100 years of changing August sea ice... Total data set available from 1850 through 2017.

Summary of methods: https://carbonbrief.org/guest-post-piecing-together-arctic-sea-ice-history-1850

Guest post: Piecing together the Arctic’s sea ice history back to 1850 - Carbon Brief

Scientists have constructed a record of Arctic sea ice history going back to 1850 - from some rather unusual sources.

Carbon Brief