Palaeoclimate evidence of vulnerable permafrost during times of low sea ice
Palaeoclimate evidence of vulnerable permafrost during times of low sea ice
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DOI：
10.1038/s41586-019-1880-1
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发表时间：
2020-01
期刊：
影响因子：
64.8
通讯作者：
A. Vaks;A. Mason;S. F. M. Breitenbach;Aleksandr Kononov;Alexander V. Osinzev;M. Rosensaft;A. Borshevsky;Oksana Gutareva;Gideon M. Henderson
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文献类型：
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作者：
A. Vaks;A. Mason;S. F. M. Breitenbach;Aleksandr Kononov;Alexander V. Osinzev;M. Rosensaft;A. Borshevsky;Oksana Gutareva;Gideon M. Henderson
北极的气候变化正在迅速发生，预测表明，到本世纪中期，夏季海冰将完全消失。北方半球的永久冻土（永冻层）对变暖的敏感性不太清楚，其长期趋势比海冰更难监测。在这里，我们使用古气候数据表明，当北极海冰存在时，西伯利亚永久冻土对变暖具有鲁棒性，但当北极海冰不存在时，它就很脆弱。位于连续多年冻土南部边缘的西伯利亚洞穴中的碳酸盐沉积物（洞穴沉积物）的铀铅年代学揭示了上覆地面没有永久冻结的时期。洞穴沉积物记录始于150万年前（Ma），当时赤道到两极的热量输送更大，导致北方半球更温暖。洞穴沉积物的生长表明，洞穴遗址的永久冻土在当时是不存在的，从大约1.35 Ma开始，随着北方半球的冷却，永久冻土变得更加频繁，并在大约0.4 Ma后永久化。这一历史反映了北冰洋全年海冰的情况，在大约0.4 Ma之前基本上不存在，但自该日期以来一直存在。当海冰存在时，永久冻土的坚固性，以及当海冰不存在时，永久冻土的脆弱性增加，可以用热量和水分输送的变化来解释。海冰的减少可能导致北极空气变暖，这可能导致内陆地区变暖。开放的北极沃茨也增加了水分的来源，增加了西伯利亚的秋季降雪，使地面免受冬季低温的影响。这些过程解释了0.4 Ma前北极无冰和永久冻土融化之间的关系。如果这些过程在现代气候变化期间继续下去，未来夏季北极海冰的损失将加速西伯利亚永久冻土的融化。
Climate change in the Arctic is occurring rapidly, and projections suggest the complete loss of summer sea ice by the middle of this century. The sensitivity of permanently frozen ground (permafrost) in the Northern Hemisphere to warming is less clear, and its long-term trends are harder to monitor than those of sea ice. Here we use palaeoclimate data to show that Siberian permafrost is robust to warming when Arctic sea ice is present, but vulnerable when it is absent. Uranium–lead chronology of carbonate deposits (speleothems) in a Siberian cave located at the southern edge of continuous permafrost reveals periods in which the overlying ground was not permanently frozen. The speleothem record starts 1.5 million years ago (Ma), a time when greater equator-to-pole heat transport led to a warmer Northern Hemisphere. The growth of the speleothems indicates that permafrost at the cave site was absent at that time, becoming more frequent from about 1.35 Ma, as the Northern Hemisphere cooled, and permanent after about 0.4 Ma. This history mirrors that of year-round sea ice in the Arctic Ocean, which was largely absent before about 0.4 Ma (ref. ), but continuously present since that date. The robustness of permafrost when sea ice is present, as well as the increased permafrost vulnerability when sea ice is absent, can be explained by changes in both heat and moisture transport. Reduced sea ice may contribute to warming of Arctic air, –, which can lead to warming far inland. Open Arctic waters also increase the source of moisture and increase autumn snowfall over Siberia, insulating the ground from low winter temperatures, –. These processes explain the relationship between an ice-free Arctic and permafrost thawing before 0.4 Ma. If these processes continue during modern climate change, future loss of summer Arctic sea ice will accelerate the thawing of Siberian permafrost.