Future Changes of Drought and Flood Events in China under a Global Warming Scenario

Transcription

1 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2013, VOL. 6, NO. 1, 8 13 Future Changes of Drought and Flood Events in China under a Global Warming Scenario CHEN Huo-Po 1, SUN Jian-Qi 1, and CHEN Xiao-Li 2 1 Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing , China 2 Environmental Development Center of Ministry of Environmental Protection, Beijing , China Received 14 May 2012; revised 26 May 2012; accepted 11 June 2012; published 16 January 2013 Abstract This study investigates the impact of global warming on drought/flood patterns in China at the end of the 21st century based on the simulations of 22 global climate models and a regional climate model (RegCM3) under the SRES (Special Report on Emissions Scenarios) A1B scenario. The standardized precipitation index (SPI), which has well performance in monitoring the drought/ flood characteristics (in terms of their intensity, duration, and spatial extent) in China, is used in this study. The projected results of 22 coupled models and the RegCM3 simulation are consistent. These models project a decrease in the frequency of droughts in most parts of northern China and a slight increase in the frequency in some parts of southern China. Considering China as a whole, the spatial extents of droughts are projected to be significantly reduced. In contrast, future flood events over most parts of China are projected to occur more frequently with stronger intensity and longer duration than those prevalent currently. Additionally, the spatial extents of flood events are projected to significantly increase. Keywords: standardized precipitation index, drought/ flood, projection Citation: Chen, H.-P., J.-Q. Sun, and X.-L. Chen, 2013: Future changes of drought and flood events in China under a global warming scenario, Atmos. Oceanic Sci. Lett., 6, Introduction A drought is characterized by below-normal precipitation over a long period of time and is recognized as one of the main natural causes of agricultural, economic, and environmental damage. Droughts occur over most parts of the world, even in humid regions. Historical records indicate a large drying trend since the mid-1950s over many land areas, with widespread drying over much of Eurasia, northern Africa, Canada, and Alaska (Dai et al., 2004). Similar to these countries, droughts also frequently occur in China. Studies based on climate station data indicate that a severe drought affecting much of northern China in recent decades appears to be the worst since 1990 (Qian et al., 2003; Zou et al., 2005). These frequent severe droughts in China have caused significant economic and societal losses and have been a concern of both the government and general public. In a warm future climate, from a global perspective, Corresponding author: CHEN Huo-Po, chenhuopo@mail.iap.ac.cn most Atmosphere-Ocean General Circulation Models (AOGCM) indicate that wet extremes are projected to become more severe in many land areas where the mean precipitation is expected to increase, and dry extremes are projected to become more severe in areas where the mean precipitation is projected to decrease (Meehl et al., 2007). On a regional scale, climate change effects are more complex, especially in China. The wet extremes are projected to become more frequent and more intense with the increase of mean precipitation under a global warming scenario (Chen and Sun, 2009; Chen et al., 2012a, b; Chen, 2012; Jiang et al., 2009; Jiang and Fu, 2012; Sun et al., 2010; Wang et al., 2012). However, less effort has been devoted to the projection of drought variations in China. Thus, the primary goal of this study is to project the behavior of droughts in China under a global warming scenario. The results from this study may be used to provide guidance on potential impacts from and develop adaptation strategies for a global warming scenario. Because of the complexity of drought, it is very difficult to objectively quantify its characteristics in terms of intensity, duration, and spatial extent (Dai, 2011). For this reason, there have been a multitude of recent investigations presenting methodologies for drought analysis and monitoring (e.g., Wang et al., 2011). Among these methodologies, objective indices are the most widely used, but subjectivity in the definition of drought have made it very difficult to establish a unique and universal drought index (Heim, 2002). Thus, numerous indices have been developed using readily available data, such as precipitation and temperature, to detect and monitor droughts during the last century (Palmer, 1965; McKee et al., 1993; Ma and Fu, 2001). Among these indices, the standardized precipitation index (SPI, McKee et al., 1993), based on a precipitation probabilistic approach, has been commonly used in recent years. Compared with other indices, SPI can be calculated at varying time scales to distinguish or monitor different drought types with respect to different usable water resources, which explain the wide acceptance of the SPI in research and operational works in recent years (Min et al., 2003). This wide acceptance is also the main reason that most scientists recommended the SPI as the standard index available worldwide to track meteorological droughts (Hayes et al., 2011). Due to the symmetrical feature of SPI values, this index can also be used as a tool to detect and monitor flood risk (Seiler et al., 2002). Hence, this study will use the SPI to examine the im-

