19th century glacier retreat in the Alps preceded the emergence of industrial black carbon deposition on high-alpine glaciers

Michael Sigl,Jacopo Gabrieli,Margit Schwikowski,Theo M Jenk,Nerilie J Abram,Dimitri Osmont

doi:10.5194/tc-12-3311-2018

Abstract

Abstract. Light absorbing aerosols in the atmosphere and cryosphere play an important role in the climate system. Their presence in ambient air and snow changes the radiative properties of these systems, thus contributing to increased atmospheric warming and snowmelt. High spatio-temporal variability of aerosol concentrations and a shortage of long-term observations contribute to large uncertainties in properly assigning the climate effects of aerosols through time. 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 deposition of industrial black carbon to snow has been suggested as the main driver of the abrupt glacier retreats in the Alps. The basis for this hypothesis was model simulations using elemental carbon concentrations at low temporal resolution from two ice cores in the Alps. Here we present sub-annually resolved concentration records of refractory black carbon (rBC; using soot photometry) as well as distinctive tracers for mineral dust, biomass burning and industrial pollution from the Colle Gnifetti ice core in the Alps from AD 1741 to 2015. These records allow precise assessment of a potential relation between the timing of observed acceleration of glacier melt in the mid-19th century with an increase of rBC deposition on the glacier caused by the industrialization of Western Europe. Our study reveals that in AD 1875, the time when rBC ice-core concentrations started to significantly increase, the majority of Alpine glaciers had already experienced more than 80 % of their total 19th century length reduction, casting doubt on a leading role for soot in terminating of the Little Ice Age. Attribution of glacial retreat requires expansion of the spatial network and sampling density of high alpine ice cores to balance potential biasing effects arising from transport, deposition, and snow conservation in individual ice-core records.

Highlights

The role of aerosols in climate forcing is significant but poorly understood (Charlson et al, 1992)
Our study reveals that in advance phases (AD) 1875, the time when Refractory black carbon (rBC) ice-core concentrations started to significantly increase, the majority of Alpine glaciers had already experienced more than 80 % of their total 19th century length reduction, casting doubt on a leading role for soot in terminating of the Little Ice Age
Industrial black carbon believed to be emitted in large quantities starting in the mid-19th century had been suggested as the key external forcing responsible for an accelerated melting of European glaciers through reductions in ice-albedo and subsequent ablation (Painter et al, 2013). We examined this interpretation by presenting new, highly resolved, well replicated ice-core measurements of refractory black carbon, mineral dust, and distinctive industrial pollution tracers from the Colle Gnifetti ice core in the Alps covering the past 270 years

Summary

Introduction

The role of aerosols in climate forcing (defined as perturbation of the Earth’s energy balance relative to the preindustrial) is significant but poorly understood (Charlson et al, 1992) Aerosol emissions and their atmospheric burden vary in time and from region to region; some aerosols cause cooling while even co-emitted species can lead to simultaneous warming. M. Sigl et al.: 19th century glacier retreat in the Alps defined as an incomplete combustion product from natural biomass burning (e.g. forest fires) or anthropogenic biofuel and fossil-fuel burning. Sigl et al.: 19th century glacier retreat in the Alps defined as an incomplete combustion product from natural biomass burning (e.g. forest fires) or anthropogenic biofuel and fossil-fuel burning It is insoluble, refractory, strongly absorbs visible light, and forms aggregates of small carbon spherules. If analysed with a thermal optical method, BC is referred to as elemental carbon (EC) (Currie et al, 2002)

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