Comment on egusphere-2023-737

Abstract. Most extant small ice caps mantling low-relief Arctic Canada landscapes remained cold-based throughout the late Holocene, preserving in situ bryophytes killed as ice expanded across vegetated landscapes. As Arctic summers warmed after 1900 CE, ice caps receded, exposing entombed vegetation. The calibrated radiocarbon ages of dead moss collected near ice-cap margins (kill-dates) define when ice advanced across the site, killing the moss, and remained over the site until the year of their collection. In an earlier study we reported 94 Last Millennium radiocarbon dates on in situ dead moss collected at the margins of two upland ice complexes on northern Baffin Island, Arctic Canada. Tight clustering of those ages indicated an abrupt onset of the Little Ice Age ~1240 CE, and further expansion ~1480 CE, coincident with episodes of major explosive volcanism. Here we test the confidence in kill dates as reliable predictors of expanding ice caps by re-sampling those previously sampled ice complexes 14 years later, after ~250 m of ice recession. The probability density functions (PDF) of the more recent series of ages matches PDFs of the earlier series, but with a larger fraction of early CE ages; post 2005 CE ice recession has exposed relict ice caps that grew during earlier Common Era advances, and were preserved beneath later ice-cap growth. We compare 107 kill dates from the two ice complexes with 79 kill dates from 62 other ice caps within 250 km of the two densely sampled ice complexes. The PDF of kill dates from the 62 other ice caps cluster in the same time windows as those from the two ice complexes alone, with the PDF of all 186 kill dates documenting episodes of widespread ice expansion restricted almost exclusively to 250–450 CE, 850–1000 CE and a dense early Little Ice Age cluster with peaks at ~1240 and ~1480 CE. Ice continued to expand after 1480 CE, reaching maximum dimensions ~1880 CE, still visible as zones of limited vegetation cover in remotely sensed imagery. Intervals of widespread ice-cap expansion coincide with persistent decreases in mean summer surface air temperature for the region in a Community Earth System Modeling (CESM) fully coupled Common Era simulation, suggesting primary forcing of the observed snowline lowerings were both modest declines in summer insolation, and cooling resulting from explosive volcanism, most likely intensified by positive feedbacks from sea-ice expansion and reduced northward heat transport by the oceans. The clusters of ice cap expansion defined by moss kill-dates are mirrored in an annually resolved Common Era record of ice-cap dimensions in Iceland, suggesting this is a circum-North-Atlantic-Arctic climate signal for the Common Era. During the coldest century of the Common Era, 1780–1880 CE, ice caps mantled > 11,000 km2 of north-central Baffin Island, whereas < 100 km2 is glaciated at present. That state approached conditions expected during the inception phase of an ice age, and was only reversed after 1880 CE by anthropogenic alterations of the planetary energy balance.

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