2 NO. 1 CHEN ET AL.: FUTURE CHANGES OF DROUGHT AND FLOOD IN CHINA 9 pact of global warming on drought and flood patterns in China by comparing the projected climate ( ) in the SRES (Special Report on Emissions Scenarios) A1B experiment with the present-day control climate ( ). We mainly analyze the multi-model ensemble (MME) results from 22 AOGCMs simulations. The simulations from the regional climate model (RegCM3) that were performed by Gao et al. (2012) are also analyzed. 2 Data and methodology 2.1 Datasets The AOGCMs used in this study are part of the World Climate Research Programme s (WCRP s) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset archive at the Lawrence Livermore National Laboratory. For our study, we used 22 models for which the monthly precipitation data are available. Because of the differences in parameterization of physical and dynamical processes, the models differ one from another in spatial resolution. To obtain the MME (equally weighted average of 22 models) pattern, the original model outputs were converted to the same resolution (2.5 longitude/latitude) by employing the bi-linear interpolation technique. A high resolution climate simulation by the RegCM3 spanning from 1948 to 2100 provided by Gao et al. (2012) was also analyzed here. The horizontal grid spacing of the RegCM3 was 25 km, and the model domain covered the whole of China and the surrounding East Asia areas, with the center at 35 N, 109 E, and (east-west by north-south) grid points. Note that this simulation was driven with the six-hourly outputs from the MIROC3.2_ hires (The Model for Interdisciplinary Research on Climate) 20th century simulation (20C3M) and the A1B emission scenario runs. Thus, the results from this simulation show much comparable with the MME of AOGCMs. The details about this experiment were reported by Gao et al. (2012). To validate the model performance, we used the observed precipitation dataset at (latitude-longitude) spatial resolution developed by Xie et al. (2007); the time interval of the dataset spans from 1961 to To examine the monitoring capability of SPI, historical records from two sources were used: one is the records of the disasters that were caused by droughts and floods in China for the period from 1949 to 1999 that were compiled by the National Climate Center of China Meteorological Administration (CMA); the other is the station surface-layer soil moisture (40 stations) from the Global Soil Moisture Data Bank, with a maximum density of stations in Northeast and North China. The monthly precipitation levels spanning from 1961 to 2010 from 752 meteorological stations that were compiled by CMA are also used here for the estimation of the SPI values. 2.2 SPI as a drought index The SPI is a probability index that considers only the observed precipitation deficit occurring over a given prior time period. The calculation procedure has been explained in a number of studies (e.g., McKee et al., 1993; Lloyd- Hughes and Saunders, 2002). The assessment scales considered range from 1 to 48 months. The different time scales allows the SPI to identify drought conditions that are important for a range of meteorological, agricultural, and hydrological applications. For example, soil moisture conditions respond to precipitation deficits occurring on a relatively short time scale, whereas groundwater, streamflow, and reservoir storage generally occur on a long time scale (McKee et al., 1993). In the following, we mainly show the results of drought conditions on a three-month time scale (hereafter, the results of SPI referred to use a three-month time scale) because the main focus of this study is meteorological/agricultural drought. Generally, drought is described in terms of frequency, intensity, duration, and spatial extent. A drought event in this study is defined as the value of SPI smaller than 1.0. Onset is defined as the month when the SPI drops below 1.0, and secession as the month when SPI is last below 1.0. Duration is defined as the period between the onset and secession months, and intensity is defined as the average value of SPI during the duration. The definition of a flood event is similar to that of a drought, but the threshold value is 1.0. The spatial extent of drought (flood) for each month is calculated as the sum of the grid area where the SPI less than 1.0 (larger than 1.0), weighted with the cosine of the grid latitude. Before future drought and flood characteristics are analyzed, we first examine the capability of SPI to monitor the moisture conditions in China. Figure 1 shows two historically recorded severe droughts in China and the SPI monitoring results in the corresponding time intervals. The shadings in historical records denote the large economic losses caused by droughts. Clearly, this index successfully monitors these severe droughts in terms of their intensity, duration, and spatial extent. In addition, other severe drought cases, such as in the years of 1962, 1963, 1967, 1968, 1972, 1997, and 1999, that were recorded by CMA, are also investigated and yielded similar results. The utility of SPI can also be found in monitoring the historical severe floods in China, such as the years of 1991, 1995, and 1998 (figures not shown). In addition, the correlations of SPI with the surface soil moisture (top 50 cm) are calculated. From the variations in the surface soil moisture, we can find that the SPI shows a consistent variability with the surface soil moisture in the seasonal and inter-annual scales (figure not shown). For most stations, the correlation coefficients are significantly above the 95% confidence level, especially for Haerbin, Tianshui, and Xuzhou stations, whose soil moisture data are available for the period The correlation coefficients for Haerbin, Tianshui, and Xuzhou stations are 0.54, 0.43, and 0.41, respectively, which are all significantly above the 99% confidence level. Thus, the SPI successfully tracks the moisture conditions in China. 3 Results 3.1 Changes in drought In the present-day climate, the observed high frequ-

