Speaker: Meredith Kelly, Professor, Department of Earth Sciences, Dartmouth College

Host: Nathan Bangs

Title: Tropical high-altitude climate conditions during and after the lastglacial maximum

Abstract: The tropics are the “heat engine” of the planet and currently influence some of the largest natural climate variations through the El Niño-Southern Oscillation.  While changes in tropical climate can have globally cascading effects, the role of the tropics in past climate changes, such as Quaternary glacial-interglacial cycles, is not well understood.  A particularly important knowledge gap is the timing and magnitude of past temperature changes in tropical terrestrial regions, where few data exist relative to higher latitude locations.  I will present reconstructions of temperature during and after the last glacial maximum (LGM; ~26-19 ka), based on past glacial extents in the Rwenzori Mountain sin Uganda, Chirripó National Park in Costa Rica, and the Páramo del Sol in Colombia.  At these sites, my research group has constrained the timing of past glacial extents using surficial mapping and ¹⁰Be surface exposure dating, and inferred past climate conditions using glacial modeling and analyses of lake sediment cores.  The results show a consistent pattern of~7-9°C of cooling relative to preindustrial time at high elevations across the tropics.  While this magnitude of cooling is larger than that inferred from tropical sea-surface temperature proxies, the records can be reconciled by assuming a steepening of the lapse rate during, consistent with prior work in the Rwenzori Mountains using lake sediments.  Together, these data suggest that the lapse rate was steeper during the LGM, indicating a drier tropical atmosphere.  Post-LGM glacier recession in the tropics occurred prior to the rise in atmospheric greenhouse gases, suggesting an alternative mechanism for tropical warming at the end of the last glacial period, possibly related to atmospheric moisture.