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Tropical glaciers are essential water resources in the central Andes as vital water resources and crucial climate indicators, currently undergoing rapid retreat. However, understanding their vulnerability to the combined effects of persistent warming, short-term climate phenomena, and interannual fluctuations remains limited. Here we automate mapping of key mass balance parameters on the Quelccaya Ice Cap (QIC), the world's largest tropical ice cap. Using Landsat's near-infrared (NIR) band, we analyze snow cover area (SCA) and total area (TA) and calculate the Accumulation Area Ratio (AAR) and Equilibrium Line Altitude (ELA) over nearly 40 years (1985–2023). Between 1985 and 2022, the QIC lost ~46 % and ~34 % of its SCA and TA, respectively. We show that the QIC's loss in SCA and rise in ELA are exacerbated by El Niño events, which are strongly correlated to the preceding wet season's Ocean Niño Index (ONI). We observe lower levels of correlation to more recent El Niño events as anthropogenic climatic impacts overwhelm the natural forcing and continue to exacerbate loss at the QIC. [ABSTRACT FROM AUTHOR]

The number of Arctic glaciers with direct, long–term measurements of mass balance is limited. Here we used satellite–based observations of the glacier snowline altitude (SLA), the location of the transition between snow cover and ice late in the summer, to approximate the position of the equilibrium line altitude (ELA)–a parameter important for mass balance assessment and for understanding the response of glaciers to climate change. We mapped the snowline (SL) on a subset of 269 land–terminating glaciers above 60 °N latitude in the latest available summer, clear–sky Landsat satellite image between 1984 and 2022. The mean SLA was extracted using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM). We compare remotely–observed SLA observations with available long–term field–based measurements of ELA and with ERA5–Land reanalysis climate data. Over the last four decades, Arctic glacier SLAs have risen an average of ~152 m (3.9±0.4 m yr–1; R2=0.74, p<0.001), with a corresponding summer (June, July, August) temperature shift of +1.2 °C at the glacier locations. This equates to a 127±5 m shift per 1 °C of summer warming. However, along with warming, we observe an overall decrease in snowfall, an increase in rainfall, and a decrease in the total number of days in which the mean daily temperature is less than or equal to 0 °C. Glacier SLA is most strongly correlated with the number of freezing days, emphasizing the dual effect of multi–decadal trends in mean annual temperature on both ablation (increasing melt) and accumulation processes (reducing the number of days in which snow can fall). Although we find evidence for a negative morpho–topographic feedback that occurs as glaciers retreat to higher elevations, we show that more than 50 % of the glaciers studied here could be entirely below the SLA by 2100, assuming the pace of global warming and the mean rate of SLA rise is maintained. [ABSTRACT FROM AUTHOR]

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 sixty–six lake– based GIC records (plus one non–lake–based record from the Russian Arctic) from seven Arctic regions: Alaska; the archipelagos of the eastern Canadian Arctic; GICs peripheral to the Greenland Ice Sheet; 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 pan–Arctic compilation suggests that the majority (50% or more) of studied GICs were smaller than present or absent by ~10 ka. The regional compilations suggest even earlier GIC loss, and thus warmth, in the Russian Arctic and in Svalbard. We find the highest percentage (>90%) of Arctic GICs smaller than present or absent in the middle Holocene ~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 pan–Arctic records also suggest two periods of enhanced GIC growth in the mid–to–late Holocene, from ~4.5–3 ka and after ~2 ka. The regional records show substantial 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. [ABSTRACT FROM AUTHOR]