Future Landslide Characteristic Assessment Using Ensemble Climate Change Scenarios: A Case Study in Taiwan

Affected by climate change owing to global warming, the frequency of extreme rainfall events has gradually increased in recent years. Many studies have analyzed the impacts of climate change in various fields. However, uncertainty about the scenarios they used is still an important issue. This study used two and four multi-scenarios at the base period (1979–2003) and the end of the 21st century (2075–2099) to collect the top-ranking typhoons and analyze the rainfall conditions of these typhoons in two catchments in northern Taiwan. The landslide-area characteristics caused by these typhoons were estimated using empirical relationships, with rainfall conditions established by a previous study. In addition to counting landslide-area characteristics caused by the typhoons of each single scenario, we also used the ensemble method to combine all scenarios to calculate landslide-area characteristic statistics. Comparing the statistical results of each single scenario and the ensembles, we found that the ensemble method minimized the uncertainty and identified the possible most severe case from the simulation. We further separated typhoons into the top 5%, 5%–10%, and 10%–15% to confirm possible changes in landslide-area characteristics under climate change. We noticed that the uncertainty of the base period and the end of the 21st century almost overlapped if only a single scenario was used. In contrast, the ensemble approach successfully distinguished the differences in both the average values of landslide-area characteristics and the 95% confidence intervals. The ensemble results indicated that the landslide magnitude triggered by medium- and high-level typhoons (top 5%–15%) will increase by 24%–29% and 125%–200% under climate change in the Shihmen Reservoir catchment and the Xindian River catchment, respectively, while landslides triggered by extreme-level typhoons (top 5%) will increase by 8% and 77%, respectively. Still, the uncertainty of landslide-area characteristics caused by extreme typhoon events is slightly high, indicating that we need to include more possible scenarios in future work.


Introduction
The Intergovernmental Panel on Climate Change (IPCC) has reported that global warming will lead to climate change, which will cause sea levels to rise and climate variability [1,2]. The latter

Base Period (1979-2003)
Two scenarios for the base period are widely used in Taiwan. The first is the scenario proposed by the IPCC in the Fourth Assessment Report, 2007 (AR4), which is the Coupled Model Intercomparison Project Phase 3 (CMIP3) model under the Special Report on Emission Scenarios (SRES) A1B scenario [1]. We call this scenario "m00". The second is the scenario proposed by the IPCC in the Fifth Assessment Report, 2013 (AR5), which is the Coupled Model Intercomparison Project Phase 5 (CMIP5) model under the representative concentration pathways 8.5 (RCP8.5) [2]. This scenario is called "m01". The RCP8.5 is a hypothetical scenario that no country in the world has reduced greenhouse gas emissions. This scenario is a relatively worst-case emission scenario that can be used for a severe scenario estimation and for the conservative adjustment. In terms of rainfall, the seasonal and annual rainfall change rate under climate change of the A1B scenario is similar to those of the RCP8.5. They have the climatic characteristics that rainfall increases during the wet season and decreases during the dry season.

End of 21st Century (2075-2099)
The targeted projection window for the future climate is the last quarter of the 21st century. The model is computed from future projections by CMIP5 models under the RCP8.5. Mizuta et al. [24] classified 28 CMIP5 models into three clusters by a cluster analysis of annual-mean tropical sea surface temperature (SST) change patterns. The classified SST change patterns are featured by the zonal gradient of the change in the equatorial Pacific and interhemispheric contrast of the warming. Precipitation and atmospheric circulation responses are composited for the clusters. Based on Mizuta et al. [24], we developed four scenarios for the future SSTs. One is the multi-model ensemble mean of SST computed from all models (average value of all models), which we call "c0". The other three patterns were created using a cluster analysis, in which normalized tropical SST anomalies derived from the 28 models were grouped to avoid subjective selection of a single model, which we call clusters 1, 2, and 3, denoted as "c1", "c2", and "c3" [24]. The procedure of clustering is based on a single-linkage method, where the minimum distance between two models or groups is gradually added [25]. When the last three groups are obtained, the clustering process is terminated. The detailed classification method was described by Mizuta et al. [24], and the average SSTs in different seasons under these scenarios are shown in Figure 1.
Water 2020, 12, x FOR PEER REVIEW 3 of 20 pathways 8.5 (RCP8.5) [2]. This scenario is called "m01". The RCP8.5 is a hypothetical scenario that no country in the world has reduced greenhouse gas emissions. This scenario is a relatively worst-case emission scenario that can be used for a severe scenario estimation and for the conservative adjustment. In terms of rainfall, the seasonal and annual rainfall change rate under climate change of the A1B scenario is similar to those of the RCP8.5. They have the climatic characteristics that rainfall increases during the wet season and decreases during the dry season.

