The Cryosphere, 12, 3311–3331, 2018 https://doi.org/10.5194/tc-12-3311-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. 19th century glacier retreat in the Alps preceded the emergence of industrial black carbon deposition on high-alpine glaciers Michael Sigl1,2, Nerilie J. Abram3, Jacopo Gabrieli4, Theo M. Jenk1,2, Dimitri Osmont1,2,5, and Margit Schwikowski1,2,5 1Laboratory of Environmental Chemistry, Paul Scherrer Institut, 5232 Villigen, Switzerland 2Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland 3Research School of Earth Sciences and the ARC Centre of Excellence for Climate System Science, Australian National University, Canberra 2601 ACT, Australia 4Institute for the Dynamics of the Environmental Sciences, National Research Council (IDPA-CNR), 30172 Venice, Italy 5Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland Correspondence: Michael Sigl ([email protected]) Received: 29 January 2018 – Discussion started: 20 February 2018 Revised: 17 September 2018 – Accepted: 19 September 2018 – Published: 16 October 2018 Abstract. Light absorbing aerosols in the atmosphere and centrations started to significantly increase, the majority of cryosphere play an important role in the climate system. Alpine glaciers had already experienced more than 80 % of Their presence in ambient air and snow changes the radiative their total 19th century length reduction, casting doubt on a properties of these systems, thus contributing to increased leading role for soot in terminating of the Little Ice Age. At- atmospheric warming and snowmelt. High spatio-temporal tribution of glacial retreat requires expansion of the spatial variability of aerosol concentrations and a shortage of long- network and sampling density of high alpine ice cores to bal- term observations contribute to large uncertainties in prop- ance potential biasing effects arising from transport, deposi- erly assigning the climate effects of aerosols through time. tion, and snow conservation in individual ice-core records. Starting around AD 1860, many glaciers in the European Alps began to retreat from their maximum mid-19th century terminus positions, thereby visualizing the end of the Little Ice Age in Europe. Radiative forcing by increasing deposi- 1 Introduction tion of industrial black carbon to snow has been suggested as the main driver of the abrupt glacier retreats in the Alps. The role of aerosols in climate forcing (defined as pertur- The basis for this hypothesis was model simulations using bation of the Earth’s energy balance relative to the pre- elemental carbon concentrations at low temporal resolution industrial) is significant but poorly understood (Charlson et from two ice cores in the Alps. al., 1992). Aerosol emissions and their atmospheric burden Here we present sub-annually resolved concentration vary in time and from region to region; some aerosols cause records of refractory black carbon (rBC; using soot photom- cooling while even co-emitted species can lead to simulta- etry) as well as distinctive tracers for mineral dust, biomass neous warming. This results in large uncertainties of the as- burning and industrial pollution from the Colle Gnifetti ice cribed radiative forcing terms to short-lived aerosols in con- core in the Alps from AD 1741 to 2015. These records allow trast to greenhouse gas forcing (Bond et al., 2013; Dubovik precise assessment of a potential relation between the tim- et al., 2002). ing of observed acceleration of glacier melt in the mid-19th Black carbon (BC) has a unique and important role in century with an increase of rBC deposition on the glacier the climate system because it absorbs solar radiation even caused by the industrialization of Western Europe. Our study at very low concentrations, influences cloud formation, and reveals that in AD 1875, the time when rBC ice-core con- enhances the melting of snow and ice via albedo feedbacks (Flanner et al., 2007; Hansen and Nazarenko, 2004). BC is Published by Copernicus Publications on behalf of the European Geosciences Union. 3312 M. Sigl et al.: 19th century glacier retreat in the Alps Figure 1. (a) Colle Gnifetti (CG) and Fiescherhorn (FH) ice-core drilling sites, high resolution glacier length reconstructions from the Bernese Alps (Oberer Grindelwald, Unterer Grindelwald) and French Alps (Bossons, Mer de Glace). Dashed rectangle encompasses the six 1◦ × 1◦ grids used for the comparison of gridded black carbon (BC) emission estimates (Bond et al., 2007) with ice-core BC concentrations (Fig. 6); source: Perconte (based on SRTM-Data; CC BY-SA 2.5: https://creativecommons.org/licenses/by-sa/2.5, last access: 29 July 2016), via Wikimedia Commons. (b) Drilling site of the new CG15 ice-core at 4450 m a.s.l. (source: Michael Sigl); (c) Alpine and Greenland ice-core drilling sites superimposed on present-day annual mean fossil fuel and biofuel BC emission estimates (adapted from Stohl, 2006). defined as an incomplete combustion product from natural