Results of the Impact Assessment Regarding Intensification of Disasters Due to Climate Change -Typhoons in the Future with More Advanced Global Warming -

In order to help with the implementation of adaptation measures based on the climate change impact in the future, since fiscal 2020, the ministry has been implementing a project to assess the impact of Extreme Weather similar to past typhoons that caused extensive damage in recent years when they occur in our society under more advanced global warming. The project uses the results of the Ministry of Education, Culture, Sports, Science and Technology’s climate change research programs.
The project assessed the impact of typhoons that may form at the position where the specific typhoons that actually hit Japan formed and that may take a path similar to that of the actual typhoons under the conditions of more advanced global warming.
 In addition to Typhoon Hagibis (2019), which was the subject of the FY2021 interim report, Typhoon Jebi (2018) was covered by the assessment. A supercomputer was used to simulate central pressure, rainfall amount, wind speed, and impact on flooding and storm surge of similar typhoons in our society with more advanced global warming.^*1
 
 In the future, the ministry will implement assessment using the same method for the Heavy Rain Event of July 2018, which caused extensive floods and other damage, especially in Western Japan, while conducting a survey and study on methods to assess impacts on the social economy.
 
*1  The following scenarios are used for more advanced global warming:
2°C warming scenario: Assuming a 2°C increase in the global average temperature above the pre-industrial level (around the mid-18th century) (if future warming is significantly reduced by active mitigation)
 
4°C warming scenario: Assuming a 4°C increase in the global average temperature above the pre-industrial level (around the mid-18th century) (if warming is not reduced by mitigation exceeding the current efforts)
 
 
 The result of the above is compared with the meteorological model simulation of the same typhoons under the meteorological conditions when the typhoons actually formed. (The simulation result is referred to as the “current climate” hereinafter.)
 
[Notes]

• The survey aims to assess the impact of climate change in the case when typhoons similar to past typhoons occur. It does not indicate that the simulated typhoons will occur in the future. Furthermore, heavy rain, strong wind, and other influences of a typhoon greatly vary depending on its course. We need to pay attention to the fact that this report is based on the multiple cases chosen as similar to the courses of the typhoons from among predictions of course, central pressure, rainfall, wind speed, etc. of each targeted typhoon (27 cases).

 The 2019 East Japan Typhoon that formed on October 6, 2019, made landfall on the shore of the Izu Peninsula while maintaining the “large scale and strong intensity”^*4 and then moved through the Kanto Region. It caused record-breaking heavy rain in a wide area around the Kanto Koshin and Tohoku regions. In many places in Shizuoka and Niigata Prefectures and the Kanto-Koshin and Tohoku regions, 3-, 6-, 12-, and 24-hour precipitations exceeded the largest value in recorded history.
The simulation indicates the following: in the future with the progress of global warming, typhoons will have lower central pressure, approach Japan with stronger intensity, bring more rain to the Kanto and Tohoku Regions and expand the area of flood damage, and the probability of river (fluvial) flooding will increase even in regions that have rarely experienced inundation.

Targeting the eight river systems (Arakawa, Tamagawa, Tonegawa, Chikumagawa (Shinanogawa), Nakagawa, Kujigawa, Abukumagawa and Narusegawa (Yoshidagawa) river systems) that suffered especially heavy damage from Typhoon Hagibis (2019), we calculated rainfall in the area above the reference point of each river system (basin average rainfall) and the peak discharge, which is one of the indicators of flood disaster risk. The result shows a 10% in average (3 to 16%) increase for the 2°C warming scenario, and a 14 to 34% increase (23% in average) for the 4°C warming scenario. In the 4°C warming scenario, the discharge at the reference points of the eight heavily affected river systems exceeded the values of the Basic River Improvement Policy, which are the targets of the long-term River Improvement Plan.
Typhoon Hagibis (2019) caused heavy flood damage across Eastern Japan. The result suggested that similar typhoons in the future under climate change would further expand the area of flood damage and that the probability of flooding would increase even in the regions that have rarely experienced inundation.


*2  As this simulation simultaneously handles a wide area covering multiple basins, quantitative assessment of individual river flow involves significant uncertainty. Furthermore, in reality, flooding in the upstream area may decrease the flow in the downstream area, but this possibility is not considered in this simulation. The result shows the possibility of flooding based on peak flow/runoff, but it is necessary to consider the status of river levees, tide embankment, and other facilities in order to obtain more detailed results.

Typhoon Jebi (2018), which formed on August 28, 2018, made land fall in Tokushima Prefecture while maintaining a “very strong”^*4 intensity, moved through the Kinki region and proceeded to the Sea of Japan. The typhoon caused heavy rain and strong wind in a wide area around western Japan. A record-breaking maximum wind speed was observed in Osaka, Wakayama and other prefectures. The highest tidal level record was also broken in these areas as a result of a storm surge.
The simulation indicates that in the future with advanced global warming, typhoons will have lower central pressure and approach Japan with stronger intensity.
In particular, maximum wind speed will increase 8.6m/s on average in the 2°C warming scenario, and 10.2m/s on average in the 4°C warming scenario, which will further increase the risk of storm surge.

Regarding changes in tide level (the maximum deviation between the astronomical tide level and the actual tide level) due to storm surge around Osaka Bay, assuming the progress of global warming, tide levels will rise 27.5% on average (-51.7 to 127.6%) for the 2°C warming scenario, and 23.0% on average (-27.4 to 281.6%) for the 4°C warming scenario compared with the current climate. Major contributing factors are strengthened pressure-induced and wind-induced effects caused by increased wind speed due to the lower central pressure of the typhoons.
Typhoon Jebi (2018) made a new record of the highest tide level, but did not cause inundation of built-up areas due to strong protection systems. However, the simulation suggested the possibility of storm surge damage due to the increased maximum sea-level when a typhoon forms in the future under the influence of climate change.


*3  This result does not refer to the impact of storm surge on inundation. In order to understand the impact, it is necessary to consider the status of river levees, sea walls, and other protection facilities in addition to the rise of the maximum water level. Furthermore, the results of the storm surge simulation will vary depending on the course of the typhoon and other conditions.
 
*4  Intensity and scale of typhoons are classified as follows:
[Intensity (Maximum wind speed)]
  Strong: 33m/s (64kt) ≤ Maximum wind speed < 44m/s (85kt)
  Very strong: 44m/s (85kt) ≤ Maximum wind speed < 54m/s (105kt)
  Violent: 54m/s (105kt) ≤ Maximum wind speed
[Scale (Radius of the area with a wind speed of 15 m/s or more)]
  Large: 500km ≤ Wind radius < 800km
  Very large: 800km ≤ Wind radius
  Source: Website of Japan Meteorological Agency
“Classification of intensity and scale of typhoon” (in Japanese)
https://www.jma.go.jp/jma/kishou/know/typhoon/1-3.html