End of 21st Century (2075-2099)
The targeted projection window for the future climate is the last quarter of the 21st century. The model is computed from future projections by CMIP5 models under the RCP8.5. Mizuta et al. [24] classified 28 CMIP5 models into three clusters by a cluster analysis of annual-mean tropical sea surface temperature (SST) change patterns. The classified SST change patterns are featured by the zonal gradient of the change in the equatorial Pacific and interhemispheric contrast of the warming. Precipitation and atmospheric circulation responses are composited for the clusters. Based on Mizuta et al. [24], we developed four scenarios for the future SSTs. One is the multi-model ensemble mean of SST computed from all models (average value of all models), which we call "c0". The other three patterns were created using a cluster analysis, in which normalized tropical SST anomalies derived from the 28 models were grouped to avoid subjective selection of a single model, which we call clusters 1, 2, and 3, denoted as "c1", "c2", and "c3" [24]. The procedure of clustering is based on a single-linkage method, where the minimum distance between two models or groups is gradually added [25]. When the last three groups are obtained, the clustering process is terminated. The detailed classification method was described by Mizuta et al. [24], and the average SSTs in different seasons under these scenarios are shown in Figure 1.

Climate Change Data in Taiwan
The general circulation model (GCM) for climate estimation and simulation typically has a resolution of 100-200 km. It is difficult to identify the structure of a typhoon with a radius of less Water 2020, 12, 564 4 of 16 than several hundred kilometers. Due to the low spatial resolution of the model, it is impossible to reasonably simulate extreme weather systems with physical models, and it is impossible to estimate extreme weather under climate change. [26,27]. Therefore, this study used a climate projection modeled from the high-resolution (approximately 20-km horizontal resolution) atmospheric general circulation model (AGCM) developed by the Meteorological Research Institute (MRI) of the Japan Meteorological Agency (JMA) [28]. The model incorporating the climate change scenario of A1B and RCP8.5 in CMIP 3/5 [24] is called MRI-AGCM. In addition, for the simulation of the end of the 21st century, the low boundary can be driven by different SSTs mentioned in the previous section on the c0, c1, c2, and c3 scenarios. This model has been verified by simulation of typhoon generation [29,30]. However, the horizontal resolution of 20 km may not necessarily reflect the intensity of precipitation caused by the terrain effect in Taiwan. Therefore, the Weather Research and Forecasting Model (WRF), proposed by the US National Center for Atmospheric Research (NCAR), was used to dynamically downscale the MRI-AGCM data to obtain more realistic rainfall data in Taiwan with a spatiotemporal resolution of 5 km per hour. According to the downscaled rainfall data, during the base period, there are 82 and 84 typhoons in the m00 and m01 scenarios, but 45, 23, 55, and 46 typhoons in the c0, c1, c2, and c3 scenarios, respectively, in the last quarter of the 21st century (Table 1).

Calculation of Landslide-Area Characteristics
Chen et al. [16] established the relationship between rainfall conditions and landslide-area characteristics for two catchments in northern Taiwan, the Shihmen Reservoir catchment and the Xindian River catchment ( Figure 2). They found that the landslide-area characteristics in the Shihmen Reservoir catchment are highly correlated with cumulative rainfall during a typhoon event. On the other hand, the landslide-area characteristics in the Xindian River catchment are highly correlated with peak rainfall intensity during a typhoon event. The empirical relationships are as follows: For the Shihmen Reservoir catchment: For the Xindian River catchment: Water 2020, 12, 564

