21世紀的熱浪

Abstract

自1970年代開始，全球的氣溫有明顯上升的現象。對於造成這種現象的可能原因，有各種不同的說法，還在討論階段。但大部分地區一致性地溫度升高，使人類開始擔心地球未來的生存。近年來，各地瀕傳熱浪事件所造成的影響，與重大的生命財產損失，較以往多上許多。未來的21世紀，這種情況可能更加嚴重。本文希望藉由探討IPCC報告中對未來的情境模擬，瞭解2046-2065與2081-2100，熱浪事件可能具有的各種特性。 在研究21世紀的熱浪情況之前，先分析20世紀熱浪事件的特徵。本文使用二種方式定義熱浪事件，分別參照Huth et al. (2000)與Karl and Knight (1997)。由第一個方法得知當地熱浪事件發生的頻率，與每次熱浪事件的持續天數；第二個方式則幫助瞭解熱浪事件的強度。這兩種方式所定義出的熱浪事件主要都發生在夏季。分析台灣地區中央氣象局1950-2005年的16個測站資料，在大部份測站發現熱浪頻率與持續天數增加的現象，雖然只有5個測站的上升趨勢通過檢定。熱浪強度方面，不論是日最高溫或日最低溫，都有明顯增強的趨勢。 欲分析全球陸地的熱浪事件，本文使用IPCC AR4中10個模式的模擬資料，與NCEP/NCAR的再分析場當作觀測資料。同樣使用上述二種方式分析20世紀(1961-2000)熱浪事件的特徵，以瞭解模式對全球氣候場的模擬能力，再利用未來scenario A1B的情境模擬資料預測未來熱浪特徵的可能狀況。觀測資料顯示，在20世紀(1961-2000)，中高緯度地區較容易發生熱浪事件，但持續時間比較短；低緯度、近赤道陸地熱浪發生的頻率比較小，但每次熱浪的持續天數較長。各個模式的熱浪特徵有所不同，但模式系集平均的表現與觀測頗為類似，但模式的極値較觀測的小。在21世紀中(2046-2065)，全球陸地的熱浪頻率與持續天數都會增加，到21世紀末(2081-2100)，這些特徵會更加顯著。低緯度地區在21世紀將遭受到更頻繁，持續時間更長的熱浪事件。熱浪強度方面，幾乎所有地區都可以看到明顯的增強現象。20世紀強度較強的地方(低緯度地區、沙漠地區等)，21世紀的熱浪強度增強的現象會更加明顯。未來與現在熱浪特徵的變化，可能由於中低層高壓增強，特別是副熱帶地區，造成晴朗無雲、微風的現象，使地表附近的高溫無法下降。赤道地區水汽含量增加，造成溫度上升的正回饋作用。哈德雷胞勢力範圍的擴大也是可能原因之ㄧ。 IPCC AR4的模式資料，在21世紀的改變主要由於人類溫室氣體的增加。本文所使用的A1B情境模擬只是在較樂觀情況的狀態下，預測未來氣候的其中一種可能狀況，以及人類可能面對的問題。由各種不同對未來的研究結果，人類可以推斷未來的狀況，及早做出預防，或及時做出改變。

The Earth climate has been significantly warming since the late 1970s. No matter what the reason is, it is undoubtedly that the temperature in most of places on the Earth is getting higher. Heat waves, which could result in severe damages and fatality, have become more frequent than the past. This study is aimed to understand the characteristics of the heat waves in 2046-2065 and 2081-2100 based on the IPCC climate change scenario simulations.

Before focusing on the future climate, we should first understand the climate in the 20th century. Two methods, based on Huth et al. (2000) and Karl and Knight (1997), were adopted to define “Heat waves”. The former method defines the frequency and duration of heat wave; the latter one defines the intensity. “Heat waves” defined by either method mostly happen in summer. We examined the 16 CWB station datasets in Taiwan, from 1950-2005, and found that there are some evidence indicating the increasing frequency and duration of heat waves. Moreover, the intensity of the warmest 3-day temperature exhibits clear increasing trends in most stations, especially in the cities. “Heat Island Effect” is likely to contribute to this phenomenon. The same two methodologies were applied to study the heat waves in the global domain to validate the simulation results from 10 IPCC AR4 models for the period from 1961 to 2000, and to project the heat wave activity in 2046-2065 and 2081-2100. We explored the spatial characteristics of the change in the heat wave frequency, duration, and the intensity in the future climate under the IPCC A1B scenario. In the 20th century (1961-2000), the diagnostic results based on the NCEP/NCAR Reanalysis indicate that the heat waves occurred most frequently in the middle and high latitude region in both hemispheres. The occurrences of heat waves in the lands near the equator were fewer but lasting longer. Most models show the similar patterns, though the values were not so accurate. The frequencies of heat wave will be doubled in the middle of the 21st century, and become even higher at the end of the 21st century. In addition, the countries near the equator, which are warm in the present time, will encounter more frequent and longer heat wave events in the future time. The evident increases in the highest maximum and minimum temperature show that the intensity of heat wave will significantly increase. Temperature in all land areas will arise 2~4 degree by the end of the 21st century. The characteristics of Heat Wave may be reduced by the change of atmospheric circulation. The geopotential heights in middle and low levels show positive anomalies by the end of the 21st century, especially the subtropics. The anomalies may result a clear skies, weak wind situation, make the high temperature near the surface prolonged. The expansion of Hadley cell may be one of the factors. These results may no represent the true changes that will occur, but will be useful in assessing the possible anthropogenic impact on the future heat wave activity.