3 10 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 6 Figure 1 The historically recorded severe droughts (left panels) in China and the distributions of the mean SPI (three-month, right panels) in the corresponding time intervals: 1965 (top panels), and 1978 (bottom panels). The left panels sourced from the records of the disasters that were caused by droughts and floods in China for the period from 1949 to 1999 that were compiled by the CMA. ency centers of drought are primarily located over the north of China, especially over the Northeast China, North China, and the east of Northwest China, and the low frequency centers are over the regions of Xinjiang and the lower reach of the Yangtze River (figure not shown). Most GCMs successfully simulate this spatial pattern of drought occurrence, although many differences exist between models. The MME result priors to the individual models and captures the spatial characteristic of increased occurrence in the north and reduced occurrence in the south, despite the underestimation of drought frequency in China (figure not shown). This feature is also well reproduced by the RegCM3, and the simulated magnitudes of drought occurrences are much closer to the observation compared to GCMs; in addition, the RegCM3 better simulates the regions of Southwest China and the west of Tibet Plateau, where the frequencies are also underestimated by RegCM3 (figure not shown). Overall, both the MME and the RegCM3 simulations of drought events in China are reasonable. Figure 2 shows the plots of projected differences between the three drought variables in the A1B and the 20C3M. The left panels show the MME changes in the frequency, mean intensity, and mean duration, while the right panels show the corresponding changes simulated by the RegCM3. From Fig. 2a, it is clear that the MME projected drought frequency tends to be increased over some parts of the south of China and the west of Tibet Plateau, which is supported by a high degree of model consistency. On the other hand, drought is expected to occur less frequently over the north of China, where the model consistency is high, and the decreased magnitude is much larger than the increased magnitude in the south of China. By comparing Fig. 2b with Fig. 2a, we find that the RegCM3 exhibits similar change patterns with the MME, but with larger changing magnitudes. In percentage terms, the MME and the RegCM3 consistently project that the drought recurs with less frequency at the end of the 21st century than in the present-day climate, and the averaged frequency in China is projected to decrease by 24.6% (all the GCMs show consistent changes with the MME) and 21.1%, respectively. In addition, the pattern of difference simulated by MIROC3.2_hires model is also compared with the RegCM3 and a similar result can be obtained (figure not shown). Therefore, China will experience fewer drought events under a global warming scenario. This result is substantially consistent with the previous study (Zhang and Sun, 2012), which predicts more annual mean precipitation minus evaporation across China in the future. The different patterns of the intensity from the MME and the RegCM3 (Figs. 2c and 2d) consistently reveal that the south of China and the Tibet Plateau droughts, which are anticipated to be more frequent, will be more intense. This is more apparent over the southeast coast and the west of Tibet Plateau. The degree of model consistency is also high over these regions, as good as that for the frequency. Note that the intensity is projected to strengthen over some parts of Northeast China as well, in a region that exhibited negative frequency change. On the other hand, the strong weakening trend predicted with a high level of consistency from both the RegCM3-MME and inter-gcms is dominant over North China and some parts of Northwest China. Considering China as a whole, the

