Global glacial inception threshold from positive degree-day modelling

Glaciations of the Pleistocene have left a global imprint, expanding from polar plains to equatorial mountains on all continents. This glacial record has been systematically researched for nearly two centuries. However, its diversity, as well as fieldwork logistics, ice-flow modelling challenges and paleoclimate unknowns have often constrained paleoglacier studies to remain regional.Here, we present a global, 30 arcsec resolution map of temperature change needed to initiate glacier growth, hereafter glacial inception threshold. Using downscaled climatologies from CHELSA-2.1 and CHELSA-W5E5 over the period 1981–2010, a positive-degree-day snow accumulation and melt model is applied globally with temperature anomalies ranging from +5 to -20 K to compute the glacial inception threshold.Glacial inception accelerates for temperature anomalies below -3 K in North America and Asia, and below -5 K in Europe and South America, but remains limited to localized highlands for all tested temperatures in Africa and Oceania. The inception threshold is locally sensitive to the choice of input climate dataset, particularly in Central Asia and Patagonia. Under 1981–2010 conditions, mountain glaciers form at altitudes following a camel curve peaking at 7 km in the tropics, and lowering to about 1 km along the polar circles. An inception threshold of 0 represents contemporaneous accumulation limits, and indeed this pattern is comparable to modelled equilibrium lines for all glaciers on Earth from the Open Global Glacier Model (OGGM) and the Global Glacier Evolution Model (GloGEM).Under Last Glacial Maximum climate from the Paleoclimate Modelling Intercomparison Project Phase 4 (PMIP4), glacial inception areas expand to lower elevations. Validation against paleo-equilibrium line reconstructions from small glaciers in the Cordilleras shows good agreement in the mid-latitudes but underpredicted glaciation in the tropics. Further comparison to global glacial extents and cosmogenic isotope dates show that our glacial inception threshold map reproduces many known glacier and ice-sheet inception centres, while also hinting at potentially undocumented mountain glaciations. While our map does not account for glacier expansion from ice flow, we hope it will help identify potential targets for future field and modelling studies, and provide a foundation towards global paleoglacier research.