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.
The role of aerosols in climate forcing (defined as perturbation of the Earth's energy balance relative to the pre-industrial) 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. This results in large uncertainties of the ascribed radiative forcing terms to short-lived aerosols in contrast to greenhouse gas forcing (Bond et al., 2013; Dubovik et al., 2002).
Black carbon (BC) has a unique and important role in the climate system because it absorbs solar radiation even at very low concentrations, influences cloud formation, and enhances the melting of snow and ice via albedo feedbacks (Flanner et al., 2007; Hansen and Nazarenko, 2004). BC is 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. Per unit mass, BC has the highest light absorption of all abundant aerosols in the atmosphere (Bond et al., 2013). Given that carbonaceous aerosols in the atmosphere present a continuum of varying physical and chemical properties, their quantification is strongly related to the analytical method used. A wide range of terminologies has developed in the scientific community to characterize BC and related carbonaceous aerosols, and we follow the terminology recommendations recently put in place (Petzold et al., 2013). Refractory black carbon (rBC) will be used instead of black carbon for reporting concentrations derived from our laser-based incandescence method, while the general term black carbon (BC) is used for a qualitative description when referring to light-absorbing carbonaceous substances in atmospheric aerosol. If analysed with a thermal optical method, BC is also referred to as elemental carbon (EC) (Currie et al., 2002).
While natural sources such as forest fires dominated the global BC burden in the pre-industrial atmosphere, current emissions are largely driven by industrial, energy related sources (Bond et al., 2013). The modern burden is highest in heavily industrialized and populated regions including China, India, and Europe (Fig. 1). Trends in BC emissions estimated from bottom-up approaches (i.e. from fuel consumption data) suggest large changes during the industrial era (Bond et al., 2007; Lamarque et al., 2010), which were recently largely confirmed by continuous measurements of BC in Greenland ice cores (Bauer et al., 2013; Koch et al., 2011; Y. H. Lee et al., 2013; McConnell et al., 2007). However, multiple source regions contribute in varying degrees to the BC deposition over Greenland, hampering attribution of the observed trends to individual emission source areas (Hirdman et al., 2010; Liu et al., 2011).
Together with mineral dust and other absorbing organic aerosols, BC deposited
on snow and ice can lead to increased melt rates and changes in melt onset
due to reductions in surface albedo. These effects are further enhanced by
subsequent snow albedo feedbacks such as an increase in the water content and
surface accumulation of impurities (Flanner et al., 2009; Hansen and
Nazarenko, 2004). The best estimate for industrial era global forcing of BC
is
To quantify trends and magnitudes of climate forcing from BC in the atmosphere (direct and indirect effect) and cryosphere (snow-albedo effect) climate-model simulations are widely used (Bond et al., 2013; Lamarque et al., 2013; Shindell et al., 2013). These rely strongly on energy-consumption based estimates of BC emissions that are highly uncertain (see Fig. 8 in Bond et al., 2007) and thus need to be evaluated against independent ice-core based observations (Bauer et al., 2013; Y. H. Lee et al., 2013). Those comparisons allow identification of mismatches and can subsequently help to improve parameterization and model performance (Lamarque et al., 2013).
