). Similar to many other countries,          searchers mainly focused on long-term forecasting by apply-
Bangladesh will face tremendous challenges from climate             ing intelligent methods, earth models of intermediate com-
change, particularly since it is mostly an agrarian country with    plexity (EMICs), energy balance models (EBMs), global/
a high population. Therefore, since its agriculture and water       general circulation models (GCMs), providing regional cli-
are the most vulnerable sectors to climate change which can         mates for impacts studies (PRESIS) regional model, support
lead to floods, droughts, tornados, cyclones, tidal surges and      vector regression, fuzzy inference system, hybrid approach,
soil salinities, climate change scenarios need to be incorporat-    the combination of artificial neural network, fuzzy logic and
ed into the development and planning activities of the country      wavelet functions. However, these methods/models have their
(Rahman 2013). The IPCC has reported and highlighted the            own difficulties, complexity as well as large data require-
need for more detailed information about climate change on          ments. Moreover, it is well known that the potential of any
regional and local scales, which is of particular interest to       global and regional climate model is limited in simulating
various nations and economic groups (IPCC 2014).                    local climatic (rainfall and temperature) characteristics (Dash
Therefore, country-level information about climate variability      et al. 2013). Furthermore, global and regional models are
and change is needed to adapt with the changes and formulate        complicated and not easy to use because of huge data require-
appropriate strategies to overcome these problems. On the           ment and current knowledge of the physical climate system
other hand, spatial distribution of meteorological data is be-      (IPCC 2001). Therefore, many efforts have been made in the
coming important as inputs to spatially explicit landscape,         statistical modelling of temperature and rainfall data using
regional, and global models (Li et al. 2006; Rahman et al.          historical records (Hansen et al. 2006; Rahmstorf et al.
2014). Spatial technologies, such as geographic information         2007). Time series models (Lee and Sohn 2007) have become
system (GIS) and numerical modelling techniques, have been          quite popular due to its trend-detecting capability. Among
developed as powerful tools for ecological and environmental        others, autoregressive integrated moving average (ARIMA)
assessments (Paudyal 1996; Boyle et al. 1998; Krivtsov 2004;        time series model is one of the most important to forecast data
Rahman and Saha 2007; Rahman and Saha 2008; Rahman                  statistically (Romilly 2005). In general, ARIMA processes
and Saha 2009; Rahman et al. 2009; Rahman et al. 2014;              have several advantages over other methods including its
Rahman et al. 2015). On the other hand, within the context          capability to simulate the path of forecasting, its rich infor-
of the general climate discussion, the evaluation of climate        mation on time-related changes and the consideration of
time series is growing in importance. Predicting weather con-       serial correlations between observations (Yurekli et al.
ditions using previous data is one of the real uses of simula-      2007). Previously, the ARIMA models were most notably
tion. Most of the weather forecasters use this information pub-     used in various scientific and engineering applications but
lished by weather bureaus and these kinds of simulations can        not too often for the prediction of temperature and rainfall
help predict weather conditions. Numerical weather prediction       trends over tropical areas. Recently, however, the ARIMA
for forecasting involves complicated numeric computer               model has been used by Baker-Austin et al. (2012) to pro-
models to predict weather accurately by taking many param-          ject sea surface temperature (SST) data in the Baltic Sea up
eters into account. Effective management of climate change          to year 2050.
Climate change in Bangladesh: a spatio-temporal analysis
    In reviewing the relevant studies in Bangladesh, there is          average), respectively. January is the coldest and April and
limited information about past climate change at the national          May are the hottest months in Bangladesh (Fig. 2a).
level (Met Office 2011). In the past, studies were carried out            The historical average rainfall of the country is 2428 mm
mainly on the trend of climate change over Bangladesh                  per year (BMD 2013). and the rainfall is very much seasonal
(Divya 1995; Mia 2003; Karmakar and Shrestha 2000;                     in Bangladesh (Fig. 2b), which varies from 1400 to 4400 mm.
