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Climatic Natural Variability and Interdecadal Variation
|Abstract:||地球氣候不斷的變化，呈現多重時間尺度的特性。過去一百多年的地球氣候除了有長達百年的暖化趨勢，亦呈現明顯的年代際變化，如20世紀初至1940年代的暖化趨勢，1950年代至1970年代的冷卻趨勢，以及1980年代以後更明顯的暖化趨勢。降雨也呈現明顯的年代際變化，而且區域性強。 IPCC第四次評估報告對過去數十年全球暖化加速的看法，忽略了年代際振盪的貢獻。過去30年剛好處於大西洋多年代振盪朝正相位（亦即溫度上升）發展的時期，加上全球溫度上升趨勢，因此顯得暖化速率特別快。無論檢視過去氣候變化或推估未來氣候變遷趨勢，吾人都需考慮年代際的影響，因為年代際振盪造成的是近期影響，人為溫室效應則是遠期影響。年代際振盪的影響不僅不可忽視，甚至可能提供較為準確的近期氣候推估。IPCC 已經將年代預報納為重要的研究方向，並將於第五次評估報告首次提出研究成果。 此項觀察與長期暖化趨勢的存在並不牴觸。年代際振盪以北半球最為明顯，南半球的海溫與全球海面高度皆呈現顯著的長期上升趨勢與微弱的年代際振盪。理論上，如果人為溫室效應確實影響了地球氣候，海水溫度與熱容量是最容易延續該訊息的氣候變數，這是因為海水的熱容量遠大於氣體與土壤，具有較長的「記憶」。南半球的海溫與全球海面高度的顯著長期上升趨勢很可能反應了此一人為影響。 人造溫室氣體是過去數十年氣溫上升的主因之一。最近的觀測顯示，自京都議定書以來，全球溫室氣體的排放不但無法抑制，反而加速成長。人類排放的溫室氣體造成的溫室效應已經十分明顯，而且大多數氣候模式都顯示人為溫室效應的影響大於其他已知的自然因素，因此過去一百多年來的溫度上升有一部分受到人為溫室效應的影響是極其可能的。依據IPCC的情境推估，在2100年暖化程度為1.1°C至6.4°C。此一暖化幅度遠大於年代際振盪造成的溫度變化，也大於火山爆發帶來的短期冷卻。即使考慮未來如果發生像造成小冰期的Maunder Minimum，其造成的全球冷卻效應約為0.3°C，仍遠小於人為溫室效應造成的暖化。 整體而言，如果溫室氣體的增加無法抑制，全球暖化程度將遠遠超過自然變化的幅度，無任何已知的過去百年來的自然變化可以抵銷。地球史上曾發生過的毀滅性災難如果再次發生，或許可以抑制全球暖化趨勢。但是，這類事件無法預知也非人類所樂見。抑制全球暖化以減緩對地球生態環境的可能衝擊，勢在必行。因此，如何抑制溫室氣體的排放，降低大氣溫室氣體濃度的增加速率，是目前人類面臨的最大的課題與挑戰。|
Earth climate changes all the time in multiple time scales. In addition to the centennial warming trend in the past one hundred years, the global-mean surface temperature fluctuated in multidecadal time scales, e.g., the warming from the beginning of the 20th century to 1940s, cooling between 1950s and 1970s, followed by a dramatic increase after 1980s. Rainfall also exhibited in interdecadal time scales but with significant regional characteristics. The conclusion of the IPCC AR4 about the accelerating warming trend in the past few decades may have ignored the possible contribution of the multidecadal oscillation. In the past 30 years, the Atlantic Multidecadal Oscillation (AMO) happened to be in the developing stage of positive phase (i.e., warming tendency). Its superposition on the centennial warming trend likely led to the impression of accelerating warming trend. It is important to seriously consider the effect of multidecadal oscillation when examining past climate change and projecting future climate change. The multidecadal oscillation contributes relatively short period of climate variability, while the anthropogenic greenhouse effect leads to long-term climate variation. Multidecadal fluctuation, which is likely to contribute to more accurate projection of near-future climate, cannot be ignored. Study in multidecadal fluctuation and decadal prediction is one of the key issues and will be reported in the IPCC Fifth Assessment Report. The observation of multidecadal fluctuation and centennial warm trend do not contradict with each other. While multidecadal oscillation was most significant in the extratropical Northern Hemisphere, both mean surface temperature in the Southern Hemisphere and global-mean sea level have been exhibiting mostly long-term upward trends in the past one hundred years. Theoretically speaking, the anthropogenic warming signals will be retained more clearly in the ocean than over the land, because of the larger heat capacity of sea water and the longer climate “memory”. It is likely that the significant warming trends seen in the mean surface temperature in the Southern Hemisphere and the global-mean sea level reflect the anthropogenic warming effect. Anthropogenic greenhouse effect is one of the major reasons for the warming in the past few decades. Recent observation reveals that the anthropogenic greenhouse gas emission has not been suppressed since the Kyoto Protocol; instead it has been increasing more rapidly since then. The warming due to anthropogenic greenhouse effect is evident and most of climate models indicate that the impact of anthropogenic greenhouse effect is larger than those caused by observed natural factors. It is fair to conclude that the global warming observed in the past one hundred years was partially caused by the anthropogenic greenhouse effect. IPCC AR4 reported that the projected future warming, based one different emission scenarios, will be 1.1-6.4°C, which is much larger than the amplitudes of known multidecadal fluctuation and the shorter-term cooling effect of known volcanic eruption. If an event equivalent to Little Ice Age due to Maunder Minimum occurs in the future, it may cause 0.3°C cooling, which is still smaller than the projected anthropogenic warming. In summary, the anthropogenic global warming will be larger than the natural variability observed in the past one hundred years if the emission of anthropogenic greenhouse gas cannot be suppressed. If a catastrophic event that occurred in the Earth history occurred in the future, it might be able to suppress the projected warming trend. However, such an event cannot be predicted and is not what we want, either. Suppressing anthropogenic greenhouse emission has to be achieved in order to minimize the impact of future global warming on the ecological environment on Earth. How to suppress the emission of anthropogenic greenhouse gases though carbon reduction and mitigation to decelerate the increasing rate of greenhouse concentration in the atmosphere is the most important issue and the greatest challenge for the mankind.
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