Thousands of fossilized Metasequoia trees that grew near 80 degrees N latitude during the middle Eocene (~45 Ma) in Nunavut, Canada beg questions concerning how these forests could thrive in an Arctic four-month continuous-light growing season. The spectacular preservation of these fossils allowed for d¹⁸O and dD analyses of cellulose, techniques previously relegated to wood < ~30,000 years old. From this, Eocene atmospheric moisture levels were determined to be 20 to 120 % higher than modern Arctic levels, indicating an atmosphere that thermally insulated the polar region by impeding surface re-radiation, and increased the trees' radiation use efficiency (RUE) during the short growing season.