4 NO. 1 CHEN ET AL.: FUTURE CHANGES OF DROUGHT AND FLOOD IN CHINA 11 Figure 2 Multi-model ensemble differences of (a) frequency (per two decades), (c) the mean intensity, and (e) the mean duration (months) of drought events between the period of in the A1B scenario and the period of in the 20C3M experiments. (b), (d), and (f) are the projected differences of the RegCM3 simulation. The dots indicate that the changes from at least 50% of the AOGCMs are consistent with the MME results. averaged intensity is projected to be slightly intensified by 3.84% (RegCM3) and 0.20% (MME). However, out of 22 GCMs, there are only 15 models that exhibit a similar change with the MME. Similarly, in the case of the frequency changes, the MME and the RegCM3 consistently indicate that drought is projected to last for longer durations in some parts of the south of China and the west of Tibet Plateau and for shorter durations in the other regions than in the present-day climate (Figs. 2e and 2f). Considering China as a whole, the averaged-duration is projected to be reduced by 4.7% in the RegCM3 and 9.7% in the MME. The simulations from 20 GCMs exhibit a similar change with the MME. Figure 3 shows the variations in the proportion of the land area under drought in China over the twentieth and twenty-first centuries. From the observation, the drought area tends to decrease over the last fifty years. This de- creasing trend is well reproduced by the RegCM3 and MME results, and this trend even continues throughout the twenty-first century, which is counter to the significant increase of drought area under the global warming scenario on the global scale (Burke et al., 2006). By the end of the 21st century, the percentage of drought area decreases to 12.2% (RegCM3) and 12.4% (MME), while they are 16.8% and 18.3% in the period Changes in flood In the present-day climate (figures not shown), the flood events are mainly concentrated over the regions of Northeast China, the east of Tibet Plateau, and the Yangtze River, while lower frequency of flooding can be found in the other regions, especially in North China. This observed spatial pattern of flood frequency is well reproduced by the RegCM3, and the locations and magnitudes of the high frequency centers are all well simulated. The

5 12 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 6 Figure 3 The proportion of the area of drought for the 20C3M and the A1B experiments. The observed result is also shown here. A nine-year low-pass filter has been performed on each time series, and the shadings are the ranges of ±1 standard deviation of the 22 AOGCMs, which represents the uncertainties from the models. GCM simulations are also compared with the observation, and the results indicate that although there are many differences between the GCMs, the MME of these 22 models successfully simulate the observations. Similar to the drought case, the frequency is also underestimated by the MME. A similar diagnosis with Fig. 2 is performed for the flood cases (figure not shown). The results indicate that the flood frequency tends to respond to the mean precipitation increase, and the projected results from the Reg- CM3 and the MME consistently predict an extensive increase over China. This increase is more apparent over the north of China. However, there is no consistency over some parts of the south of China between the RegCM3 and the MME. The flood frequency simulated by the RegCM3 is projected to decrease over the regions of Tibet Plateau and Southwest China, while most GCMs project an increase. This decrease by the RegCM3 may be associated with a significant decrease of the mean precipitation in these regions (Gao et al., 2012). Accompanying the increase of flood occurrence, the mean intensity exhibits a strengthening and the mean duration exhibits a lengthening nearly everywhere of China. That is, the occurrence of flood in China is expected to be more frequent, more intense, and much longer under a global warming scenario. For clarity, the area-averaged changes in the flood variables are calculated. By the end of the 21st century, the frequency, intensity, and duration simulated by the RegCM3 are projected to increase by 69.1%, 13.2%, and 26.5%, respectively, relative to the period of Similarly, all of the GCMs also predict significant increases, especially in the MME results. The frequency, intensity, and duration are projected to increase by 59.2%, 8.8%, and 21.2%, respectively. The proportion of flood area in China is also estimated (figure not shown). An observed upward trend is reported over the last several decades, which is well reproduced both by the RegCM3 and the MME, although their simulated magnitudes are much smaller than the observation. Because of the significant increase of precipitation, this increasing trend in the flood area will continue throughout the 21st century, which means more widespread areas will experience flood events. The simulation from the Reg- CM3 indicates that the proportion of the flood area in China increases from 11.2% in the period of to 25.4% in the period of The MME predicts a similar change, with a projected increase in the flood area from 13.2% to 23.6%. 4 Conclusion Based on the SRES emission scenario A1B for atmospheric greenhouse gasses and aerosols, a number of climate models and the RegCM3 simulations have been performed to project the drought and flood risk changes over the 21st century. These simulations consistently indicate that most parts of China will experience more frequent, more intense, and much longer flood events under a global warming scenario. The spatial extent of flood events is also reported to be significantly enlarged. In contrast to the flood events, droughts are projected to be less frequent over most parts of China and the area over which droughts occur will be reduced. The above results are mainly based on the three-month SPI values that are calculated only from precipitation variations. However, the surface air temperature is another important factor for drought and flood occurrences. Thus, in future work, the temperature will also be considered to further investigate the change in the characteristics of drought and flood events. Acknowledgements. We sincerely thank the two anonymous reviewers for their helpful comments and suggestions on this manu-

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