Mountain glaciers are retreating worldwide and are projected to further
shrink with the expected increase in global surface temperatures due to
increasing greenhouse gas concentrations (Mernild et al., 2013; Oerlemans,
2005; Zemp et al., 2006). While the currently observed mass loss is global in
scale and attributed to anthropogenic greenhouse gas emissions, the onset of
melting during the 19th century was asynchronous for many mountain regions
(e.g. between Scandinavia and the Alps; Imhof et al., 2012; Larsen et al.,
2013). Observations place the start of the retreat in the Western Alps from
1860 to 1865 after glaciers reached their maximum extent around 1850–1855
(Nussbaumer and Zumbühl, 2012; Zumbühl et al., 2008). The retreat was
rapid and synchronous among different documented glaciers. By 1880, glacier
tongues had retreated by several hundred metres in length (Nussbaumer and
Zumbühl, 2012). Using early instrumental temperature and precipitation
data, a combination of high spring temperatures and reduced autumn
precipitation was suggested as the main drivers of the observed glacier
retreat (Steiner et al., 2008; Zumbühl et al., 2008). In an alternative
hypothesis, early industrial BC deposited on snow and ice of Alpine glaciers
was held responsible for the rapid melting, involving a snow-albedo feedback
(Painter et al., 2013). This hypothesis built on model simulations to
estimate snow albedo forcing from two ice-core based reconstructions of BC
(i.e. EC) from Fiescherhorn glacier (Jenk et al., 2006) and Colle Gnifetti
(Thevenon et al., 2009). Starting in the 1870s, both records show an initial
2–3-fold increase of BC concentrations rising from a mostly natural
background of 9 ng g
Transient changes in external natural (e.g. volcanic eruptions) and anthropogenic climate forcing (e.g. greenhouse gases, tropospheric aerosols) occurred during the emergence of industrialization in Europe (Eyring et al., 2016; Jungclaus et al., 2017). To isolate the often complex relationships between glacier fluctuations and meteorological forcing and to identify the mechanisms responsible for glacier retreat in the second half of the 19th century requires comprehensive modelling efforts (e.g. Lüthi, 2014; Zekollari, 2017; Goosse et al., 2018). Underpinning such efforts, accurate and precise delineation of external forcing (e.g. volcanic eruptions), potential feedbacks (e.g. BC deposition on snow) and cryosphere changes (e.g. variations in glacier front positions) is critically important.
The snow-albedo feedback hypothesis formulated by Painter et al. (2013) was a first effort to attempt this but was limited predominantly by the available BC data at that time (Thevenon et al., 2009; Jenk et al., 2006). The available data were of relatively low time-resolution (Thevenon et al., 2009; Jenk et al., 2006), and the dating of the Colle Gnifetti ice core was at that time not based on annual-layer dating constrained by historic age markers (Jenk et al., 2009; see Fig. S1 and Table S1 in the Supplement) and thus was highly uncertain in Thevenon et al. (2009). Measurements of EC may be subject to artifacts related to losses during filtration, interferences from mineral dust, or the pyrolysis of organic compounds (see Lack et al., 2014; Lim et al., 2014, for details). Large sample size requirements (0.2–1 kg) for EC quantification with traditional thermal techniques, however, made it impossible to analyse replicate core sections in order to demonstrate the repeatability of the results (Thevenon et al., 2009; Jenk et al., 2006).
Here, we set out to re-evaluate the timing of industrial BC deposition in ice cores from the Alps by using a new, more accurately dated record of rBC at a much higher time resolution (sub-annual). In addition, we measure distinctive tracers of anthropogenic pollution (e.g. bismuth, sulfate, lead, ammonium) and compare all records with the most highly resolved history of glacier length changes of four glaciers in the Western Alps currently available (Nussbaumer and Zumbühl 2012).
Two 82 m long ice cores (CG03A, CG03B) from the European Alps, which are
surrounded by highly industrialized countries (e.g. Germany, Italy, France),
representing some of the main emitters of 19th to 21st century fossil-fuel
industrial BC (Bond et al., 2007), were drilled in 2003 a metre apart on Colle
Gnifetti (Monte Rosa, 4450 m a.s.l., 45
The Colle Gnifetti site has produced a number of impurity and pollution records, highlighting the strong impact of human activity on the atmospheric composition over Central Europe during recent decades and centuries, including records of sulfate and nitrate (Döscher et al., 1995; Schwikowski et al., 1999a), ammonium (Döscher et al., 1996), carbonaceous aerosols (Lavanchy et al., 1999; Thevenon et al., 2009), trace elements such as lead, copper, cadmium, zinc, plutonium (Barbante et al., 2004; Gabrieli and Barbante, 2014; Gabrieli et al., 2011; Schwikowski et al., 2004), and organic pollutants (Gabrieli et al., 2010; Kirchgeorg et al., 2013). Temporal variability of mineral dust, including long-range transported dust from Africa, was investigated by Bohleber et al. (2018), Gabbi et al. (2015), Gabrieli and Barbante (2014), Wagenbach and Geis (1989), and Wagenbach et al. (1996) using calcium, sulfate, iron, and barium as mineral dust proxies.