estimated from bottom-up approaches (i.e. from fuel con- biomass burning (e.g. forest fires) or anthropogenic biofuel sumption data) suggest large changes during the industrial and fossil-fuel burning. It is insoluble, refractory, strongly era (Bond et al., 2007; Lamarque et al., 2010), which were absorbs visible light, and forms aggregates of small carbon recently largely confirmed by continuous measurements of spherules. Per unit mass, BC has the highest light absorp- BC in Greenland ice cores (Bauer et al., 2013; Koch et al., tion of all abundant aerosols in the atmosphere (Bond et al., 2011; Y. H. Lee et al., 2013; McConnell et al., 2007). How- 2013). Given that carbonaceous aerosols in the atmosphere ever, multiple source regions contribute in varying degrees present a continuum of varying physical and chemical prop- to the BC deposition over Greenland, hampering attribution erties, their quantification is strongly related to the analytical of the observed trends to individual emission source areas method used. A wide range of terminologies has developed (Hirdman et al., 2010; Liu et al., 2011). in the scientific community to characterize BC and related Together with mineral dust and other absorbing organic carbonaceous aerosols, and we follow the terminology rec- aerosols, BC deposited on snow and ice can lead to increased ommendations recently put in place (Petzold et al., 2013). melt rates and changes in melt onset due to reductions in Refractory black carbon (rBC) will be used instead of black surface albedo. These effects are further enhanced by subse- carbon for reporting concentrations derived from our laser- quent snow albedo feedbacks such as an increase in the water based incandescence method, while the general term black content and surface accumulation of impurities (Flanner et carbon (BC) is used for a qualitative description when re- al., 2009; Hansen and Nazarenko, 2004). The best estimate ferring to light-absorbing carbonaceous substances in atmo- for industrial era global forcing of BC is C0:13 W m−2, but spheric aerosol. If analysed with a thermal optical method, values for regions with seasonal snow cover (e.g. the Arc- BC is also referred to as elemental carbon (EC) (Currie et tic, European Alps, Tibetan Plateau) are much higher (Bond al., 2002). et al., 2013). Industrial BC deposition has been suggested as While natural sources such as forest fires dominated the being responsible for observed Arctic warming in the 1940s global BC burden in the pre-industrial atmosphere, current and recent years (Flanner, 2013; Flanner et al., 2009; Quinn emissions are largely driven by industrial, energy related et al., 2008) but recent surface albedo decreases (i.e. dark- sources (Bond et al., 2013). The modern burden is high- ening) of the Greenland ice sheet occurred in the face of est in heavily industrialized and populated regions including a widespread decrease in BC deposition based on multiple China, India, and Europe (Fig. 1). Trends in BC emissions ice cores (Keegan et al., 2014; McConnell et al., 2007), sug- The Cryosphere, 12, 3311–3331, 2018 www.the-cryosphere.net/12/3311/2018/ M. Sigl et al.: 19th century glacier retreat in the Alps 3313 gesting a small role for light absorbing impurities in causing 1870s, both records show an initial 2–3-fold increase of BC these changes (Polashenski et al., 2015). In the Himalayas the concentrations rising from a mostly natural background of combined increased deposition of mineral dust and industrial 9 ng g−1 to more than 20 ng g−1, before the highest values black carbon was suggested to play a role in the observed (37 ng g−1) were reached during the early 20th century. glacier retreat during the past decades (Flanner and Zender, Transient changes in external natural (e.g. volcanic erup- 2005; Kaspari et al., 2011; Lau et al., 2010; W. S. Lee et tions) and anthropogenic climate forcing (e.g. greenhouse al., 2013). In contrast to climate effects from direct radia- gases, tropospheric aerosols) occurred during the emergence tive forcing (Bond and Sun, 2005; Penner et al., 1998) and of industrialization in Europe (Eyring et al., 2016; Jungclaus cloud effects (Haywood and Boucher, 2000; Lohmann and et al., 2017). To isolate the often complex relationships be- Feichter, 2005) that are short lived and effective only during tween glacier fluctuations and meteorological forcing and to the brief atmospheric lifetime of the aerosols (from days to identify the mechanisms responsible for glacier retreat in the a week), BC-induced changes in the snow cover persist for second half of the 19th century requires comprehensive mod- longer periods of time ranging from weeks-to-months. They elling efforts (e.g. Lüthi, 2014; Zekollari, 2017; Goosse et al., are most pronounced during the spring and summer, when in- 2018). Underpinning such efforts, accurate and precise de- solation and seasonal snowmelt reach a maximum (Flanner lineation of external forcing (e.g.