Shahid 2009; Shahid 2010; Shahid 2011; Rahman 2013;                    The highest rainfall occurs in June, July, and August. More
Hasan et al. 2014; Rahman and Lateh 2015). However, there              than 75 % of the total rainfall in Bangladesh occurs during the
is no such detail on spatial temporal analysis of potential cli-       monsoon season, caused by winds blowing from the Southern
mate change, particularly on temperature and rainfall, from            Hemisphere from mid-May to September, which accumulates
recent data in Bangladesh. Moreover, few studies are available         moisture and deposits copious amounts of precipitation over
that predict rainfall and temperature changes over Bangladesh          the South Asian continent. In respect to the global warming
using climate or statistical models, and in addition, those are        and climate change, Bangladesh is one of the most vulnerable
mostly focused on long-term projections (May 2004; Dash                countries in the world due to its least capacity to address the
et al. 2006; Immerzeel 2007; Kripalani et al. 2007; Islam              devastating impacts (IPCC 2007). Recently, Bangladesh is
2009; Rahman et al. 2012a). However, impact communities                experiencing higher temperatures, more variability in rainfall,
are more interested on shorter time scales prediction, such as         more extreme weather events and sea level rise. Bangladesh is
one or two decades as well as at the regional and local scales in      highly vulnerable, because it is low-lying, located on the Bay
order to assess the impacts of climate change and to develop           of Bengal in the delta of the Ganges, Brahmaputra and
suitable adaptation and mitigation policies (Giorgi 2005).             Meghna and also densely populated. Since agriculture is the
Unfortunately, the optimum time and spatial scales for climate         mainstay of the economy of Bangladesh, its agriculture and
change are not well studied in Bangladesh with recent data.            water sectors are very sensitive to impacts of the climate
Thus, in this paper, the temperature and rainfall data from the        change. Thus, detail study about spatio-temporal analysis of
last four decades (1971 to 2010) in Bangladesh were analysed           temperature and rainfall over Bangladesh is a key issue and
in order to evaluate the magnitude of their changes statistically      short-term prediction of these two main climatic parameters is
and spatially. Specifically, trend, variability and spatial patterns   important for adaptation and planning to cope up the problems
of the mean, mean minimum and mean maximum temperatures                due to climate change at present and future as well.
and annual, pre-monsoon and post-monsoon rainfalls were
assessed and analysed. A short-term (one decade, 2010–
2020) prediction of temperature and rainfall using an ARIMA            3 Data and methods
time series model was evaluated and analysed spatially using
GIS. This was done since the first phase of research in climate        The monthly dataset of the minimum and maximum temper-
change is to use scientific criteria to verify that a ‘change’ has     atures and rainfall from 34 stations in Bangladesh during the
actually been taken place (Lorentzen 2014) and later modelling         period 1971–2010 that was used and analysed in this study
the variations of climate change in order to make dependable           was provided by the Bangladesh Meteorological Department
forecasts for sound environmental policies (Romilly 2005).             (BMD 2013). and the locations of the 34 weather stations are
                                                                       shown in Fig. 1. However, out of 34 weather stations, data
                                                                       from 29 stations was available from the last 40 years while
2 Study area                                                           from the remaining five stations, namely Tangail (1987–
                                                                       2010), Kutubdia (1985–2010), Chuadanga (1989–2010),
The spatial extent of Bangladesh is between 20° 34′ N to 26°           Mongla (1989–2010) and Sayedpur (1991–2010), data was
38′ N latitude and 88° 01′ E to 92° 41′ E longitude (Fig. 1)           only available from the past 20 to 26 years. Therefore, a
with an area of 144,000 km2. Bangladesh has a sub-tropical             shorter series of measurements (20 to 26 years) was used
humid climate characterized by wide seasonal variations in             during analysis and we considered these five stations in our
rainfall, moderately warm temperature and high humidity.               study because of no other stations were located in those par-
Three distinct seasons can be recognized in Bangladesh, (i)            ticular areas. After processing the data from each station, there
the dry winter/post-monsoon season from November to                    was some missing data, which amounted to less than 2 %.
February, (ii) the pre-monsoon hot summer season from                  When missing data was found, this was replaced by the aver-
March to May and (iii) the rainy monsoon season from June              age value of the same month but from the previous and sub-
to October (Rashid 1991). The historical average temperature           sequent years. The data was visually examined using histo-
of the country is 25.75 °C, with a range of 18.85 to 28.75 °C          grams for any potential outliers as well as compared with
(monthly average). The average minimum and maximum                     neighbouring weather stations. However, no significant anom-
temperatures are 21.18 and 30.33 °C, and varies from 12.5              alies were found. Additionally, subjective double mass curve
to 25.7 °C (monthly average) and 25.2 to 33.2 °C (monthly              (Kohler 1949) and the objective student’s T test (Panofsky and
                                                                                                                Rahman M.R., Lateh H.