Ice cores, parameters, and analytical methods.
The top 57.2 m of CG03B comprising 1635 discrete samples (cross section area
For the parallel ice core CG03A a wide range of additional elemental and
chemical components of aerosols had been analysed using ion-chromatography
(IC, Dionex) and inductively coupled plasma mass spectrometry
(ICPMS, Agilent 7500), enabling ice-core dating (Jenk et al., 2009)
and detailed characterization of dust and pollution aerosols (Gabrieli and
Barbante, 2014). Trace element analyses were performed at University of
Venice using the ICPMS in continuous flow mode, achieving an effective
sampling resolution of approximately 0.5 cm water equivalent (Gabrieli and
Barbante, 2014). Here we present measurements of trace metals (i.e. bismuth,
Bi), typically emitted by coal burning and other industrial processes
(McConnell and Edwards, 2008) and of chemical tracers (i.e. calcium,
The CG03B ice core was dated against the chronology of the CG03A core (Jenk
et al., 2009) using the major ion records obtained for both cores to align
the records. In total, 221 stratigraphic links were established between these
two records between AD 1741 and 2003, which is close to the number of annual
layers identified originally in CG03A. Linear interpolation was used to date
the ice between the stratigraphic tie-points. Differences in the depths for
common time markers are found to be less than 13 cm at most (Table S1). The
chronology of CG03A (Fig. S1; Table S1) was originally derived by
annual-layer-counting predominantly using the
In the absence of direct BC measurements during the last few centuries,
gridded emission inventories from bottom-up approaches, (e.g. Bond et al.,
2007; Lamarque et al., 2010) are widely used to estimate emissions and
aerosol loading. These are the final products of a wide range of estimates of
activity (e.g. fuel consumption) combined with emission factors (e.g. grams
of BC emitted per mass of fuel burned derived from controlled burning of fuel
types under laboratory conditions) and thus carry large uncertainties
(Bond et al., 2013). While the general emission trends from these inventories
could be confirmed through comparison to existing ice-core reconstructions
(Jenk et al., 2006; Junker and Liousse, 2008; Lavanchy et al., 1999), a more
detailed evaluation was hampered by the relatively large error ranges
inherent in both these reconstruction approaches. For this study, we deploy
the BC emission estimates from fossil-fuel and biofuel burning (available at
5-year resolution) from Bond et al. (2007), for (1) the OECD countries in
Europe and (2) the mean of the grid cells 45–47
Glacier fluctuations in the European Alps are among the best documented worldwide, as glaciers are situated in densely populated areas. Painter et al. (2013) used five glaciers from the Western and Eastern Alps to analyse the 19th century changes of their terminus positions, with Unterer Grindelwald glacier providing the densest observation frequency during the 19th century among these glaciers. For this study, we compile four glacier length reconstructions from the Western Alps (all situated in close proximity to the ice-core site) including Mer de Glace, Oberer Grindelwald and Unterer Grindelwald glacier, and Bossons glacier (Nussbaumer and Zumbühl, 2012), the latter offering the highest observation density during the mid-to-late 19th century with annual data coverage between AD 1850 and 1899 of 78 %. To analyse trends in glacier length variability relative to the increase of industrial black carbon deposition at Colle Gnifetti and Fiescherhorn (Jenk et al., 2006), we filled missing terminus position data by linear interpolation and constructed a stacked glacier length curve by averaging the terminus positions of all four glaciers.
To determine the timing when rBC concentrations exceeded their natural
variability, suggesting an additional, industrial emission source, we
performed a time-of-emergence (ToE) analysis on annually averaged rBC
concentrations. The ToE is formally defined by Hawkins and Sutton (2012) as
the mean time at which the signal of change emerges from the noise of natural
variability. We followed the methods of Abram et al. (2016) in defining the
threshold of emergence value as the earliest occurrence where the
signal-to-noise ratio exceeds the value 2 (industrial rBC signal is
distinguishable from zero at a 95 % confidence level). We consider the
time period AD 1741 to 1840 as the pre-industrial reference period during which
human emissions of light absorbing rBC in Central Europe was minimal,
restricted to occasional forest fires and residential wood burning
(henceforth summarized as biomass burning BB). To discriminate large rBC
values caused by BB within the pre-industrial period (AD 1741–1850) we
employed a fire detection algorithm adapted from Fischer et al. (2015) and
replaced the correspondent BC concentration values for detected “fire
activity” years with their 11-year running medians (BC
Combined (CG03 and CG15) Colle Gnifetti rBC concentration record including the original analysis (black) and replicate samples from parallel core sections (red) between AD 1741 and 2015.
