Climate change in the Arctic is occurring rapidly, and projections suggest the complete loss of summer sea ice by the middle of this century¹. 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 Hemisphere². 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. ³), 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^4,5,6, which can lead to warming far inland⁷. Open Arctic waters also increase the source of moisture and increase autumn snowfall over Siberia, insulating the ground from low winter temperatures^8,9,10. 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.

北极的气候变化正在迅速发生，预测表明到本世纪中叶夏季海冰将完全消失¹. 北半球永久冻土（永久冻土）对变暖的敏感性尚不清楚，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很强的抵抗力，但在没有北极海冰时则很脆弱。位于连续永久冻土南部边缘的西伯利亚洞穴中碳酸盐矿床（洞穴）的铀铅年代学揭示了上覆地面未永久冻结的时期。speleothem 记录开始于 150 万年前 (Ma)，当时赤道到极地的热传输增加导致北半球变暖². 洞穴的生长表明当时洞穴遗址的永久冻土不存在，随着北半球的冷却，从大约 1.35 Ma 开始变得更加频繁，并且在大约 0.4 Ma 之后永久冻土。这段历史反映了北冰洋全年海冰的历史，在大约 0.4 Ma 之前基本上不存在海冰（参考文献³），但自该日期以来一直存在。当存在海冰时，永久冻土的坚固性，以及当没有海冰时，永久冻土的脆弱性增加，可以通过热量和水分输送的变化来解释。海冰减少可能导致北极空气变暖^4,5,6，这可能导致远内陆地区变暖⁷. 开放的北极水域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 8,9,10}。这些过程解释了无冰北极与 0.4 Ma 之前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消失将加速西伯利亚永久冻土的融化。