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The recent retreat of nearly all glaciers and ice caps (GICs) located in Arctic regions is one of the most clear and visible signs of ongoing climate change. This paper synthesizes published records of Holocene GIC fluctuations from lake archives, placing their recent retreat into a longer-term context. Our compilation includes 66 lake-based GIC records (plus one non-lake-based record from the Russian Arctic) from seven Arctic regions: Alaska, Baffin Island in northeastern Canada, Greenland, Iceland, the Scandinavian peninsula, Svalbard, and the Russian high Arctic. For each region and for the full Arctic, we summarize evidence for when GICs were smaller than today or absent altogether, indicating warmer-than-present summers, and evidence for when GICs regrew in lake catchments, indicating summer cooling. Consistent with orbitally driven high boreal summer insolation in the early Holocene, the full Arctic compilation suggests that the majority (50 % or more) of studied GICs were smaller than present or absent by ∼10 ka. We find the highest percentage ( >90 %) of Arctic GICs smaller than present or absent in the middle Holocene at ∼ 7–6 ka, probably reflecting more spatially ubiquitous and consistent summer warmth during this period than in the early Holocene. Following this interval of widespread warmth, our compilation shows that GICs across the Arctic began to regrow and summers began to cool by ∼6 ka. Together, the Arctic records also suggest two periods of enhanced GIC growth in the middle to late Holocene from ∼ 4.5–3 and after ∼2 ka. The regional records show variability in the timing of GIC regrowth within and between regions, suggesting that the Arctic did not cool synchronously despite the smooth and hemispherically symmetric decline in Northern Hemisphere summer insolation. In agreement with other studies, this implies a combined response to glacier-specific characteristics such as topography and to other climatic forcings and feedback mechanisms, perhaps driving periods of increased regional cooling. Today, the direction of orbital forcing continues to favor GIC expansion; however, the rapid retreat of nearly all Arctic GICs underscores the current dominance of anthropogenic forcing on GIC mass balance. Our review finds that in the first half of the Holocene, most of the Arctic's small GICs became significantly reduced or melted away completely in response to summer temperatures that, on average, were only moderately warmer than today. In comparison, future projections of temperature change in the Arctic far exceed estimated early Holocene values in most locations, portending the eventual loss of most of the Arctic's small GICs.

Larocca, L. J. and Axford, Y.: Arctic glaciers and ice caps through the Holocene:a circumpolar synthesis of lake-based reconstructions, Clim. Past, 18, 579–606, https://doi.org/10.5194/cp-18-579-2022, 2022.

Globally, mass loss from glaciers and ice caps (GICs) is accelerating (Hugonnet et al., 2021). Between 2000 and 2019, GICs worldwide lost a mass of 267±16 Gt yr −1 , which is equivalent to 21±3 % of the observed sea-level rise (Hugonnet et al., 2021). Notably, the roughly 50 000 GICs located in Arctic regions accounted for ∼70 % of this recent loss (Hugonnet et al., 2021, Table 1; Vaughan et al., 2013). By the end of the century, mean surface air temperature in the Arctic is expected to warm by 2.2–8.3 ∘ C – a rate that is amplified relative to the global mean (Collins et al., 2013, temperature anomalies relative to the 1986–2005 reference period). Accordingly, regionally differentiated global-scale projections of GIC mass change find Arctic GICs to be the largest contributors to forecasted global ice volume loss by 2100 (Radić and Hock, 2011; Radić et al., 2014). The continued wastage of Arctic GICs is expected to have a myriad of sociocultural and economic ramifications for Arctic communities, including major alterations to hydrological systems at a local scale, potentially affecting water availability, quality, and downstream aquatic ecosystems (Huntington et al., 2019; IPCC, 2007).

The current and projected rapid changes to the Arctic cryosphere are even more striking when considered within a longer-term context (e.g., Kaufman et al., 2009; Fisher et al., 2012; Miller et al., 2013). GIC fluctuations over the Holocene have been reconstructed using a mix of discontinuous and continuous proxies including the mapping and dating of glacial moraines, lichenometry and tree ring records, and proglacial lake and speleothem records (Solomina et al., 2015). Overall, these records indicate that many Arctic GICs were small or had completely melted away in the early to middle Holocene in response to orbitally forced summer warmth in the Northern Hemisphere (Solomina et al., 2015). These records also show that many of the GICs melting away today reformed during the middle to late Holocene as summer temperature cooled from the insolation-driven Holocene maximum (McKay et al., 2018; Solomina et al., 2015). Recent anthropogenically driven warming has sharply reversed this long-term, insolation-driven, cooling trend and GIC expansion, and it is expected to cut short the life spans of Arctic GICs that in many cases have existed for several thousand years.

While relatively widely studied, many questions linger about Holocene climate. Globally, climate simulations and proxy reconstructions fundamentally disagree on the overall direction of mean annual temperature trends through the Holocene, a significant data–model discrepancy coined the Holocene temperature conundrum (Liu et al., 2014). Furthermore, to date, there have been few published multi-proxy syntheses of Holocene climate specific to the Arctic, and most have been focused on sub-regions or narrower time periods (e.g., Kaufman et al., 2004, 2009; Kaplan and Wolfe, 2006; Briner et al., 2016; McKay et al., 2018; Axford et al., 2021). Although chiefly driven by symmetrical orbital forcing, synthesis studies of temperature-sensitive proxy data suggest that the timing and magnitude of the Holocene Thermal Maximum (HTM) were spatially and temporally asymmetrical across the Arctic (e.g., Kaufman et al., 2004; Kaplan and Wolfe, 2006; Briner et al., 2016). Likewise, the onset and rate of summer cooling in the Arctic in the middle to late Holocene did not occur concomitantly (McKay et al., 2018). These syntheses reaffirm the notion that the Arctic does not behave as a distinct climatological unit and that the climatic responses to insolation and other forcings through the Holocene were complex (McKay et al., 2018). An improved understanding of the regional and Arctic-wide patterns of multi-millennial Holocene temperature changes may elucidate the driving factors that control regional climates and thus help anticipate the local-scale consequences of future Arctic warming.

Recent observations have confirmed that GICs respond sensitively and quickly on decadal timescales, primarily to changes in summer temperature and, to a lesser extent, accumulation season precipitation (Oerlemans, 2005; Koerner, 2005; Bjørk et al., 2012). Sediment records from glacial lakes offer invaluable continuous archives of GIC variations over the Holocene, recording their presence and absence on the landscape and in some cases more subtle variations in GIC size over time. Records of GIC fluctuations from lake sediments can be considered a relatively straightforward qualitative proxy for summer temperature. GICs' exceptional sensitivity to modest summer temperature changes make their individual archives important indicators of regional climate and their combined archives an important gauge of broad, large-scale climate trends through the Holocene (Kelly and Lowell, 2009; Solomina et al., 2015). Here, we synthesize published lake-based GIC records ( n =66 , plus one non-lake-based record from Franz Josef Land, included due to a dearth of records from the Russian Arctic) to assess regional- and Arctic-scale summer temperature trends through the Holocene. Our review covers seven geographical regions above 58 ∘ N from which such records are available: Alaska, Baffin Island (Canada), Greenland, Iceland, the Scandinavian peninsula, Svalbard, and the Russian high Arctic. For each region, we summarize evidence for when GICs were smaller than today or absent altogether, documenting the timing of warmer-than-present summer conditions. In addition, we summarize evidence of GIC regrowth in lake catchments, documenting summer cooling and, specifically, when equilibrium-line altitudes (ELAs) first lowered to intersect the local topography.