过去四十年来,北极的升温速度比地球平均快了2-4倍,引发北极海冰和陆冰迅速消融,北极航道日益进入公共议程。相较于北极,人们对于南极的关注有限,实际上,南极的气候变化更为复杂。
有别于传统认知,自1979年有卫星监测以来,直到2014年,南极海冰以每十年1%的速度在增长,2012-2014年间的三个9月,美国国家航空和航天局都曾观测到破纪录的海冰最大范围,一度让一些人声称自己找到了否定气候变化的有力证据。南极的温度究竟如何变化?南极的海冰增加了吗?极地生物又将何去何从?
北极还有冰吗?
极地的冰分陆冰和海冰。在陆地上的陆冰包括冰盖和冰架,由几千年的积雪慢慢冻结而成。冰盖厚度可以接近4公里,冰盖融化会直接引发海平面上升。冰盖延伸出海岸成为冰架或陆缘冰,冰架一小部分浮在海面上,在海面下的部分可以深达1公里。再往外延伸在海面上的是海冰,由海水结冰形成,在北极海冰被外围的陆地圈住,但南极海冰则相反,在秋冬季如果有很强的风从南极中心向外吹,海冰就会不断向外扩散,开阔的海面也会不断再次结冰。
图2 南极不同类型的冰 | 图源:http://glaciology.weebly.com/
图4 北极的海冰融化时间越来越长 | 图源[17]
南极有没有变热?
图5 南极地图,南极大陆可以分成三大块,南极半岛在西部,由于距离大陆近,从阿根廷最南边坐船约2天可达,是南极科考站聚集的地区。此外西边的大陆称为西南极,以横贯南极山脉为界,东边2/3的大陆称为东南极,占地辽阔,南极中心的南极极点也在这半边,东南极靠近大洋洲和印度洋,从澳大利亚坐船大约10天可以到达冰区 | 图源:美国国家海洋和大气管理局
图6 1958–2012年南极不同地区的温度变化,红色代表每十年的升温在0.4-0.6摄氏度,黄色代表升温不明显,在0.1-0.2摄氏度,蓝色代表温度有所下降 | 图源:美国国家海洋和大气管理局
图7 南半球环状模(Southern Annular Mode,简称SAM)主要描述环绕南半球中高纬度(南纬40 – 50度,澳大利亚最南边的塔斯马尼亚岛的位置)的西风带的活动。图中的三条带箭头的黑线就是SAM,一般位置如图所示,但SAM会随着气候的变化发生异常,整体向北或是向南平移,影响包括澳大利亚、南极洲在内的南半球的气候,例如当地的降水量、气温等等。| 图源 [30]
南极的冰有没有增加?
图8 1979年有卫星监测纪录以来,历史上海冰范围的最大最小纪录都发生在近十年:两次海冰范围最大纪录在2013、2014年,两次最小纪录2017、2018年。今年夏天南极夏季海冰范围又一次达到最小值 | 图源[36]
1. 强风促进海冰形成 [37]。如前所述,海冰形成期如果有很强的离岸风把冰往外吹,可以加速海冰形成和扩散。由于温室气体增加,加上拉尼娜现象等因素,过去三十年SAM持续增强 [38],可以帮助海冰扩散,降低东南极海面温度,从而增加海冰总体覆盖率。
2. 南极冰冻层的融化维持了海冰的相对稳定 [39]。和海冰不一样,南极的冰架已经发生了大面积崩裂和融化,一种理论说这些融化的冰水汇集在海面,像天然隔热层一样,让我们在南极看到了相对稳定的海冰覆盖。
海冰变化是大气环流、海流3D运动、以及当地小气候的综合结果,还需要考虑海水-大气-海冰三者的相互作用,也是由于这样的复杂性,再加上目前我们对南极海冰的观测数据仍然有限,模式研发也不够成熟,因此我们对南极海冰的未来究竟如何仍有很多未知。戴爱国说,“虽然现在看起来南极的海冰变化不大,但气候模型预测在接下来的几十年里如果南极温度升高,会有大量的海冰流失。”
和海冰的趋势不同的是,南极的陆冰越来越不稳定了。2021年8月的政府间气候变化专门委员会第六次评估报告显示,如果全球温度上升2-3摄氏度,南极西部的冰盖将全部消失。而南极海边的冰架崩解也在日益严峻。
图9 温暖的次表层水加速冰川融化 | 图源[46]
极地生物将何去何从?
图10 北极生态圈图示 | 图源:Polar Bear International
图11 南极磷虾捕捞区和捕捞量 | 图源[47]
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