Description: Records of Neoglacial glacier activity in the Arctic constructed from moraines are often incomplete due to a preservation bias toward the most extensive advance, usually the Little Ice Age. Recent warming in the Arctic has caused extensive retreat of glaciers over the past several decades, exposing preserved landscapes complete with in situ tundra plants previously entombed by ice. The radiocarbon ages of these plants define the timing of snowline depression and glacier advance across the site, in response to local summer cooling. Although most dead plants recently exposed by ice retreat are rapidly removed from the landscape by erosion, where erosive processes are unusually weak, dead plants may remain preserved on the landscape for decades. In such settings, a transect of plant radiocarbon ages can be used to construct a near-continuous chronology of past ice margin advance. Here we present radiocarbon dates from the first such transect on Baffin Island, which directly dates the advance of a small ice cap over the past two millennia. The nature of ice expansion between 20 BCE and ~1000 CE is still uncertain, but episodic advances at ~1000, ~1200, and ~1500 CE led to the maximum Neoglacial dimensions ~1900 CE. We employ a two-dimensional numerical glacier model to reconstruct the pattern of ice expansion inferred from the radiocarbon ages and to explore the sensitivity of the ice cap to temperature change. Model experiments show that at least ~0.44 °C of cooling over the past 2 ka is required for the ice cap to reach its 1900 margin, and that the period from ~1000 to 1900 CE must have been at least 0.25 °C cooler than the previous millennium; results that agree with regional climate model simulations. However, ~3 °C of warming since 1900 CE is required to explain retreat to its present position, and, at the same rate of warming, the ice cap will disappear before 2100 CE.

Description: Benthic foraminifera stable isotopes values from Shatsky Rise 1209 and 1210. A synthesis of benthic foraminifera d13C values from the mid-Pliocene warm period and early Pliocene and OC3 products pre-Industrial water column d13C and Late Holocene core top benthic foraminifera d13C.

Description: Modelling output from a Late Pliocene and Early Pliocene geochemically enabled CESM 1.2.2.1. The CESM 1.2.2.1 code used to run the climate simulations is available from https://svn-ccsm-models.cgd.ucar.edu/cesm1/release_tags/cesm1_2_2_1. The code modifications made to CESM 1.2.2.1 to include carbon isotopes following ref. 38 are available from https://github.com/nburls/FordEtAl2022_CISOmods. Figures with coastal outlines (for example, Figs. 1 and 2 and Extended Data Figs. 1 and 2) were created in Matlab with the M_maps package71.