Brier 1968; Shahid 2010) were performed to test homogeneity            distribution of temperature and rainfall and their trends
of temperature and rainfall time series data. There were no            and variability, inverse distance weighted (IDW) interpola-
statistically significant variations existing in the time series       tion technique and geographical information systems (GIS)
data. Besides, in 1971, most of the stations have missed large         were applied. Integrated land and water information system
number of observed data. Thus, data for the year 1971 was              (ILWIS), a GIS and image processing software (ILWIS
replaced by the data from 1970 in the data set.                        2005). was used for this purpose. ILWIS and IDW were also
     A linear regression analysis using the least square method was    used to generate the spatial pattern of the forecasted temper-
applied to detect any trends in temperature and rainfall time series   ature and rainfall. In this study, autoregressive integrated
data and the confidence level of 95 % was taken as the threshold.      moving average (ARIMA) time series model, which was
Details of the linear regression and least square methods can be       popularized by Box and Jenkins (1976), was used and the
found in Moore and McCabe (2003). To measure the tempera-              monthly dataset of 34 stations was used to simulate temper-
ture and rainfall variability, a coefficient of variation (Cv) was     ature and rainfall during 2011–2020. The details of the
used and calculated for the period of 1971 to 2010. The Cv             ARIMA model can be found in Box and Jenkins (1976),
represents the ratio of the standard deviation (σ) to the mean         Yurekli et al. (2007) and Brockwell and Davis (1991).
(x ), which is useful for comparing the degree of variation from      However, ARIMA models are, in theory, the most general-
one data series to another. Often, the coefficient of variation is     ized class of models for forecasting time series. In the
expressed as a percentage and can be represented by Eq. 1.             ARIMA (p, d, q) processes, p is an auto regressive (AR)
         σ                                                             retrospectively weighted series, d is a difference lag (I) to
C v ¼   100                                                   ð1Þ    achieve series stationary, and q is a retrospectively weighted
         x
                                                                       moving average (MA) random error series. The appropriate
  For trend and variability analysis, Excel software (Excel            values of autoregressive order p and moving average q are
version 2010) was used. For mapping the spatial                        chosen by examining the autocorrelation function (ACF)
and partial autocorrelation function (PACF) of the time se-          the temperature and rainfall for the decade of 2001–2010 and
ries. ARIMA model can be represented by Eq. 2.                       then the forecasted values were compared with the actual ob-
                                                                     served values for the decade of 2001–2010. After validation
ϕðBÞ∇d Y t ¼ θðBÞεt                                            ð2Þ   and acceptance, finally, the temperature and rainfall were fore-
                                                                     casted for the decade 2011–2020 using the validate model.
where
                                                                     The overall methodology of the study is shown in Fig. 3.
ϕðBÞ ¼ 1  ϕ1 B  ϕ2 B2  …  ϕp Bp                        ð2  1Þ
θðBÞ ¼ 1  θ1 B  θ2 B2  …  θq Bq                        ð2  2Þ
                                                                     4 Results and discussion
where Yt and εt represent time series and random error terms at
time t, respectively. B is the backward shift operator, ∇d de-
                                                                     4.1 Trends of temperature and rainfall
scribes differencing operation to data series to make the data
series stationary, d is the number of differencing. ϕp and θq are
                                                                     The annual mean, mean minimum and mean maximum tem-
the model parameters, and the ϕ(B) and θ(B) are of order p and
                                                                     peratures and annual, pre-monsoon (MAM months) and post-
q. The time series often contained seasonal effects, to which
                                                                     monsoon (NDJF months) rainfall were considered for analysis
seasonal differencing is often used to remove these effects.
                                                                     in this study. From the last 40 years (1971–2010), weather
Thus, ARIMA models have two general forms: non-
                                                                     data analysis shows that the annual mean, mean minimum
seasonal ARIMA (p, d, q) and multiplicative seasonal
                                                                     and mean maximum temperatures of Bangladesh were
ARIMA (p, d, q) (P, D, Q)S, where p, d and q are non-
                                                                     25.83, 21.21 and 30.44 °C, respectively. The obtained annual
seasonal parts of the model, and P, D and Q are seasonal parts
                                                                     mean, mean minimum and mean maximum temperatures
of the model (Box and Jenkins 1976). which can be represent-
                                                                     showed a positive trend of about 0.020, 0.018 and 0.022 °C
ed by Eq. 3.