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where A T is total landslide area (m 2 ), L is landslide-area ratio (%), A M is maximum landslide area (m 2 ), N is number of landslides, E is cumulative rainfall (mm), and I p is peak rainfall intensity (mm/h). Although the empirical approach is an effective method to identify landslide-area characteristics, it is evaluated by the interpolation or extrapolation. According to these formulas, when rainfall conditions are weak, unreasonable negative values of landslide-area characteristics may be calculated, and it can be considered that the rainfall conditions cannot trigger landslides.
Water 2020, 12, x FOR PEER REVIEW 6 of 20 Figure 2. Geographic location of the study area. 3 show the results of average rainfall conditions (cumulative rainfall and peak rainfall intensity) and estimated landslide-area characteristics of the two catchments during top typhoons under different climate change scenarios for the base period and the end of the 21st century. For the base period in the Shihmen Reservoir catchment, the highest cumulative rainfall of 1183.69 mm appeared in the scenario of m00, resulting in 896 landslides, with a maximum landslide area of 9.49 × 10 5 m 2 , a total landslide area of 6.30 km 2 , and a Each climate change scenario will have different weights on the empirical models for estimating landslide-area characteristics. The most important factor is the future SST change in each scenario (Figure 1), affecting the circulation of the ocean and the atmosphere and leading to different rainfall patterns. Then the changes of cumulative rainfall and peak rainfall intensity will be different in different regions, which will greatly affect the local landslide-area characteristics. Chen et al. [16] estimated the impact of climate change on landslide-area characteristics for the two catchments but only used single scenarios, m01 and c0, for the base period and the end of the 21st century, respectively. As we mentioned in Section 2, climate change is so full of uncertainty that there are many possible scenarios for the present and the future. The ensemble method is one of the most effective approaches for dealing with many possible climate change scenarios. In this method, the data from all scenarios are combined to obtain the most reliable predictions for the present and future [31][32][33]. For example, there is a total of 10 typhoons in a single scenario so that if the top 10% typhoons according to a certain rainfall condition are taken, there will be only one typhoon. On the other hand, using the ensemble method to consider all scenarios together, the number of typhoons is assumed to be 100, and the top 10% typhoons will have as many as 10 times the number of typhoons (10 typhoons). In this study, we used two scenarios (m00 and m01) for the base period and four scenarios (c0, c1, c2, and c3) for the end of the 21st century. Based on the cumulative rainfall for the Shihmen Reservoir catchment and peak rainfall intensity for the Xindian River catchment during typhoon events, we chose the top 15% typhoons for each scenario and for the ensembles (Table 1). We then estimated the landslide-area characteristics during those typhoons and evaluated the effects of climate change. 3 show the results of average rainfall conditions (cumulative rainfall and peak rainfall intensity) and estimated landslide-area characteristics of the two catchments during top typhoons under different climate change scenarios for the base period and the end of the 21st century. For the base period in the Shihmen Reservoir catchment, the highest cumulative rainfall of 1183.69 mm appeared in the scenario of m00, resulting in 896 landslides, with a maximum landslide area of 9.49 × 10 5 m 2 , a total landslide area of 6.30 km 2 , and a landslide-area ratio of 0.81%. In the Xindian River catchment, the highest peak rainfall intensity of 52.85 mm/h appeared in the scenario of m01, resulting in 181 landslides, with a maximum landslide area of 1.18 × 10 5 m 2 , a total landslide area of 0.71 km 2 , and a landslide-area ratio of 0.15%. In addition, for the end of the 21st century in the Shihmen Reservoir catchment, the highest cumulative rainfall of 1327.07 mm appeared in the scenario of c3, resulting in 996 landslides, with a maximum landslide area of 1.08 × 10 6 m 2 , a total landslide area of 6.97 km 2 , and a landslide-area ratio of 0.89%. In the Xindian River catchment, the highest peak rainfall intensity of 71.17 mm/h appeared in the scenario of c0, resulting in 291 landslides, with a maximum landslide area of 1.90 × 10 5 m 2 , a total landslide area of 1.14 km 2 , and a landslide-area ratio of 0.24%. Details of each typhoon can be found on the website (https://drive.google.com/file/d/1CYkCYBOvWK3Jm4gyaJBNf3i5uenO8U_h/view?usp=sharing).

