geosciences Article Mapping the Loss of Mt. Kenya’s Glaciers: An Example of the Challenges of Satellite Monitoring of Very Small Glaciers Rainer Prinz 1,* ID , Armin Heller 2, Martin Ladner 2,3 ID , Lindsey I. Nicholson 4 and Georg Kaser 4 1 Department of Geography and Regional Science, University of Graz, Heinrichstrasse 36, 8010 Graz, Austria 2 Institute of Geography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria; [email protected] (A.H.); [email protected] (M.L.) 3 Austrian Alpine Club, Olympiastrasse 37, 6020 Innsbruck, Austria 4 Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria; [email protected] (L.I.N.); [email protected] (G.K.) * Correspondence: [email protected]; Tel.: +43-316-380-8296 Received: 31 March 2018; Accepted: 5 May 2018; Published: 11 May 2018 Abstract: Since the last complete glacier mapping of Mt. Kenya in 2004, strong glacier retreat and glacier disintegration have been reported. Here, we compile and present a new glacier inventory of Mt. Kenya to document recent glacier change. Glacier area and mass changes were derived from an orthophoto and digital elevation model extracted from Pléiades tri-stereo satellite images. We additionally explore the feasibility of using freely available imagery (Sentinel-2) and an alternative elevation model (TanDEM-X-DEM) for monitoring very small glaciers in complex terrain, but both proved to be inappropriate; Sentinel-2 because of its too coarse horizontal resolution compared to the very small glaciers, and TanDEM-X-DEM because of errors in the steep summit area of Mt. Kenya. During 2004–2016, the total glacier area on Mt. Kenya decreased by 121.0 × 103 m2 (44%). The largest glacier (Lewis) lost 62.8 × 103 m2 (46%) of its area and 1.35 × 103 m3 (57%) of its volume during the same period. The mass loss of Lewis Glacier has been accelerating since 2010 due to glacier disintegration, which has led to the emergence of a rock outcrop splitting the glacier in two parts. If the current retreat rates prevail, Mt. Kenya’s glaciers will be extinct before 2030, implying the cessation of the longest glacier monitoring record of the tropics. Keywords: glacier monitoring; glacier inventory; satellite remote sensing; Pléiades satellite images; Sentinel-2; TanDEM-X; DEM; Mount Kenya; tropical glacier 1. Introduction Glaciers act as low pass filters of climate variability [1], and are, therefore, key indicators of climate change [2]. For this reason, glacier monitoring is implemented in the Global Climate/Terrestrial Observing System (GCOS/GTOS) via a Global Hierarchical Observing Strategy (GHOST) [3]. Following the tiers defined in this strategy, long-term observations of glaciers currently demonstrate historically unprecedented global glacier area and mass losses [4], which pose challenges to existing monitoring networks, such as glacier disintegration, increasing debris cover, and complete extinction of glaciers [5]. To keep pace with the rapid ongoing changes, glacier monitoring at appropriate spatial and temporal scales becomes increasingly important [6,7]. Enhanced Earth observing systems facilitate glacier monitoring in most remote areas [8], however, the variations of very small glaciers (<0.1 km2) in complex mountainous terrain remain a challenge for detection [9]. The glaciers on Mt. Kenya (0◦90 S, 37◦180 E) are among the best studied tropical glaciers. Glacier changes have been reported since the late 19th century, including (sub) decadal mappings Geosciences 2018, 8, 174; doi:10.3390/geosciences8050174 www.mdpi.com/journal/geosciences Geosciences 2018, 8, 174 2 of 14 Geosciences 2018, 8, x FOR PEER REVIEW 2 of 14 from 1934 onwards, and surface mass balance measurements from 1979 to 1996, and from 2010 to 2014 [10–The12] (andglaciers references on Mt. Kenya therein). (0°9′ S, The 37°18 last′ E) glacierare among inventory the best studied on Mt. tropical Kenya glaciers. was carried Glacier out in changes have been reported since the late 19th century, including (sub) decadal mappings from 1934 2004 [13], showing that between 1899 and 2004, the total area of 18 glaciers decreased from 1.64 km2 to onwards, and surface mass balance measurements from 1979 to 1996, and from 2010 to 2014 [10–12] 2 − 0.27 km(and (references84%), while therein). 8 glaciers The last vanishedglacier inventory completely on Mt. [Kenya14]. The was most carried recent out in mapping 2004 [13], ofshowing the largest glacierthat (Lewis, between Figure 1899 and1) was 2004, performed the total area in of 2010 18 glaciers [ 15,16 decreased], which from confirmed 1.64 km² the to 0.27 strong km² retreat (−84%), rates, indicatingwhile that8 glaciers between vanished 1934 completely and 2010 the[14]. glacier The most lost recent 90% (79%) mapping of its of volumethe largest (area) glacier [12 ].