過(guò)去四十年來(lái)，北極的升溫速度比地球平均快了2-4倍，引發(fā)北極海冰和陸冰迅速消融，北極航道日益進(jìn)入公共議程。相較于北極，人們對(duì)于南極的關(guān)注有限，實(shí)際上，南極的氣候變化更為復(fù)雜。
有別于傳統(tǒng)認(rèn)知，自1979年有衛(wèi)星監(jiān)測(cè)以來(lái)，直到2014年，南極海冰以每十年1%的速度在增長(zhǎng)，2012-2014年間的三個(gè)9月，美國(guó)國(guó)家航空和航天局都曾觀測(cè)到破紀(jì)錄的海冰最大范圍，一度讓一些人聲稱自己找到了否定氣候變化的有力證據(jù)。南極的溫度究竟如何變化？南極的海冰增加了嗎？極地生物又將何去何從？
北極還有冰嗎？
極地的冰分陸冰和海冰。在陸地上的陸冰包括冰蓋和冰架，由幾千年的積雪慢慢凍結(jié)而成。冰蓋厚度可以接近4公里，冰蓋融化會(huì)直接引發(fā)海平面上升。冰蓋延伸出海岸成為冰架或陸緣冰，冰架一小部分浮在海面上，在海面下的部分可以深達(dá)1公里。再往外延伸在海面上的是海冰，由海水結(jié)冰形成，在北極海冰被外圍的陸地圈住，但南極海冰則相反，在秋冬季如果有很強(qiáng)的風(fēng)從南極中心向外吹，海冰就會(huì)不斷向外擴(kuò)散，開(kāi)闊的海面也會(huì)不斷再次結(jié)冰。
圖2 南極不同類型的冰 | 圖源：http://glaciology.weebly.com/
圖4 北極的海冰融化時(shí)間越來(lái)越長(zhǎng) | 圖源[17]
南極有沒(méi)有變熱？
圖5 南極地圖，南極大陸可以分成三大塊，南極半島在西部，由于距離大陸近，從阿根廷最南邊坐船約2天可達(dá)，是南極科考站聚集的地區(qū)。此外西邊的大陸稱為西南極，以橫貫?zāi)蠘O山脈為界，東邊2/3的大陸稱為東南極，占地遼闊，南極中心的南極極點(diǎn)也在這半邊，東南極靠近大洋洲和印度洋，從澳大利亞坐船大約10天可以到達(dá)冰區(qū) | 圖源：美國(guó)國(guó)家海洋和大氣管理局
圖6 1958–2012年南極不同地區(qū)的溫度變化，紅色代表每十年的升溫在0.4-0.6攝氏度，黃色代表升溫不明顯，在0.1-0.2攝氏度，藍(lán)色代表溫度有所下降 | 圖源：美國(guó)國(guó)家海洋和大氣管理局
圖7 南半球環(huán)狀模（Southern Annular Mode，簡(jiǎn)稱SAM）主要描述環(huán)繞南半球中高緯度（南緯40 – 50度，澳大利亞最南邊的塔斯馬尼亞島的位置）的西風(fēng)帶的活動(dòng)。圖中的三條帶箭頭的黑線就是SAM，一般位置如圖所示，但SAM會(huì)隨著氣候的變化發(fā)生異常，整體向北或是向南平移，影響包括澳大利亞、南極洲在內(nèi)的南半球的氣候，例如當(dāng)?shù)氐慕邓?、氣溫等等?/span>| 圖源 [30]
南極的冰有沒(méi)有增加？
圖8 1979年有衛(wèi)星監(jiān)測(cè)紀(jì)錄以來(lái)，歷史上海冰范圍的最大最小紀(jì)錄都發(fā)生在近十年：兩次海冰范圍最大紀(jì)錄在2013、2014年，兩次最小紀(jì)錄2017、2018年。今年夏天南極夏季海冰范圍又一次達(dá)到最小值 | 圖源[36]
1. 強(qiáng)風(fēng)促進(jìn)海冰形成 [37]。如前所述，海冰形成期如果有很強(qiáng)的離岸風(fēng)把冰往外吹，可以加速海冰形成和擴(kuò)散。由于溫室氣體增加，加上拉尼娜現(xiàn)象等因素，過(guò)去三十年SAM持續(xù)增強(qiáng) [38]，可以幫助海冰擴(kuò)散，降低東南極海面溫度，從而增加海冰總體覆蓋率。
2. 南極冰凍層的融化維持了海冰的相對(duì)穩(wěn)定 [39]。和海冰不一樣，南極的冰架已經(jīng)發(fā)生了大面積崩裂和融化，一種理論說(shuō)這些融化的冰水匯集在海面，像天然隔熱層一樣，讓我們?cè)谀蠘O看到了相對(duì)穩(wěn)定的海冰覆蓋。
海冰變化是大氣環(huán)流、海流3D運(yùn)動(dòng)、以及當(dāng)?shù)匦夂虻木C合結(jié)果，還需要考慮海水-大氣-海冰三者的相互作用，也是由于這樣的復(fù)雜性，再加上目前我們對(duì)南極海冰的觀測(cè)數(shù)據(jù)仍然有限，模式研發(fā)也不夠成熟，因此我們對(duì)南極海冰的未來(lái)究竟如何仍有很多未知。戴愛(ài)國(guó)說(shuō)，“雖然現(xiàn)在看起來(lái)南極的海冰變化不大，但氣候模型預(yù)測(cè)在接下來(lái)的幾十年里如果南極溫度升高，會(huì)有大量的海冰流失?！?/span>
和海冰的趨勢(shì)不同的是，南極的陸冰越來(lái)越不穩(wěn)定了。2021年8月的政府間氣候變化專門(mén)委員會(huì)第六次評(píng)估報(bào)告顯示，如果全球溫度上升2-3攝氏度，南極西部的冰蓋將全部消失。而南極海邊的冰架崩解也在日益嚴(yán)峻。
圖9 溫暖的次表層水加速冰川融化 | 圖源[46]
極地生物將何去何從？
圖10 北極生態(tài)圈圖示 | 圖源：Polar Bear International
圖11 南極磷蝦捕撈區(qū)和捕撈量 | 圖源[47]
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