No leading role for industrial black carbon in forcing 19 th century glacier retreat in the Alps

Light absorbing aerosols in the atmosphere and cryosphere play an important role in the climate system. Their 15 presence in ambient air and snow changes radiative properties of these media, 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 1860 AD, many glaciers in the European Alps began to retreat from their maximum mid-19 century terminus positions, thereby visualizing the end of the Little Ice Age in Europe. Radiative forcing by increasing deposition of 20 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 were model simulations using elemental carbon concentrations at low temporal resolution from two ice cores in the Alps. Here we present sub-annually resolved, well-replicated 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 25 ice core in the Alps from 1741-2015 AD. These records allow precise assessment of a potential relation between the timing of observed acceleration of glacier melt in the mid-19 century with an increase of rBC deposition on the glacier caused by the industrialization of Western Europe. Our study reveals that in 1875 AD, the time when European rBC emission rates started to significantly increase, the majority of Alpine glaciers had already experienced more than 80% of their total 19 century length reduction. Industrial BC emissions can, therefore, not been considered as the primary forcing for the rapid 30 deglaciation at the end of the Little Ice Age in the Alps. BC records from the Alps and Greenland also reveal the limitations of bottom-up emission inventories to represent a realistic evolution of anthropogenic BC emissions since preindustrial times.