                                                                     per year, respectively. These results differ with those of Jones
ϕðBÞϕðBs Þð1  BÞd ð1  Bs ÞD Y t ¼ C þ θðBÞθðBs Þεt           ð3Þ
where
(1995), Ahmad and Warrick (1996), Shahid (2010), and Met                 mean maximum temperature (0.03–0.06 °C per year) was no-
Office Hadley Centre (Met Office 2011). For example, Jones               ticed in the southern, southeastern and northeastern parts
(1995) found no significant change in the annual mean mini-              (Fig. 4c). It was also found that the mean temperature trend
mum and mean maximum temperatures of Bangladesh, while                   was remarkably positive (0.02–0.042 °C per year) in the
Ahmad and Warrick (1996) mentioned that the trend of the                 southern, southeastern, northeastern and extreme northwest-
mean temperature of Bangladesh was 0.5 °C over 100 years                 ern parts of the county (Fig. 4a). The maximum positive trend
(0.005 °C per year). Shahid (2010) found that the mean, mean             was noticed at Cox’s Bazar, Sylhet and Dinajpur stations for
minimum and mean maximum temperatures increased by                       the mean temperature. On the other hand, a maximum upward
0.0097, 0.0091 and 0.0102 °C per year, respectively, in the              trend was found at the Rangpur and Dinajpur stations for the
last 50 years (1958–2007). McSweeney et al. (2010) men-                  mean minimum and Cox’s Bazar and Sitakunda stations for
tioned that the surface air temperature in Bangladesh has                the mean maximum (Fig. 4). A downward trend of the mean
warmed at a rate of 0.002, 0.007 and 0.012 °C per year for               minimum temperature was observed at the Rangamati station,
the annual, June–August (JJA), and September–November                    situated in the southeastern part of Bangladesh (Fig. 4b). A
(SON) months, respectively. According to Met Office                      downward trend of the mean minimum temperature in this
Hadley Centre (Met Office 2011), that since 1960, the mean               area is unknown and further studies are needed to investigate
temperature trend was positive during both the summer and                the causes and their influences on the seasonal patterns. Thus,
winter seasons in Bangladesh at 0.019 and 0.024 °C per year,             it may be said that the northern, northwestern, southern, south-
respectively. The difference between the present and previous            eastern and, to some extent, central parts of the country are
findings might be due to the use of long-term (50 years) data            warming at an alarming rate. This may be linked to the impact
as well as the absence of recent data in their analysis which            of global warming and climate change.
was included in this study. The rate of change of temperature                On the other hand, data analysis during the 1971–2010
was more accelerated in the last 30 years (Table 1). The shorter         period shows that the average annual, pre-monsoon (MAM
analysis datasets exhibit faster warming and is clearly found in         months) and post-monsoon (NDJF months) rainfall were
Table 1 that maximum temperature was warmed dramatically                 2387, 446 and 77 mm, respectively. Analysis by least square
over the last 30-year period. Moreover, the changing rate of             fitting for the rainfall data reveals the following results: during
the maximum temperature was higher than that of the mini-                the investigated period, the annual rainfall rose by 7.13 mm
mum temperature (0.022 vs. 0.018 °C per year for 1971–                   per year (a total of 14 %), while pre-monsoon and post-
2010). Therefore, recent data analysis is likely a key issue to          monsoon rainfall declined by −0.75 mm (total of 17 %) and
find out the valid trends of the climatic parameters.                    −0.55 mm (total of 39 %) per year, respectively. Thus, the
   The spatial pattern of temperature trend is shown in Fig. 4,          general trend was positive for the annual and negative for
which was generated using calculated trend statistics for the            seasonal rainfall in the country. The greatest decrease in rain-
34 weather stations, IDW and GIS. As seen in Fig. 4, all of the          fall over the year occurred during the post-monsoonal season
weather stations presented a positive trend, except at the               (Table 1). Analysis further shows that the rate of rainfall
Rangamati and Sitakunda stations for the mean minimum                    change (annual and seasonal) decelerated in the last 30 years
and the Mymensingh station for the mean maximum temper-                  (Table 1). Previous studies showed that there was a positive
ature. The spatial pattern of mean minimum temperature indi-             trend in the annual (+5.5 mm per year) and pre-monsoonal (+
cates a 0.02 to 0.06 °C upward trend per year in the northern,           2.47 mm per year) rains and changes to rainfall during the
northwestern, northeastern, central and central southern parts           post-monsoon season were not significant (Shahid 2010). A
of the country (Fig. 4b), while an extremely high trend for the          study by OECD (2003) pointed out a positive trend of precip-
                                                                         itation annually as well as during the pre-monsoon and post-
                                                                         monsoon seasons, but there was no appreciable change during
Table 1 Trends of mean, mean minimum, mean maximum                       the winter in Bangladesh. Meanwhile, Ahmed and Alam
temperatures and annual, pre- and post-monsoon rainfalls of Bangladesh   (1999) mentioned that there was little change in winter rainfall
                                                                         and an upward trend in rainfall during the other seasons in
Temperature (°C/year)   1971–2010 (40 years)    1981–2010 (30 years)
                                                                         Bangladesh. According to McSweeney et al. (2010), the mean
  Mean                  0.020                   0.024                    rainfall over Bangladesh has decreased by 13.2 mm per de-
  Mean minimum          0.018                   0.021                    cade (6 %) between 1960 and 2003, but this was not statisti-
  Mean maximum          0.022                   0.028                    cally significant. However, a positive trend (+3.4 %) was ob-
Rainfall (mm/year)      1971–2010 (40 years)    1981–2010 (30 years)     served in the months MAM and a negative trend (−1.7 %) in
  Annual average        7.130                   −5.616                   the months June–August (JJA) between 1960 and 2003.
