(Money quote: Here we show marine proxy evidence for the disappearance of perennial sea-ice in the southern Lincoln Sea during the Early Holocene, which suggests a widespread transition to seasonal sea-ice in the Arctic Ocean.)
New evidence that polar bears survived 1,600 years of ice-free summers in the early Holocene
New evidence indicates that Arctic areas with the thickest ice today probably melted out every year during the summer for about 1,600 years during the early Holocene (ca. 11.3-9.7k years ago), making the Arctic virtually ice-free. As I argue in my new book, this means that polar bears and other Arctic species are capable of surviving extended periods with ice-free summers: otherwise, they would not be alive today.
Last Ice Area and Lincoln Sea
An illustration of the Last Ice Area in the Arctic, which is currently covered in perennial ice (2-4m thick) that does not melt out every summer.
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Figure S5 from the Detlef paper (below) shows that reduced sea ice during the early Holocene was widespread, with evidence for seasonal ice in the Barents, Beaufort and Laptev Seas as well as NE Greenland and the Lincoln Sea between northern Greenland and Ellesmere Island.
Evidence from several sources indicates that the Eemian produced conditions even warmer than documented during the early Holocene and they lasted longer, as explained in the excellent summary by Leonid Polyak and colleagues (2010). During the early portion of the Eemian at least (ca. 130-120k years ago), summer temperatures were about 5–8 degrees Celsius warmer than today and the Arctic was virtually ice-free. At about 120k years ago, there is evidence from Finland and the Norwegian Sea off Norway that a cooling event lasting 500-1,000 years broke the long stretch of warmth (Helmens et al. 2015).
Not only did polar bear survive these two extended periods when ice-free summers prevailed, but the Eemian warm summers came only about 10,000 years after the bears arose as a unique species. This makes polar bear survival through the Eemian even more impressive than most scientists acknowledge. The polar bears’ ability to store excess energy as fat in the spring and metabolize it later when needed must have been fine-tuned by natural selection during this challlenging time
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https://polarbearscience.com/2023/06/16/new-evidence-that-polar-bears-survived-1600-years-of-ice-free-summers-in-the-early-holocene/Seasonal sea-ice in the Arctic’s last ice area during the Early Holocene
https://www.nature.com/articles/s43247-023-00720-wAccording to climate models, the Lincoln Sea, bordering northern Greenland and Canada, will be the final stronghold of perennial Arctic sea-ice in a warming climate. However, recent observations of prolonged periods of open water raise concerns regarding its long-term stability. Modelling studies suggest a transition from perennial to seasonal sea-ice during the Early Holocene, a period of elevated global temperatures around 10,000 years ago. Here we show marine proxy evidence for the disappearance of perennial sea-ice in the southern Lincoln Sea during the Early Holocene, which suggests a widespread transition to seasonal sea-ice in the Arctic Ocean. Seasonal sea-ice conditions were tightly coupled to regional atmospheric temperatures. In light of anthropogenic warming and Arctic amplification our results suggest an imminent transition to seasonal sea-ice in the southern Lincoln Sea, even if the global temperature rise is kept below a threshold of 2 °C compared to pre-industrial (1850–1900).
Defining the “Ice Shed” of the Arctic Ocean's Last Ice Area and Its Future Evolution
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021EF001988
