米兰科维奇冰期旋回理论:挑战与机遇

米兰科维奇理论认为,北半球高纬夏季太阳辐射变化是驱动第四纪冰期旋回的主因。这个理论的核心是单一敏感区的触发驱动机制,即北半球高纬气候变化信号被放大、传输进而影响全球。最近,由于大量高分辨率及精确定年的气候变化记录的获得,从以下4个方面构成了对米氏理论的挑战:1)一些低纬地区并没有明显的10万年冰量周期,而是以2万年岁差周期为主,表明北半球冰盖的扩张、收缩变化并没有完全控制低纬区的气候变化;2)在最近几次冰消期时,南半球和低纬区的温度增高,要早于北半球冰盖的融化,表明冰消期的触发机制并非是北半球高纬夏季太阳辐射;3)大气CO₂浓度在第2冰消期的增加同南极升温相一致,表明该时大气CO₂浓度增加亦有可能早于北半球冰盖消融;4)南半球的末次冰盛期有可能早于北半球。这就说明单一敏感区触发驱动机制已难以圆满解释所有观察事实,天文因素控制下轨道尺度气候变化机制研究正面临理论突破的新需求和新机遇。

THE MILANKOVITCH THEORY OF PLEISTOCENE GLACIAL CYCLES: CHALLENGES AND CHANCES

Milankovitch theory holds that climatic changes of the Pleistocene have been forced by cyclic variations in summer insolation around 65°N of the northern hemisphere. The signatures originated in this region can be amplified by positive feedbacks and propagated into other parts of the climate system through processes such as changes in atmospheric carbon dioxide concentrations and North Atlantic Deep-Water formation. Oxygen isotopic records of deep-sea sediments have shown that episodic growth and decay of the polar ice sheets of the northern hemisphere was dominated by a period of about 100000 years during the Late Pleistocene. Other climatic records derived from the high and middle latitudes of both hemispheres also have a strong 100000-year period, thus broadly supporting the Milankovitch theory. However, recent studies show that in some parts of the tropical region on the globe, a 20000-year precession cycle is predominant during the Late Pleistocene climate changes, suggesting the importance of local insolation forcing. Phase-relation studies indicate that during the last terminations, the warming of the Antarctica and the equatorial region occurred several thousand years earlier than the melting of the ice sheets of the northern hemisphere. An earlier increase in atmospheric CO₂ concentrations has also been recognized for the penultimate termination. These lines of evidence are inconsistent with the ice-volume forcing model that was implied by the Milankovitch theory. Some accurately-dated climate records even show an earlier Last Glacial Maximum in the southern hemisphere relative to that in the northern hemisphere. All these facts mentioned above suggest that the forcing mechanism for the Pleistocene glacial cycle could have been more complicated than that held by the Milankovitch theory.