  Pre-monsoon           −0.750                  −4.769                   Conversely, the Met Office Hadley Centre pointed out that
  Post-monsoon          −0.550                  −1.296                   there has been a negligible positive trend in total precipitation
                                                                         over Bangladesh since 1960 (Met Office 2011). However,
Climate change in Bangladesh: a spatio-temporal analysis
significant changes in rainfall have been observed in recent       per year) in the northwestern, southern and southwestern parts
decades as well as the overall variability increasing and ex-      (Fig. 5c). On the other hand, most of the weather stations also
treme weather events have been observed in Bangladesh re-          showed negative trends for the pre-monsoon rainfall, except
cently. Previous studies mostly analysed data for the long-        for the stations located at the extreme northwestern, northeast-
term and started from 1958 and did not include very recent         ern and southeastern parts of the country (Fig. 5b). The max-
data, which may explain slight differences with the current        imum downward trend of pre-monsoon rain (−2.10 to
study. However, the present study corroborates with the            −10.88 mm per year) was observed in the central, central
trend in rainfall of South Asia obtained by Solomon et al.         western and central southern parts (Fig. 5b). This implies that
(2007) in which it was found that seasonal precipitation sig-      a dry weather or drought condition occurred during the pre-
nificantly decreased in southern Asia.                             and post-monsoon seasons over a wide area of Bangladesh,
   The spatial trend of rainfall shows a regional disparity as     particularly in the northwestern and southwestern areas.
well as variations of rainfall in Bangladesh (Fig. 5). When        Though the findings of the present study agree with those of
comparing the districts, an upward trend of annual rainfall        Shahid (2010), in which there was a positive trend in annual
was highest (9 to 43 mm per year) in the hill districts, located   rainfall, it does not agree with the spatial pattern of seasonal
in the southeastern part of Bangladesh. However, a negative        rainfall trends in Bangladesh. Shahid (2010) found a signifi-
trend of annual rainfall was found at the Rajshahi, Ishurdi,       cant upward trend of annual rainfall in the western part; how-
Faridpur, Madaripur and Patuakhali stations (Fig. 5a). The         ever, in the present study, a significant upward trend was
post-monsoon rainfall presented a negative trend at all of the     mostly observed in the southeastern and northern parts of
weather stations, and the rate was highest (−1.70 to −3.4 mm       Bangladesh. In the last decade, low rainfall was recorded in
the northwestern, western, central and central southern parts       Spatial pattern depicted that the annual rainfall variability was
and high rainfall was recorded in the southeastern and north-       the highest (between 22.25 and 32.80 % per year) in the north-
eastern parts (BMD 2013). The present study agrees with the         western, northern and eastern parts (Fig. 7a). On the other
findings of the pre-monsoon spatial trend of rainfall obtained      hand, the maximum pre-monsoon (between 46 to 60 % per
by Shahid (2010) in which there was an increase to the pre-         year) and post-monsoon (between 83 to 100 % per year) rain-
monsoon rainfall in the extreme northwestern and southeast-         fall variability was highest in the southern coastal and south-
ern parts of Bangladesh.                                            eastern hill districts of Bangladesh. The pre-monsoon rainfall
                                                                    variability was also remarkable in the northwestern and south-
                                                                    western parts (Fig. 7b). The maximum rainfall variability, par-
4.2 Variability of temperature and rainfall                         ticularly in pre- and post-monsoon seasons, may be attributed
                                                                    to the relative dryness observed in those parts of the country
The variability of the temperature shows that the mean, mean        during the period, since generally, low rainfall areas experi-
minimum and mean maximum variabilities were 0.081, 0.025            ence greater variability (Rahman and Lateh 2015). Of course,
