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Deciphering the climate changes that influenced the glacial fluctuations of the last millennium requires documenting the spatial and temporal patterns of these glacial events. Here, we estimate the change in equilibrium line altitudes (ELAs) between the most prominent glacial advance of the last millennium and the present for four alpine glaciers located in different climatic regimes along the Andes. For each glacier, we reconstruct scenarios of climatic conditions (temperature and precipitation anomalies) that accommodate the observed ELA changes. We focus on the following glaciers: an alpine glacier in the Cordillera Vilcanota (13°S), Tapado glacier (30°S), Cipreses glacier (34°S), and Tranquilo glacier (47°S). Our results show that the range of possible temperature and precipitation anomalies that accommodate the observed ELA changes overlap significantly at three of the four sites (i.e. Vilcanota, Cipreses, and Tranquilo). Only Tapado glacier exhibits a set of climate anomalies that differs from the other three sites. Assuming no change in precipitation, the estimated ELA changes require a cooling of at least 0.7°C in the Cordillera Vilcanota, 1.0°C at Tapado glacier, 0.6°C at Cipreses glacier, and 0.7°C at Tranquilo glacier. Conversely, assuming no change in temperature, the estimated ELA changes are explained by increases in precipitation exceeding 0.52 m yr-1 (64% of the annual precipitation) in the Cordillera Vilcanota, 0.3 1 m yr-1 (89%) at Tapado glacier, 0.22 m yr-1 (27%) at Cipreses glacier, and 0.3 m yr-1 (27%) at Tranquilo glacier. By mapping the ELA changes and modeling the potential climate forcing across diverse climate settings, we aim to contribute toward documenting the spatial variability of climate conditions during the last millennium, a key step to decipher the mechanisms underlying the glacial fluctuation that occurred during this period. [ABSTRACT FROM AUTHOR]

Glaciers on the world's highest tropical mountains are among the most sensitive components of the cryosphere, yet the climatic controls that influence their fluctuations are not fully understood. Here we present the first 10Be ages of glacial moraines in Africa and use these to assess the climatic conditions that influenced past tropical glacial extents. We applied 10Be surface exposure dating to determine the ages of quartz-rich boulders atop moraines in the Rwenzori Mountains (~1°N, 30°E), located on the border of Uganda and the Democratic Republic of Congo. The 10Be ages document expanded glaciers ca. 23.4 and 20.1 ka, indicating that glaciers in equatorial East Africa advanced during the global Last Glacial Maximum (ca. 26-19.5 ka). A comparison of these moraine ages with regional paleoclimate records indicates that Rwenzori glaciers expanded contemporaneously with dry and cold conditions. Recession from the moraines occurred after ca. 20.1 ka, similar in timing to a rise in air temperature documented in East African lake records. Our results suggest that, on millennial time scales, past fluctuations of Rwenzori glaciers were strongly influenced by air temperature. [ABSTRACT FROM AUTHOR]

The last glacial-interglacial transition (ca. 18--11 ka) was interrupted by abrupt climate events that differed in each hemisphere. During the Antarctic Cold Reversal (ca. 14.5--12.9 ka), the Southern Hemisphere high and mid latitudes cooled, while the Northern Hemisphere warmed. The pattern of change then reversed during the Younger Dryas (ca. 12.9--11.7 ka), which was characterized by cold conditions in much of the Northern Hemisphere. Well-dated paleoclimate records serve to reveal the possible mechanisms for these events. Here we present a reconstruction of the late glacial fl uctuations of Quelccaya Ice Cap, located in the southern tropics, based on 38 new radiocarbon ages. Quelccaya was retreating from its Last Glacial Maximum (ca. 21 ka) extent by ca. 17.2 ka, and was located upvalley from its late glacial moraines by 13.6--12.8 ka. Quelccaya experienced a signifi cant readvance that culminated at 12.5--12.4 ka, and then receded several kilometers to near, or within, its late Holocene extent by ca. 11.6 ka. This record provides the most detailed evidence yet of glacier fl uctuations in the southern tropics during late glacial time. [ABSTRACT FROM AUTHOR]

Reconstructions of Holocene lake water isotopic composition based upon subfossil aquatic organic material offer new insights into Arctic climate. We present quantitative estimates of warmth during the Holocene Thermal Maximum in northwest Greenland, inferred from oxygen isotopes of chironomid head capsules and aquatic moss preserved in lake sediments. δ 18 O values of chironomids from surface sediments of multiple Greenland lakes indicate that these subfossil remains record the δ 18 O values of the lake water in which they grow. Our lake water δ 18 O reconstruction is supported by downcore agreement with δ 18 O values in aquatic moss and chironomid remains. δ 18 O of both organic materials from Secret Lake decrease after 4 ka (ka = thousands of years ago) by 3‰ into the Neoglacial. We argue that lake water at Secret Lake primarily reflects precipitation δ 18 O values, which is strongly correlated with air temperature in NW Greenland, and that this signal is biased towards summer and early autumn conditions. Other factors may have influenced Secret Lake δ 18 O values through the Holocene, including evaporation of lake water and changing seasonality and source of precipitation. The maximum early Holocene summer and early autumn-biased temperature anomaly at Secret Lake is 2.5–4 °C warmer than present from 7.7 (the beginning of our record) to ∼6 ka. The maximum late Holocene cold anomaly (which includes the Little Ice Age) is 1.5–3 °C colder than present. These ranges of possible temperature anomalies reflect uncertainty in the δ 18 O – temperature relationship for precipitation at the study site through the Holocene. [ABSTRACT FROM AUTHOR]

Improving the late Quaternary paleoclimate record through climate interpretations of low-latitude glacier length changes advances our understanding of past climate change events and the mechanisms for past, present, and future climate change. Paleotemperature reconstructions at low-latitude glaciers are uniquely fruitful because they can provide both site-specific information and enhanced understanding of regional-scale variations due to the structure of the tropical atmosphere. We produce Little Ice Age (LIA) and Younger Dryas (YD) paleoclimate reconstructions for the Huancané outlet glacier of the Quelccaya Ice Cap (QIC) and low-latitude southern hemisphere regional sea surface temperatures (SSTs) using a coupled ice-flow and energy balance model. We also model the effects of long-term changes in the summit temperature and precipitiation rate and the effects of interannual climate variability on the Huancané glacier length. We find temperature to be the dominant climate driver of glacier length change. Also, we find that interannual climate variability cannot adequately explain glacier advances inferred from the geomorphic record, necessitating that these features were formed during past colder climates. To constrain our LIA reconstruction, we incorporate the QIC ice core record, finding a LIA air temperature cooling at the ice cap of between ∼0.7 °C and ∼1.1 °C and ∼0.4 °C and regional SSTs cooling of ∼0.6 °C. For the YD paleoclimate reconstructions, we propose two limits on the precipitation rate, since the ice core record does not extend into the Pleistocene: 1) the precipitation rate scales with the Clausius-Clapeyron relationship (upper limit on cooling) and 2) the precipitation rate increases by 40% (lower limit on cooling), which is an increase about twice as great as the regional increases realized in GCM simulations for the period. The first limit requires ∼1.6 °C cooling in ice cap air temperatures and ∼0.9 °C cooling in SSTs, and the second limit requires ∼1.0 °C cooling in ice cap air temperatures and ∼0.5 °C cooling in SSTs. Our temperature reconstructions are in good agreement with the magnitude and trend of GCM simulations that incorporate the forcing mechanisms hypothesized to have caused these climate change events. [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]