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  • 27Aug

      是自然波動還是人為現象?

      全球溫度上升已是一個不爭的事實。除了上文提到的全球平均溫度的百年趨勢外,就筆者執筆為止,自從我們擁有較為可靠的溫度記錄的一八七零年至今,十個最熱的年份皆於一九九六年後發生,而最熱的二十五年皆於最近這三十年內發生!

      其實不用科學家告訴我們,世界各地的人們已經感受到,酷熱天氣出現的次數較過往更多,持續的時間亦更長,溫度亦較過往更高。而在不少地方,山林大火明顯較過往更頻密,而火勢亦更為猛烈。

      自有氣象記錄以來,二零零五年是全球溫度最高的一年。但早於二零零三年,歐洲便已經歷了一趟「百年一遇」的可怕熱浪。在短短個多星期內,在意大利、法國、德國、西班牙等地,因熱浪直接或間接影響而死亡的人數超過了五萬之多。一些專家警告,這種在傳統氣候學上應是「過百年才一遇」的反常天氣,將來很可能會成為「數十年一遇」甚至「十年一遇」……。(筆者執筆時,莫斯科的氣溫竟高達前所未聞的四十三度!)

      但從科學的角度看,把全球暖化認定為人類活動的結果,是否過於輕率的一個結論呢?要知自然界的變化總有各種波動和反覆,也許我們現在看到的暖化現象,只不過是自然波動的一部分而已?

      不錯,氣候的變遷(Climatic Changes)完全可以是一種自然的現象。相信大家都聽過「冰河紀」(Ice Ages)這回事吧。過去數百萬年來,地球上曾經出現過多次的冰河紀。期間最冷的時候,整個歐洲和大半個北美洲皆被厚厚的冰雪所覆蓋。而最暖的時候,即使鱷魚這種熱帶生物也可在歐洲北部出沒。

      最後一次冰河紀約於一萬三千至一萬二千年前左右退卻,而人類的農業革命則於一萬二千至一萬一千年前左右發生。科學家把冰河紀與冰河紀之間的溫暖時期稱為「冰河間期」(Inter-glacial)。也就是說,人類文明的發展,至今都是在一個溫暖怡人的「冰河間期」中進行的。

      冰河紀的更迭一般以數萬至數十萬年為單位。但即使以數千甚至短至數百年為單位,自然氣候也可以出現明顯的變化。在歐洲氣候史的研究中,兩個最著名的變化是公元八至十三世紀的「中世紀溫暖期」(Medieval Warm Period),以及由十五至十九世紀初的「小冰河紀」(Little Ice Age)。在前一段時期,北歐的維京人在格陵蘭建立了殖民地,並在其上進行農耕;而在後一段時期,倫敦的泰晤士河在冬天會結成堅冰,而民眾可在上面舉辦各種遊藝活動。

      由此看來,我們今天所目睹的全球暖化現象,是否也只是自然界波動的一部分,而與人類的活動無關呢?

      這是一個十分合理的疑問,而科學家對此亦做了大量的研究工作。就冰河紀而言,深入的研究顯示,起因是地球的自轉和公轉運動都存在著一些周期性的變化,而在海、陸分佈形態和季節更迭的影響下,這些變化遂導致陸地上冰川的周期性擴張與消減。這便是冰河紀成因的著名「天文學說」(Astronomical Theory of Ice-ages Formation)。

      事實上,按照這個學說,我們如今所身處的「冰河間期」已經過了一半。也就是說,地球未來的氣候,會一步一步的變得冷起來。無獨有偶,大約自一九四零至一九七零年這數十年間,科學家發覺地球的溫度竟下降了近 0.4 度!一時間,「我們正步向另一個冰河紀!」這種聳人聽聞的宣稱甚囂塵上,而有關全球暖化的威脅則無人問津。(有關這 0.4 度的降溫我們將於稍後再作探討。)

      讓我們回到自然氣候變化的成因之上。對於好像「中世紀溫暖期」及「小冰河紀」等時間尺度短得多,而變化幅度也小得多的現象,科學家嘗試用特大火山爆發所噴出的火山灰遮蔽了部分陽光,以及太陽輻射的極微小變化(以千分之一的幅度計算)等因素來加以解釋。電腦模擬的結果顯示,這兩項因素確能成功地解釋過去近千年來的地球溫度變化 —— 如果我們不考慮最近這數十年的話。

      有科學家曾根據氣象記錄(在一八七零年之後的時段)以及一系列間接的途徑(在一八七零年之前的時段)所重建得出的過去百年的全球溫度變化記錄,繪畫曲線並製成圖表。接著他們利用數值模型(Numerical Models)透過電腦模擬演算所得的結果,再繪製比較曲線。但期間只考慮自然因素如火山灰的影響和太陽輻射變化的影響等。他們發覺,在1970年為止,模擬演算得出的曲線與實際的溫度變化十分脗合。但自1980年以後,兩條曲線即出現明顯的分歧。