and 0.017 °C per year, respectively. With the exception of the      the southern coastal and southeastern hill districts have early
mean temperature variability, the variability of the mean min-      post-monsoon depressions or cyclone rain effects, which have
imum and mean maximum was not remarkable in Bangladesh              been increasing in recent decades that could also explain the
during the investigated period of 1971–2010. However, the           high rainfall variability in the area. It may be noted here that in
spatial pattern of the mean minimum temperature variability         Bangladesh, tropical cyclones occur during the pre- and post-
shows that a higher variability (>0.027 to 0.051 °C per year)       monsoon seasons. A study on Bangladesh rainfall has shown
was observed mainly in the northwestern, northern and north-        that, in general, there was decrease in rainfall during El-Niño
eastern parts (Fig. 6b). The spatial variability in the mean        years in all the seasons (the pre-monsoon, the monsoon and
maximum temperature was higher (between 0.017 and                   the post-monsoon) (Ahmed et al. 1996), which may be linked
0.026 °C per year) at the northeastern, eastern and southeast-      to high rainfall variability. Further, in a study by Rajeevan
ern parts of the country (Fig. 6c) compared to other parts of the   et al. (2008), it was shown that the inter-decadal and seasonal
country. In the case of the spatial pattern of the mean temper-     variability of rainfall might be linked to the variations and
ature variability, maximum variability (0.11 to 0.25 °C per         anomalies of sea surface temperatures (SST) over the equato-
year) was found in the northeastern, southern and southeastern      rial Indian Ocean, particularly the East Indian Ocean, which is
parts (Fig. 6a). Indeed, some parts of the investigated area        associated with global warming.
showed very low variability of the mean minimum and mean
maximum temperatures (<0.02 °C per year) [Fig. 6b, c].
Further, variability analysis showed that rainfall was erratic      4.3 Forecasting of temperature and rainfall (2011–2020)
during the pre-monsoon season (44.84 % per year) and be-
came much more erratic during the post-monsoon season               It was mentioned earlier in this study that we forecasted (short-
(85.25 % per year) during the last 40 years in Bangladesh.          term) temperature and rainfall using ARIMA time series anal-
The annual rainfall variability was found to be 22.59 % per         ysis model. To do so, ARIMA Expert Modeller of SPSS sta-
year (range of 15.20–32.80 %) during the investigated period.       tistical analysis software was used to perform the automatic
best fitted simulation of temperature and rainfall data for each     indicting validation of forecasting by the model. For other
station. Therefore, in a continuous sequence from January to         stations, we also found similar statistical comparisons. It
December (30 years), observed temperature and rainfall data          may be noted here that in Fig. 8d, three horizontal clusters
were used separately for each station as a single time series        were noticed mainly because of seasonal variations in the data
variable (360). We validated the forecasted values for the de-       set. Thus, it is apparent that the ARIMA Expert Modeller,
cade 2001–2010 with observed values of the same decade first         which automatically fitted the best ARIMA model for simu-
and then, finally forecasted the temperature and rainfall for the    lation for the specific time series data set, can be applied with
decade of 2011–2020. When comparing the observed values              adequate accuracy to forecast temperature and rainfall in the
and the forecasted values for the 2001–2010 period and visu-         area.
ally inspecting the plot, it was quite evident that the forecasted       Here, temperature (mean, mean minimum and mean
values were very close to the observed values (Figs. 8 and 9).       maximum) and rainfall (annual, pre-monsoon and post-
In Fig. 8, the observed mean maximum and mean minimum                monsoon) of Bangladesh during the second decade (2011–
temperatures from the Rajshahi station were compared (as a           2020) of the twenty-first century was forecasted and yearly
sample/evident) with ARIMA model forecasted values                   statistics were calculated from the monthly data set and then
(Fig. 8a, c). Between these two data sets, of observed and           decadal statistics were calculated from the yearly statistics.
