Search

Warming following the Last Glacial Maximum was interrupted by abrupt, millennial-scale climate reversals. Registered in high-latitude proxy records, these reversals were apparently anti-phased between hemispheres. Although the signals of these late-glacial (~15-11 ka) climate events are found far beyond the poles, the thermal footprint of these events around the globe remains enigmatic. This is particularly true in low-latitude regions, where terrestrial temperature records are relatively rare. Here we present a beryllium-10 chronology of glacier fluctuations from the equatorial Rwenzori Mountains, Uganda, which elucidates the timing and magnitude of deglacial warming in tropical Africa during late-glacial time. In the Bujuku valley, a suite of moraines that mark the maximum late-glacial ice extent in the valley date to ~15-14 ka. Up valley from these deposits, a lateral moraine dated to ~11.8 ka marks the last preserved late-glacial ice margin. In the Nyamugasani valley, beryllium-10 ages suggest a similar pattern of glacier fluctuations. Expanded glacier conditions are marked by perched boulders on bedrock dated to ~14 ka. After ~12 ka, glacier retreat was punctuated by stillstands or readvances until ~11.2 ka. In both the Bujuku and Nyamugasani valleys, ice apparently retreated rapidly during the earliest Holocene. Overall, the chronology indicates that glaciers in the Rwenzori were more extensive during the southern hemisphere Antarctic Cold Reversal (ACR; ~14.7-13.0 ka) than during the northern hemisphere Younger Dryas (YD; ~12.9-11.7 ka), yet the moraine sequence does not necessarily align with the timing of either late-glacial event. The Rwenzori data are similar to chronologies reported from the South American tropics, where expanded glaciers during the ACR are recognized across the region. This similarity suggests that glaciers across the tropics responded to a common forcing during late-glacial time, most likely temperature. Potential mechanisms to induce such change include radiative greenhouse-gas forcing and tropical ocean dynamics. [ABSTRACT FROM AUTHOR]

Tropical mountain environments, such as the Rwenzori Mountains in equatorial Africa, are thought to be particularly sensitive to climate change. Ongoing warming in the Rwenzori is impacting local environments and communities through glacial retreat, fires, and flooding. Paleoclimate reconstructions from elsewhere in Africa suggest considerable warming accompanied glacier retreat during the last glacial termination, from ∼21 thousand years before present (ka) through the early to mid-Holocene. Quantifying these changes has been difficult but could help to assess future impacts in the Rwenzori. Here, we present a ∼21 thousand-year (kyr) temperature reconstruction based on the relative abundance of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from Lake Mahoma (2,990 m above sea level; m asl) in the Rwenzori Mountains, Uganda. Our record, paired with existing Rwenzori glacial moraine 10Be exposure ages, suggests that deglacial warming and glacial retreat began by ∼20 ka and accelerated at ∼18–18.5 ka. The timing of the onset of rapid warming matches the timing of the post-glacial rise in radiative forcing from atmospheric greenhouse gases (GHGs) from Antarctic ice cores (Brook et al., 1996; Marcott et al., 2014; Monnin et al., 2004; Schilt et al., 2010). Our temperature reconstruction registers ∼4.9 °C warming from the Last Glacial Maximum (LGM) to the late Holocene. This increase is larger than the average ∼2-4 °C warming observed in records from lower elevation sites in tropical East Africa, but similar to that observed at other high-elevation sites in this region. The increased warming at higher elevations thus confirms that the temperature lapse rate steepened during the LGM over this region. Our results also indicate ∼3 °C of warming during the mid-Holocene relative to the late Holocene. This suggests that the freezing-level height rose above Rwenzori summit elevations at that time, likely causing complete deglaciation of the Rwenzori Mountains from ∼5 to 7 ka. The mid-Holocene is thus a potential analog for the glacial and environmental changes that these mountains are likely to experience in the coming 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. • We produce a 21 kyr temperature record from alpine Lake Mahoma, Uganda. • Rwenzori glaciers and vegetation are very sensitive to past temperature change. • ∼3 °C warming during middle Holocene caused complete deglaciation in Rwenzori. • Regional temperature records indicate a steeper tropical lapse rate during the LGM. • Tropical lapse rate shoaled and stabilized during the middle and late Holocene. [ABSTRACT FROM AUTHOR]