(Lewis, A climate sensitivityFigure study1) was based performed on energy in 2010 balance [15,16], modellingwhich confirmed found the that strong glacier retreat recession rates, indicating since the latethat 19th centurybetween can be 1934 primarily and 2010 attributed the glacier lost to atmospheric 90% (79%) of dryingits volume resulting (area) [12]. in reducedA climate cloudiness, sensitivity study snowfall, and albedobased on [17 energy]. Similar balance studies modelling proved found the that climate glacier proxy recession potential since the of tropicallate 19th glacierscentury can [18 be–20 ] as theyprimarily capture climate attributed signals to atmospheric from the tropicaldrying result mid-troposphere,ing in reduced cloudiness, where our snowfall, understanding and albedo is scarce and controversial[17]. Similar studies [21,22 proved]. Although the climate Mt. Kenya’s proxy potential glaciers of have tropical limited glaciers socioeconomic [18–20] as they relevance capture [23], climate signals from the tropical mid-troposphere, where our understanding is scarce and they have the longest record of glacier monitoring in the tropics. Recent reports of substantial glacier controversial [21,22]. Although Mt. Kenya’s glaciers have limited socioeconomic relevance [23], they changeshave [ 24the,25 longest] demand record an of update glacier of monitoring Mt. Kenya’s in the glaciers tropics. inventory, Recent reports offering of substantial unique witnessing glacier of the impendingchanges [24,25] deglaciation demand an of update an entire of Mt. massif. Kenya’s glaciers inventory, offering unique witnessing of Directthe impending glacier observationsdeglaciation of on an Mt.entire Kenya massif. ceased in 2014 [24], leaving remote sensing as the only option forDirect a continuous glacier observations monitoring on strategy. Mt. Kenya In ceased this study, in 2014 we [24], use leaving very high remote resolution sensing (0.5as the m) only satellite imagesoption from for Pl aé iadescontinuous tri-stereo monitoring scenes strategy. (acquired In th inis 2016), study, whichwe use are very processed high resolution to an (0.5 orthophoto m) and tosatellite a digital images elevation from Pléiades model tri-stereo (DEM) ofscenes the summit(acquired area.in 2016), From which the are orthophoto, processed weto an derive a neworthophoto glacier inventoryand to a digital of Mt. elevation Kenya model and quantify(DEM) of the glacier summit area area. changes From the by orthophoto, comparison we to the derive a new glacier inventory of Mt. Kenya and quantify glacier area changes by comparison to the previous inventory, which was derived from the last complete glacier mapping in 2004. For quantifying previous inventory, which was derived from the last complete glacier mapping in 2004. For glacier volume changes, we subtract Lewis Glacier DEMs from 2016 and 2010. Additionally, to explore quantifying glacier volume changes, we subtract Lewis Glacier DEMs from 2016 and 2010. the capabilityAdditionally, of available to explore remote the capability sensing of available data to monitor remote sensing the changes data to ofmonitor Mt. Kenya the changes glaciers, of Mt. we test the PlKenyaéiades glaciers, orthophoto we test and the DEM Pléiades against orthophoto freely availableand DEM products,against freely like available Sentinel-2 products, satellite like images and theSentinel-2 TanDEM-X-DEM. satellite images and the TanDEM-X-DEM. Figure 1. Lewis Glacier above Lewis Tarn seen from its late 19th century lateral moraine on 01 March Figure 1. Lewis Glacier above Lewis Tarn seen from its late 19th century lateral moraine on 2011. Note the steep walls of the two highest peaks of the massif, Batian (5199 m) and Nelion (5188 01 March 2011. Note the steep walls of the two highest peaks of the massif, Batian (5199 m) and m), which rise to the upper left corner. Photo credit R. Prinz. Nelion (5188 m), which rise to the upper left corner. Photo credit R. Prinz. 2. Materials and Methods 2. Materials and Methods 2.1. Pléiades Tri-Stereo DEM and Orthophoto 2.1. Pléiades Tri-Stereo DEM and Orthophoto The Pléiades system (operated by the Centre National d’Études Spatiales) consists of two Theidentical Pléiades optical system satellites (operated Pléiades by 1A the and Centre 1B, positioned National with d’É atudes 180° phase Spatiales) shift in consists a sun-synchronous of two identical optical satellites Pléiades 1A and 1B, positioned with a 180◦ phase shift in a sun-synchronous orbit at 694 km altitude enabling a daily acquisition of any point on Earth. During a single fly Geosciences 2018, 8, x FOR PEER REVIEW 3 of 14 orbit at 694 km altitude enabling a daily acquisition of any point on Earth. During a single fly in longitudinal-scanGeosciences 2018, 8, 174 mode (along-track), image triplets (tri-stereo) can be taken in a few seconds in3 ofthe 14 direction of forward, backward, and near perpendicular (nadir). Access to the Pléiades scenes was granted via a mapping project of the Austrian Alpine Club in longitudinal-scan mode (along-track), image triplets (tri-stereo) can be taken in a few seconds in the (http://www.gis.tirol/AV.MAP/).