Timing and Magnitude of Last Deglacial Warming in High-Elevation Regions of Eastern Equatorial Africa

Abstract:

Tropical mountains, such as the Rwenzori Mountains, Uganda in eastern equatorial Africa (EEA), are particularly sensitive to anthropogenic climate change. Recent warming in the Rwenzori is negatively impacting local environments and communities through glacial retreat, fires, and flooding, but historical climate observations are extremely limited. Reconstructions of East African temperatures can improve our understanding of climate change in this sensitive region. The Last Glacial Maximum (LGM; 25-19 kyr) and subsequent deglaciation are an important target for paleoclimate data-model comparisons, as they are the last time warming occurred due to rising greenhouse gases (GHGs). Here, we present a 21.5 kyr temperature reconstruction based on the relative abundance of branched glycerol dialkyl glycerol tetraethers (brGDGTs), from Lake Mahoma (2990 m) in the Rwenzori. Our record and Rwenzori glacial moraine exposure ages indicate significant agreement during the deglaciation and suggest deglacial warming and glacial retreat began at about 21 kyr but intensified at 18 kyr, which corresponds with increased radiative forcing from GHGs. Our temperature reconstruction records ~5ºC warming from the LGM to late-Holocene. This increase is larger than the average ~3ºC warming observed in brGDGT records from lower elevation sites in EEA, but similar to warming observed at other high-elevation sites in EEA. This confirms previous work suggesting that the temperature lapse rate steepened during the LGM over this region. Our results also indicate ~3ºC warming during the mid-Holocene, which suggests the freezing level height was high enough to deglaciate the Rwenzori Mountains. A similar warming, and therefore deglaciation, is likely within the next few decades. Overall, the timing and magnitude of temperature change observed in our record has important implications for climate model projections of future warming in tropical Africa.