forecasted, the coefficient of determination (R2) was found          Afterwards, a spatial interpolation technique was followed
to be +0.956 and +0.970 (highly positive) for the mean max-          using IDW and GIS to generate the spatial pattern of the
imum and mean minimum temperatures, respectively, at the             forecasted temperature and rainfall. The spatial variation
Rajshahi station (Fig. 8b, d). Again, when comparing the ob-         of the mean, mean minimum and mean maximum tempera-
served and forecasted values of the annual average rainfall          tures in 2011–2020 are predicted to be 25.11–27.30, 20.04–
during the 2001–2010 period, this also showed a highly pos-          23.06 and 29.78–32.08 °C, respectively (Fig. 10). Within
itive (+0.904) coefficient of determination (R2) (Fig. 9b),          this decade (2011–2020), the temperature is predicted to
Fig. 9 Comparison between the observed and forecasted rainfall of Rajshahi station, January 2001–December 2010. a Time plot and b scatter plot of the
observed monthly average rainfall and ARIMA forecasted value (mm)
rise by about 0.18, 0.20 and 0.16 °C for the mean, mean                      will receive low rainfall, which is predicted to continually
minimum and mean maximum temperatures, respectively,                         decrease during the 2011–2020 period (Fig. 11b). On the
compared to previous decade of 2001–2010. The forecasted                     other hand, during the post-monsoon season, the north-
warming of 0.18 °C per decade (mean temperature) during                      western, western and northern districts of the country will
2011–2020 is consistent with the fourth and fifth IPCC’s                     receive very low rainfall and this will likely continue to
predictions of 0.2 and 0.17 °C per decade, respectively                      decrease (Fig. 11c). By 2020, the post-monsoon rainfall
(IPCC 2007; IPCC 2014). It is also comparable in magni-                      will decrease by about 13 %, compared to 1971 (Table 2).
tude to other forecasts (Lean and Rind 2009). Comparative                    However, the data predicts that during the post-monsoon
statistics of climate change predictions in Bangladesh are                   season, the southwestern and central parts of the country
given in Table 2. Our study indicates faster warming than                    will receive slightly more rainfall than other parts
others and more of a decrease to post-monsoon rainfall. The                  (Fig. 11c). This is mainly because of high variability of
results obtained in this study may be an indication of the                   post-monsoonal rains in these parts of the country.
global change to the seasonal rainfall response.                                 In summary, the trends of temperature and rainfall during
   Prediction of rainfall shows a decrease of about                          the 1971–2010 period enhanced the attention of the region
−153 mm (−15.3 mm per year) rainfall in the decade of                        by climate change, particularly with regard to the trend of
2011–2020, compared to the previous decade 2001–2010.                        increased temperature. The results indicate that climate
Moreover, during the period of 2011–2020, a remarkable                       change apparently shows much stronger warming in
low rainfall is projected for the Rajshahi, Bogra, Jessore                   Bangladesh than what was mentioned by the global view
and Kustia districts, compared to the others parts of the                    published in the IPCC report (IPCC 2007; IPCC 2014). In
country (Fig. 11a). However, by 2020, the annual rainfall                    Bangladesh, starting from 1971, the mean temperature in-
is projected to increase by about 5.5 % (Table 2), com-                      creased by about 0.20 °C per decade in the last 40 years.
pared to the rainfall in 1971 which is consistent with the                   This is in contrast with the global mean temperature, which
IPCC’s prediction of a 5–6 % increase of rainfall by 2030                    increased by 0.074 °C ± 0.02 °C per decade in the last
(IPCC 2007). During the pre-monsoon season, the north-                       100 years (1906–2005) and 0.13 °C ± 0.03 °C per decade
western, southwestern and western parts of the country                       in the last 50 years (from 1956 to 2005) (IPCC 2007). Based
on the Global Historical Climatology Network (GHCN) da-                  temperatures increased between 0.80 and 2.4 °C over the
tabase, global linear trends of 0.10 °C per decade in the                1971–2010 period. Conversely, this study exhibited a
twentieth century have accelerated since 1950 to 0.16 °C                 14 % (+7.13 mm per year) upward trend in the annual rain-
p er de c ad e ( I P C C 2 0 07 ) . M o r e o v e r, w a r m i n g i n   fall and 17 % (−0.75 mm per year) and 39 % (−0.55 mm per
Bangladesh was even faster than the 1979–2012 period with                year) downward trend in pre-monsoon and post-monsoon
global warming rates of 0.16 °C per decade (Morice et al.                rainfall, respectively, during 1971–2010. The rainfall was
2012). Furthermore, the fifth assessment of the IPCC stated              also erratic during the pre-monsoon season, which then in-
that the average temperature of the Earth has risen by                   creased during the post-monsoon season. Furthermore, dry
0.12 °C per decade during the period 1951–2012 (IPCC                     weather or drought conditions were noticed during the pre-
2014). Again, the investigated temperature trends in this                and post-monsoon seasons over a wide area of Bangladesh,
study appear to be higher than most other studies in                     particularly in the northwestern and southwestern parts of
Bangladesh so far (Section 4.1). This supports the observa-              the country during the post-monsoon season. In these areas,
tion that an increase of temperature has taken place more in             rainfall decreased by about 50–70 % during the 1971–2010
