A paleoclimatic perspective on the 21st-century glacier loss on Kilimanjaro, Tanzania

Assessing the significance of current glacier loss on Kilimanjaro, Tanzania, demands a wellconstrained temporal perspective. That context is provided by direct measurements, ancillary observations of the ice fields and the analyses of the ice cores collected from them. Ice retreat mechanisms observed there today are consistent with the preservation of the oldest ice, 11.7 ka, in the central deepest part of the Northern Ice Field (NIF). This ice-core derived paleoclimate history published by Thompson and others (2002) is further confirmed by more recent paleoclimate records from tropical East Africa. Mounting evidence suggests that the (anticipated) loss of the entire NIF will be unprecedented within the past 10 000 years. New evidence bears directly on the mechanisms driving the current ice loss. Measurements made in 2000 on the NIF document that air temperature at 0.5 and 1.5m above the surface remained below –58C, while a surface temperature of 0.08C was sustained for up to 8 hours d under clear conditions, consistent with observations of melting on all Kilimanjaro summit ice fields. The linear relationship between oxygen and hydrogen isotopic ratios for all six ice cores drilled in 2000 lies very close to the global meteoric waterline and does not support sublimation (evaporation) as a major driver of ice loss today or in the past on Kilimanjaro. RECENT RETREAT OF THE KILIMANJARO ICE FIELDS Another paper in this issue (Thompson and others, 2011) discusses the widespread retreat of modern glaciers throughout the tropics. Assessing the significance of this recent glacier loss requires placing it in a temporal perspective derived from the best data available. Ice-core derived climate reconstructions from a number of low-latitude glaciers contribute substantially to such a database. The current shrinking and thinning of the ice fields on the summit of Kibo, the central peak of Kilimanjaro, Tanzania, are placed in a longer-term context using both direct observations and the reconstruction of Holocene climatic variations from the suite of six cores drilled there in 2000 (Thompson and others, 2002, 2009). Constructing the paleoclimate histories from these ice cores was a challenge due to the lack of annual resolution and the removal of the most recent five decades of record by surface ablation. Here we review the climate history provided by the Kilimanjaro ice cores, discuss recent observations and address the significance of the current retreat of its summit glaciers. Osmaston (2004) presented an overview of past glaciations on Kilimanjaro, with the first known glaciations dated back 500 ka BP. Based on evidence from cosmogenic Cl dating of moraine boulders, he concluded that the main glaciation occurred from 17 to 20 ka BP, or around the Last Glacial Maximum. From his map of the moraines on the summit of Kibo, the maximum ice coverage is calculated as 116.6 km, >60 times larger than at present (1.82 km) (Thompson and others, 2009). There is no evidence that this ice survived the Bølling interstadial ( 14.6–14.0 ka BP, calibrated), and C dates on material from near the bottom of the Northern Ice Field (NIF) core 3 (NIF3) suggest that the basal ice dates to the early Holocene ( 9–10 ka BP) (Thompson and others, 2002). Comparison with the oxygen isotope (dO) records from six geographically dispersed low-latitude, high-elevation ice cores (Thompson and others, 2005) suggests that glacier growth on Kilimanjaro began early in the African Humid Period ( 14.5–5.5 ka BP). However, as the C dates were measured on samples of small mass, additional lines of evidence must be considered to determine whether it is physically possible for the interior ice of the NIF to be of early Holocene age. It is instructive to examine the processes currently contributing to the shrinking of these ice fields. For example, Figure 1 documents the ice loss between 1999 and 2008 on Furtwängler Glacier (FWG), a small ice field in the middle of the Kibo crater that covered 57 149m in 2000 and only 35 024m in 2007. In 1999 the FWG margin was an intact wall 10m high (Fig. 1a); however, by 2006 the margin had retreated and collapsed, leaving a narrow remnant of the wall (Fig. 1b and c). Over the next 2 years, all the ice along this section of the margin disappeared (Fig. 1d and e), demonstrating how rapidly these processes are occurring. The ablation occurs primarily by melting and by wind-driven sublimation. Similar processes are underway along the vertical margins of all these ice fields, including the NIF, the highest and largest ice field on Kibo. A photograph taken in 1999 from Uhuru summit peak looking north toward the NIF, with FWG in the foreground (Fig. 2a), shows a section of the vertical margin of the NIF containing three ‘steps’ on Annals of Glaciology 52(59) 2011 60