Discussion
According to the results of landslide-area characteristic estimation, we counted the landslide-area characteristics for the top 10% of typhoon events of each scenario and ensemble. Figure 3 shows the maximum, minimum, and mean values, and the 95% confidence intervals of statistical results of landslide-area characteristics for each scenario and ensemble. The maximum value represents the most severe condition of landslides triggered by the typhoon. The 95% confidence interval represents the range of statistical uncertainty from the mean value. It appears that the range of maximum and minimum values and the range of the 95% confidence interval differed greatly among the different scenarios. Sometimes the range of values was very small, but the uncertainty was very large (e.g., the c1 scenario for the Shihmen Reservoir catchment), which means that the simulation by the scenario can only catch a small part of the possibility in current and future periods, and the results are still questionable (Figure 3a-d). On the other hand, sometimes the range of uncertainty was small, but the range of values was also small (e.g., the c2 scenario for the Xindian River catchment). The uncertainty of simulation from this kind of scenario is quite low, but the possible most severe condition cannot be grasped (Figure 3e-h). This may be due to the fact that model characteristics of those models, grouped in the c2 scenario, only reflect a narrow possibility of future climate, resulting in a very low climate variability. The SST of c2 scenario was moderate among the groups (Figure 1), so that no severe rainfall events could be observed in future climate simulations, which may also be one of the reasons. Generally speaking, the ensemble approach not only minimized the uncertainty (i.e., narrowing the 95% confidence interval), but also identified the worst case from the simulation [34,35]. From the results of the ensembles for the top 10% typhoons during the base period and the end of the 21 st century, on average, the total landslide area, landslide-area ratio, maximum landslide area, and number of landslide in the Shihmen Reservoir catchment will increase from 3.53 ± 0.60 to 4.02 ± 0.58 km 2 , 0.46 ± 0.08 to 0.52 ± 0.07%, (4.06 ± 1.18) × 10 5 to (5.03 ± 1.13) × 10 5 m 2 , and 485 ± 89 to 559 ± 86, while those in the Xindian River catchment will increase from 0.31 ± 0.09 to 0.57 ± 0.11 km 2 , 0.07 ± 0.02 to 0.12 ± 0.02%, (5.33 ± 1.54) × 10 4 to (9.57 ± 1.74) × 10 4 m 2 , and 81 ± 24 to 146 ± 27, respectively.
Here we further separated typhoon events into the top 5%, 5%-10%, and 10%-15% to confirm possible changes in landslide-area characteristics from the base period to the end of the 21st century. Previous studies in Taiwan often used m01 and c0 as simulation scenarios for the base period and the end of the 21st century, and they explored the impact of climate change [16,36,37]. Figure 4 shows the landslide-area characteristics of the top 5%, 5%-10%, and 10%-15% typhoons during the base period and the end of the 21st century in the Shihmen Reservoir catchment and the Xindian River catchment simulated only with the m01 and c0 scenarios. If we only look at the average values of landslide-area characteristics (red and blue lines in Figure 4), there are indeed some differences between the base period and the end of the 21st century. We can use these differences to assess the change ratios of landslide-area characteristics affected by climate change in the future. However, if we consider the uncertainty of the simulations, the 95% confidence intervals at the base period and the end of the 21st century almost overlap (red and blue areas in Figure 4), which means that this result cannot really explain the difference between the present and the future. Chen et al. [16] also pointed out that the future is full of uncertainty, for changes in human activities and the efficiency of carbon reduction will bring additional effects, which may lead to different possibilities. More climate change scenarios must be incorporated, and the ensemble method must be employed to achieve more reliable results.     Figure 5 shows the landslide-area characteristics of the top 5%, 5%-10%, and 10%-15% typhoons during the base period and the end of the 21st century in the Shihmen Reservoir catchment and the Xindian River catchment simulated by scenario ensembles. It is very clear that the ensemble approach successfully distinguished the difference between the base period and the end of the 21st century in the average values of landslide-area characteristics and 95% confidence intervals. From the top 10%-15% to the top 5% typhoon events, the uncertainty of landslide-area characteristics showed an increasing trend, indicating that the more extreme the event, the greater the uncertainty [38]. In the Shihmen Reservoir catchment, the 95% confidence intervals of the base period and the end of the 21st century still partially overlapped at the top 5% typhoons. In contrast, in the Xindian River catchment, they were completely separated. Chen et al. [16] indicated that under the impact of climate change, both the landslide severity and the landslide frequency will increase more in the Xindian River catchment than in the Shihmen Reservoir catchment, owing to the increased possibility of the occurrence of a strong typhoon and the higher rainfall intensity during a typhoon event [20][21][22]. This is why the changes in the landslide-area characteristics of the Xindian River catchment during extreme typhoon events under climate change are more significant than those of the Shihmen Reservoir catchment. Based on the robust ensemble approach, the landslide-area ratios in the Shihmen Reservoir catchment triggered by the top 10%-15%, 5%-10%, and 5% typhoons during the base period will be 0.27 ± 0.01%, 0.34 ± 0.04%, and 0.57 ± 0.09%, while those in the last quarter of the 21 st century will be 0.35 ± 0.02%, 0.42 ± 0.02%, and 0.62 ± 0.12%. In the Xindian River catchment, the landslide-area ratios triggered by the top 10%-15%, 5%-10%, and 5% typhoons during the base period will be 0.02 ± 0.01%, 0.04 ± 0.01%, and 0.09 ± 0.03%, while those in the last quarter of the 21st century will be 0.06 ± 0.00%, 0.09 ± 0.01%, and 0.16 ± 0.03%. The landslide-area ratios triggered by medium-and high-level typhoons (top 5%-15%) will increase by 24%-29% and 125%-200% from the base period to the end of the 21st century in the Shihmen Reservoir catchment and the Xindian River catchment, respectively, while those triggered by extreme-level typhoons (top 5%) will increase by 8% and 77%, respectively (Table 4). Under climate change, the landslide severity in the Xindian River catchment will increase more than that in the Shihmen Reservoir catchment, which indicates that rainfall intensity will increase more than cumulative rainfall. Changes in the SST under climate change will mainly result in more concentrated rainfall. Therefore, in the future with the change of landslide characteristics, special attention needs to be paid to the problem of serious surface erosion.
In the climate change research, changes in related hydrological hazards have traditionally been analyzed with climate change simulated by a specific scenario [16,36,37]. However, the future is full of uncertainties that changes in human activities and the efficiency of reducing carbon emissions will bring additional impacts, which may result in different possibilities. In addition, a single model may be affected by the model characteristics, leading to more biased results. Climate scientists have therefore come up with many models and scenarios to simulate various future possibilities. There is no exact declaration as to which result is the true future, but the ensemble method can discuss all possibilities together and effectively reduce uncertainty. This study used all available climate change scenarios in Taiwan and applied the ensemble method, providing the most reliable estimation on future landslide characteristics for the study area. This study can also contribute to other regions of Taiwan or areas other than Taiwan as a reference approach for related hazard assessment under climate change. Even though this study reduced the uncertainty in assessing the impact of climate change with the ensemble approach, the uncertainty of landslide-area characteristics caused by extreme typhoon events (top 5%) was still slightly high. Therefore, more climate change scenarios should be included to improve the accuracy of assessing climate change impacts. Recently, the Coupled Model Intercomparison Project 6 (CMIP6) provided new possibilities for climate change scenarios and should be considered in future works [39]. Moreover, landslide characteristics can be evaluated not only by empirical methods, but also by other methods, such as numerical simulations [40,41] and statistical methods [42][43][44], which can be used to integrate the results of all methodologies to obtain more reliable results.

