Extreme Events and Disasters are the Biggest Threat to Taiwan Typhoon Morakot

Transcription

1 Extreme Events and Disasters are the Biggest Threat to Taiwan Typhoon Morakot

2 Impacts due from Climate Change According to the statistical data of the past hundred years, the annual mean temperature of Taiwan has been increased 1.4 degree Celsius, about two folds of the global warming trend (figure 1). Similar to the global warming, the warming trend in Taiwan can be divided into two stages: from year 1921 to 1951 and from year 1961 to nowadays. However, there are differences between the warming trend in Taiwan and that of global mean. During the 50s to the beginning of the second stage, the global mean temperature was decreased but there is no significant temperature change in Taiwan. In the second stage, mean temperature in Taiwan increased 0.7 degree Celsius from year 1980 to 2001, which is greater than the trend of the past century and is consistent with the gradually increasing global warming trend in past 30 years. In Taiwan, all four seasons show warming trend. The warming is about 0.3 to 0.4 degree Celsius per year in winter time, which is higher than the warming of 0.2 degree Celsius per year in summer time. As for the precipitation, the occurrence of light rain has been decreasing and the overall precipitation intensity has been increasing in the past 50 years. According to a recent research results, with one degree Celsius increase in global mean temperature, the rainfall in extreme precipitation events in Taiwan will increase 140%. In the past 50 years, the global mean temperature has increased by 0.7 degree Celsius and the rainfall of extreme precipitation events in Taiwan has increased by 100%. For those typhoons resulting in severe disasters in Taiwan, the intensity of precipitation, not the intensity of wind speed, is the primary cause of disaster. According to the statistic of the top 30 typhoons with extreme precipitation (figure 2), which are identified based on the precipitation of the 198 typhoons striking Taiwan area from year 1970 to 2009, there is an increasing trend in the frequency of typhoon with extreme precipitation. The frequency of typhoon with extreme precipitation happened one time for every two years before year of 2000, but it increased to once a year after year of Typhoons with extreme precipitation mean increased by 1.4 within 100 years. Change in annual mean temperature annual mean temperature variation Mean temperature above average Mean temperature below average (Referring years: 1901~2000) Five-year annual moving average Figure 1. Time series of annual mean temperature for Taiwan during 1900~2006.

3 usually caused great damage and loss to life, property, and economics; and typhoon Morakot in August of 2009 brought the most extreme precipitation ever recorded. Meanwhile, with the increase in the frequency of extreme precipitation, no significant change in the annual precipitation amount is observed. The ratio of typhoon precipitation to annual precipitation increases from 15% in the 70s to 30% after year of 2000 and the difference of precipitations of rainy and dry seasons keeps increasing. This trend suggests that more precipitation, most of them are strong precipitation, is produced in rainy season and less precipitation is produced in dry season, exactly like the projection of East-Asian precipitation in IPCC AR4 report. Because the strikes of typhoon with extreme precipitation becomes more frequent, floods of low-lying land areas, malfunction urban drainage systems, slopeland disasters, soil sedimentation in reservoirs, and even the damages in public facility, also become possible causes to increase the casualty and the impact on socioeconomics. Taking into account the severe weather and rising sea level induced by climate change, Taiwan will be facing more rigorous challenges. The impact of climate change to Taiwan starts to emerge and has been aware in Taiwan. In addition to understanding the changes in regional climate and taking the responsibility of being a citizen of the world, Taiwan needs to be more aggressive to face the influence of global change on local impact. In order to reduce the impact of climate change on Taiwan area, the ongoing relative researches and projects, including using IPCC climate modeling data and downscaling technique to generate different climate scenarios of Taiwan area, creating maps of water resource and hazard risk based on the local climate scenarios, and conducting the research of risk management policy, to promote policy making authorities adjust the arrangement and the implementation. Frequency of extreme Typhoon rainfall (The Leading 30 extreme Typhoon events over Taiwan:1970~2009) 2000~ extreme Typhoon events Period 2000~ ~ extreme Typhoon events 6 extreme Typhoon events 2000~ extreme Typhoon events Freq.(1/year) Figure 2. Frequency of extreme typhoon rainfall over Taiwan in the period of 1970~2009.

