Arctic sea-ice decline archived by multicentury annual-resolution record from crustose coralline algal proxy

The most concerning example of ongoing climate change is the rapid Arctic sea-ice retreat. While just a few years ago ice-free Arctic summers were expected by the end of this century, current models predict this to happen by 2030. This shows that our understanding of rapid changes in the cryosphere is limited, which is largely due to a lack of long-term observations. Newly discovered long-lived algae growing on the Arctic seafloor and forming tree-ring-like growth bands in a hard, calcified crust have recorded centuries of sea-ice history. The algae show that, while fast short-term changes have occurred in the past, the 20th century exhibited the lowest sea-ice cover in the past 646 years.