      之後,科學家加入第三條曲線。這是科學家在自然因素以外,還加進了二氧化碳水平上升,從而加劇了大氣層的溫室效應所得出的模擬溫度。很明顯地,只有加進了這個因素,才可充份地解釋過去數十年全球迅速升溫這個現象。正因如此,科學家往往把我們今天所觀測到的暖化稱為「人為全球暖化」(Anthropogenic Global Warming)。

    (原文經編輯略作刪改)

    Posted by Eddy WC Lee @ 2:19 pm

4 Responses

WP_Cloudy
  • Y S Wai Says:

    I understand that it’s urgent in the mind of many people to promote the cause. But, can any scientific theory be really incontrovertible? Don’t misinterpret my motive. Actually I believe that most probably global warming is real. The thing is that should we talk about it in such an absolute term?

  • Eddy Lee (李逆熵) Says:

    Dear YS,

    Many thanks for your comment. You’re right that no scientific theory is “incontrovertible”. But “uncertainties” is not an excuse for inaction. Suppose you’re testing a new sports car on a desert-like flat plain, and the weather becomes foggy. Now suppose somebody tells you there’s actually a cliff somewhere nearby. Would you at least slow down the car instead of keeping your foot on the gas pedal? (Which is what we’re doing exactly in terms of spewing out more and more carbon dioxide.) Or if you’re on a pleasure boat and you’ve just arrived at a beautiful bay. Your kids are about to jump into the sea, but the captain of the boat tells you that sharks have recently been sighted in this bay. Would you do nothing and let the kids go ahead, because the appearance of sharks in the here and now is “not a certainty”?

    “Uncertainties” could actually work both ways. On the one hand, the effects of man-made global warming may be much less than what scientists expect, but on the other, they could be much worse…

    Yours Truly,
    Eddy.

  • Y S Wai Says:

    I agree that we shouldn’t do nothing just because there are uncertainties. My point is that casting the issues in absolute terms could make it religious like. (I bet that the other side of the camp do that too.) Scientific hypothesis must be falsifiable, we should not regard “cold years” as well as “hot years” as evidence of global warming. We also should be engage in cherry picking for natural disasters as evidence of global warming.

    Let’s try to imagine the worst case scenarios. If the man-made global warming and its effects are really serious, I think human beings will still exist, though a large proportion of us will be killed.

    On the other hand, if either much of the current global warming is not man-made (possible though not very likely) or its effect is not so serious, and, miraculously, we are able to cut back consumption and investment to a large extent, this cut-back will trigger global recessions, with lots of sufferings worldwide, and starvation and deaths in developing countries. That would happen in capitalism and market economies. Actually, this won’t happen because at the first sign of recession, leaders of the world will abandon any cut-back they promised.

    So, in my opinion, instead of pursuing the Kyoto Protocol type strategies, we should try comfortable conservation as proposed by some scientists (Arthur Rosenfeld?). We can try to persuade people about the safety of nuclear power; to promote increased energy efficiencies, especially in developing nations; to push the other environmental friendly devices and energy sources at a price that’s not considerably higher than that of petroleum. Then, we hope that we can get away this time. If the world population will be stabilized at 9 billions as forecast and fusion power is available(in the twenty-second century?), we may be able to solve the problem in the long term.

  • Eddy Lee (李逆熵) Says:

    Dear YS,

    Many thanks for the sharing. I hope we do have the time to follow your suggested path. But mounting evidence point to the other direction i.e. not weening ourselves of fossil fuel in the next 15-20 years or so will definitely lead to a CO2 concentration exceeding 450 ppm, and the setting in of strong feedback mechansims (e.g. arctic methane release) will lead to unavoidable catastrophe. Yes, it’s not a certainty, as nothng – as rightly pointed out by you – could be couched in absolutely terms in such complicated an issue as climate change. But would we really want to bet with the future of humanity?

    I also fully agree that the human race would survive, but not cilivization as we know it today. Pardon me for being blunt, but to contemplate an outcome where “a large proportion of us will be killed” – as if it’s just one of the outcomes of a clever gambit – is to me not morally acceptable.

    Actually, many doubts and queries such as yours will be addressed in the last part of my book. The draft of the whole book has just been finished about a week ago. Me and ET Press are currently working hard to bring it to the light of day by the end of March or early April. I’m afraid ET Press may not choose to upload all the chapters onto this blog, as this may affect the sale of the book later. :-(

    Well, I do hope you would help to spread the message among your friends – and ask them to buy the book when it comes out! :-)

    Cheers!
    Eddy.

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