4 Frequent Natural Disasters - Typhoon Taiwan is an island of the East-Asian Island Arc, sitting in between Japan and Philippines and between the largest continent (the Euro-Asia Continent) and ocean (the Pacific Ocean) in the world. The climate of Taiwan is dominated by the monsoon most of the time. Also, the location is in the earthquake belt and on the major tracks of typhoons in northwest Pacific area (figure 3), which makes Taiwan one of the vulnerable area frequently suffering from the nature disasters. 40N 30N 20N Taiwan The Central Mountain Range of Taiwan, the highest mountain range in East Asia, stretches from the north to the south part of Taiwan and has been one of the major reasons for the seasonal precipitation distribution. During wintertime, the prevailing northeast wind results in precipitation in the windward side, i.e., the north and the northeast of Taiwan. In summer, southwest Taiwan becomes the major precipitation area when the prevailing wind turns southwesterly and brings in abundant vapor from the South China Sea. The terrain also greatly influences the distribution and intensity of typhoon precipitation in Taiwan in the similar reason. There are about 3 to 4 typhoons strike Taiwan every year, which not only brings in heavy rainfall but also causes disasters with economical loss at the measure of 200 billion NT dollars per year. The water resource of Taiwan area primarily comes from the precipitation associated with Mei-Yu fronts and Typhoons. The annual mean precipitation in Taiwan is about 2550 millimeters and reaches to 4000 millimeters in the mountain area. However, because of the high density of population and the inhomogeneous distribution of precipitation, the amount of available effective water per person is less than 1/6 of the global mean. Therefore, The United Nations puts Taiwan as an area with scarce water resource. Therefore, Taiwan is still facing serious issues of water resource and water usage. Typhoon, flood, land slide, draught, and earthquake are the most happening natural disasters in Taiwan. According to Natural Disaster Hotspots A Global Risk Analysis published by the World Bank in year of 2005, Taiwan is an area at high risk for natural disaster in the world. About 73% of land and 10N EQ 120E 140E 160E Figure 3. Taiwan is located on the tracks of typhoons in northwest Pacific area. population is exposed to more than three natural hazards while 99% of land and population is exposed to two or more natural hazards. There are three major reasons for these critical threats to Taiwan: 1. The nature of vulnerability: including frequent typhoons, intensive rainfall, steep mountains and rushing rivers, high precipitation variation between rain and dry seasons, low-lying terrain of southwest coastal area, fragile geology, and loose surface soil. 2. The effect of social economical development: including flooding issue caused by urban development, high temperature and local convection enhanced by heat island effect, increases in water consumption due to rapid industrial development, the land subsidence in southwest Taiwan caused by the over pumping of the groundwater. 3. The impact by global climate change: including persistent rising of the temperature, increase in intensity of extreme precipitation events, changes in temporal and spatial distribution of precipitation, and the rise of sea level surface.

5 On Typhoon Morakot Taiwan has been making lots of effort on disaster prevention and rescue system and related technology development. In the past years, the casualty caused by typhoon has been dropping to a single digit. However, typhoons Kalmaegi and Sinlaku in 2008 and typhoon Morakot in 2009, which causes server injury and casualty, suggest that the influence of climate change may also change the nature of severe weather and extreme precipitation, resulting in more record-setting precipitation. When Typhoon Morakot struck Taiwan, from Aug 6 to Aug 10, the heavy rainfall in mountain area was over 1500 millimeters in 24 hours and the accumulated precipitation over this period was more than 3000 millimeters (figure 4), causing disasters of serious flood in southern Taiwan, overflowed levees, landslide, debris flows from slopeland; buildings buried, destroyed roads and bridges, and schools and public facilities seriously damaged. The estimated sum of loss on agriculture is more than 164 billion NT dollars. There are also 699 of death and missing people. In term of casualty and injury, it is the most severe typhoon being recorded in the history of Taiwan. Typhoon Morakot brought heavy rainfall in two to three days, which was beyond the loading of rivers and local drainage systems, resulting in flood and broken levees off the shores and the river banks. In addition, during the period of high tide, the raised sea level also impedes the draining ability of rivers. The flood area was estimated to 400 square Figure 4. The storm track, accumulated precipitation, and major regions of disaster caused by Typhoon Morakot, which struck Taiwan from Aug 6 to Aug 10.

6 On Typhoon Morakot Figure 5. Areas of floods and landslides caused by Typhoon Morakot Buried Area Figure 6. The Xiaolin Village of Kaohsiung County, the most severe damaged area, where the landside and collapse caused by the erosion of torrential rain buried the whole villiage and resulted in more than 400 death and missing people. (left: the satellite picture for Ziaolin Village before Typhoon Morakot comes, the buried areas are marked as shadow area. right: the picture of Ziaolin Village after Typhoon Morakot comes)

7 Table 1. Ratio of Typhoon Morakot precipitation to annual precipitation for major stations. County Township Annual Rainfall 08/07 08/08 08/09 08/10 08/07-08/10 08/07-08/10 vs Annual Chiayi Alishan 3, ,161 1, ,965 76% Pingtung Sandimen 3, , ,872 74% Chiayi Jhuci 3, , ,775 73% Kaohsiung Taoyuan 4, , ,790 68% Kaohsiung Liouguei 3, , ,461 78% Chiayi Fanlu 3, ,202 64% Chiayi Dapu 2, , ,156 78% Kaohsiung Jiasian 2, , ,020 71% Nantou Sinyi 3, ,929 59% Kaohsiung Maolin 3, ,404 45% Pingtung Wutai 2, ,160 40% Kaohsiung Cishan 2, % kilometers. (Figure 5) The reasons for flooding disaster include the torrential precipitation in short period of time, the crashed levees, the low-lying terrain due to land subsidence, the sedimentation of rivers, the inefficient urban draining systems, and the water release of reservoirs. Heavy precipitation of Morakot also triggered slopeland disaster, such as large scale landslide, fallen rocks, and debris flow. The slopeland disaster is the major cause of casualty and building damage. The most severely damaged area happened in the Shiaolin Village of Kaohsiung County, where the landside and collapse caused by the erosion of torrential rain buried the whole village and resulted in more than 400 death and missing people. (figure 6) Extreme precipitation is one of the main reasons for the disasters caused by Morakot. Under the influence of enhanced large-scale southwesterly flow, during the period when Morakot stuck Taiwan, a record-high 3095 mm precipitation was measured by a rain gauge while precipitation intensity over a flood frequency of 2000 years was measured in many stations. The regions with heavy precipitation Figure 7. Comparison of the maximum accumulated rainfalls in one to three days between the value caused by Typhoon Morakot and that of world record. and serious disasters mostly concentrated in towns and villages of south and southwest Taiwan. The maximum accumulated rainfalls in one to three days are comparable to that of world record, especially the amount measured on the first two days (Figure 7). Table 1. shows that, for some stations, Morakot dumped the amount of 70% annual precipitation in three days.

8 On Typhoon Morakot