Conclusions
This study used two and four climate change scenarios for the base period and the end of the 21 st century, respectively, to analyze the impact of climate change on landslide-area characteristics with the ensemble approach. In total, 166 and 169 typhoons were simulated for the base period and the last quarter of the 21st century, respectively, and rainfall conditions of the two catchments in northern Taiwan for each of the top 15% typhoon events (n = 24 and 25) were analyzed. According to the empirical relationship between rainfall conditions and landslide-area characteristics proposed by a previous study, the landslide-area characteristics triggered by each top 15% typhoon were calculated. Comparing the landslide-area characteristics triggered by the top 10% typhoons between every single scenario and the ensembles, the ensemble approach not only minimized the uncertainty, but also identified the most severe case from the simulation. This study further separated typhoon events into the top 5%, 5%-10%, and 10%-15% to confirm possible changes in landslide-area characteristics under climate change. We noticed that the uncertainty of the base period and the end of the 21 st century almost overlapped if only a single scenario was used, indicating that the difference between the present and the future could not really be explained. The ensemble approach successfully distinguished the differences between the base period and the end of the 21st century in the average values of the landslide-area characteristics and 95% confidence intervals. Based on the robust ensemble approach, the landslide magnitudes triggered by medium-and high-level typhoons (top 5%-15%) will increase by 24%-29% and 125%-200% under climate change in the Shihmen Reservoir catchment and the Xindian River catchment, respectively. In contrast, those triggered by extreme-level typhoons (top 5%) will increase by 8% and 77%, respectively.