Last Glacial Maximum Reconstructions of Rwenzori Mountain Glaciers

The magnitude of tropical cooling during the Last Glacial Maximum (LGM; similar to 19-26.5 ka) remains controversial, with sea-surface temperatures cooling by several degrees less than most temperatures reconstructed at high elevations. To explain this discrepancy, past studies proposed a steeper (increased) lapse rate-the temperature decrease with elevation-during the LGM relative to today. For instance, LGM temperatures in East Africa reconstructed from branched GDGTs from multiple elevations support an similar to 0.9 degrees C/ km increase in the lapse rate during the LGM relative to present day. Lapse rates are a critical part of the Earth's climate sensitivity and atmospheric energy transfer, and it is vital to know whether and by how much the tropical lapse rate steepened during the LGM. Here, we simulate LGM glacier extents in the Rwenzori Mountains of Uganda with and without a change in lapse rate using a range of temperature and precipitation estimates. We find that the lapse rate must have been steeper than present for glaciers to reach their LGM positions using available sea-level temperature and precipitation estimates for East Africa.