recent decades than previous ones in Bangladesh. In addi-                period. The areas under a faster warming trend coincided
tion, the highest positive trend in minimum temperature was              with the areas having this drying trend. The ARIMA time
observed in the northern, northwestern, central and central              series model predicted that the mean temperature exhibited
southern parts, while the highest positive trend was noticed             a 0.18 °C elevation during the 2011–2020 period compared
in the southern, southeastern and northeastern parts in the              with the previous decade, indicating an approximately
case of the maximum temperature. In these areas,                         0.98 °C warmer temperature in Bangladesh by 2020
compared to 1971. On the other hand, there was a prediction     temperature warmed more in the northern, northwestern,
for a 153-mm decline of annual rainfall over the 2011–2020      northeastern, central and central southern parts while the
period. Moreover, it is predicted that the pre- and post-       maximum temperature warmed more in the southern,
monsoonal rainfall would decrease at a small rate (3 and        southeastern and northeastern parts during the 1971–
5 mm, respectively) which might lead to a drying condition      2010 period. In some of these parts, the mean minimum
that would persist in the northwestern, western and south-      and mean maximum temperatures rose by more than
western parts of the country during 2011–2020. The general      2.0 °C (0.50 °C per decade), which will likely pose chal-
tendency derived by the IPCC from analysing 20 global           lenges to the population in those parts of the country. On
climate models indicates that winter precipitation in the re-   the other hand, the most significant results were related to
gion will increase and summer precipitation will decrease       the dry conditions, since during the investigated period, a
(Reiter et al. 2012), however, the analysis of rainfall data    remarkable decrease to the pre- and post-monsoonal rain-
from our study depicts decreases of both post-monsoon           falls was noticed (−0.75 and −0.55 mm per year, respec-
(winter) and pre-monsoon rainfall which do not match with       tively) with very high variability (44.84 and 85.25 % per
the IPCC’s general seasonal tendency. This is mainly due to     year, respectively). High rainfall variability is an indicator
the high spatial and temporal variability of rainfall from      of drought, and therefore, the areas under very high vari-
region to region. Even in our study, because of high spatial    ability with low rainfall, particularly the north western
and temporal variability, we observed a slightly positive       districts, are prone to drought hazard. Predictions of rain-
trend of pre-monsoon rainfall at the extreme northwestern,      fall reveal that declining rainfall will continue and a drying
northeastern and southeastern parts of the country (Fig. 5b).   condition will persist during 2011–2020 (153 mm decrease
                                                                of annual rainfall), especially during the pre- and post-
                                                                monsoon seasons. Spatial patterns of trend and variability
5 Conclusions                                                   of temperature and rainfall indicate that the northwestern,
                                                                western and southwestern parts of the country are more
In this paper, we discussed the last 40 years (1971–2010)       susceptible to climate change with respect to rising tem-
of temporal-spatial climate change in Bangladesh based on       perature, high variability and rain shortfalls, particularly
temperature and rainfall data. The ARIMA time series            for the pre- and post-monsoon rains. It is expected that
model-based future prediction of climate change for the         this study will not only help to delineate appropriate pol-
2011–2020 period was evaluated and confirmed a particu-         icies and planning to combat the impact of climate change
larly strong recent climate change in Bangladesh based on       in Bangladesh but also help to understand the regional
temperature and rainfall changes. This study also validates     climate change in this part of the South Asia. The trend
the ARIMA time series model for shorter time scale cli-         directions, magnitude and spatial patterns identified for
mate simulations and can easily be applied to more local-       both temperature and rainfall may also provide helpful
ized climate data since the ARIMA modelling focuses             information on global warming on a regional/country-
purely on the data rather than data generating processes.       level scale. Improved understanding of recent climate
However, since this model is solely based on a statistical      change helps to elucidate the impacts and vulnerability
approach, it is limited in terms of extreme and irregular       of the local population in order to implement the most
events since this model cannot predict extreme events due       appropriate practices to cope with climate change and
to any external force or natural events. For example, if a      manage the changing situation in a better way.
strong global external force of the atmosphere, like a ma-
jor volcanic eruption occurs, this might be a sound reason      Acknowledgments First author would like to acknowledge the finan-
to invalidate the forecast for that particular time.            cial support from the USM-TWAS post-doctoral fellowship programme
                                                                for the research work. Authors are also thankful to the Bangladesh Me-
Therefore, this should be considered as a limitation of         teorological Department (BMD) for providing historical data, and the
the model since the type of situation is not a regular event.   anonymous reviewers for their valuable comments and suggestions.
For the climate of Bangladesh, the changes of temperature
reflect a warming as a whole and since 1971, the climate
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