The synchronized records of CG03, CG08 and CG15 (Fig. S2) provide a
continuous record of long-term changes of rBC deposition at this site from
the pre-industrial (AD 1741) into the most recent past (AD 2015, Fig. 2).
Deposition histories for major aerosol species are highly reproducible in the
two parallel CG03 ice cores (Figs. S3, S4), indicating minimal adverse
effects of snow drift and spatial variability of aerosol deposition present
at these spatial scales. Measured rBC values at CG03B vary strongly on
intra-annual timescales, with this variability being superimposed on
longer-term trends. Replicate analyses performed at the end of the
measurement campaign confirm that the original measurements performed over 2
months are highly reproducible over a concentration range of almost 3 orders of magnitude (Figs. 2, S5). Between 1975 and 2015 (
Median concentrations for selected chemical and elemental tracers from Colle Gnifetti during pre-industrial (PI), during time periods dominated by the fossil-fuel sources coal (COAL) and petroleum products (PETROLEUM).
All concentrations are median concentrations in ng g
To analyse long-term rBC variability we calculate annual mean values by
averaging all rBC values within the respective calendar year (Fig. 3).
Excluding occasional rBC spikes (
In the pre-industrial period, ammonium (
Colle Gnifetti rBC record (black) compared with
Few other ice-core records contain precisely dated information about Central
European industrial BC emissions for the 19th century. Previous
determinations of EC concentrations from the Colle Gnifetti ice core with
various methods are characterized by coarse resolution and unknown
reproducibility (Lavanchy et al., 1999; Thevenon et al., 2009). The
Fiescherhorn FH02 ice core obtained 70 km north of CG03 in the Bernese Alps
(3900 m a.s.l., 46
The non-BB rBC record (CG03 rBC
Estimated BC emissions from Bond et al. (2007) for OECD Europe and
Western Alps (45–47
Alpine glacier lengths during the emergence of industrial BC deposition (see Fig. 7).
Whereas absolute concentrations are, depending on the specific industrial pollutant (BC, Bi), a factor of 2–4 lower in Greenland, the long-term trends are remarkably similar between the Greenland stack and CG03 (Figs. 4d, 5). Overall, concentrations of major industrial pollutants appeared to have increased earlier by roughly 10 years (in AD 1890) in Greenland compared to the Alps, most prominently visible in bismuth. This delay is consistent with industrialization having accelerated earlier in North America (McConnell and Edwards, 2008) than in the major Central European countries (e.g. Germany, Italy, France). The maximum in industrial BC emissions were synchronous between Greenland and the Alps, both peaking at approximately AD 1915. Differences exist in the long-term trends of rBC since the early 20th century maximum, with Greenland values closely approaching pre-industrial levels while remaining elevated in the Alps.
We compare our ice-core based deposition history with estimated emissions of BC from fossil-fuel and bio-fuel burning (Bond et al., 2007) which form the main input for simulating BC climate effects (direct, indirect aerosol, and ice-albedo forcing) on past climate (Flanner et al., 2007; Lamarque et al., 2013; Shindell et al., 2013). We notice that the general structure of BC and EC from the two Alpine ice cores closely resembles estimated BC emissions for both OECD Europe and the western Alpine region taken from the bottom-up inventory of Bond et al. (2007) (Fig. 6). Interpreting the ice-core long-term trends as proxies for atmospheric burden (or emissions, respectively) we identify three major differences between these datasets. First, the increase in BC to its early 20th century maximum, as deduced from the ice cores, occurred in two subsequent steps, whereas the emission inventory implies a more gradual increase throughout the 19th and early 20th century. Second, the BC inventory emissions remain fairly steady at high levels from AD 1910 to 1950 with no decrease between the two world wars as in both Alpine ice cores, and to some degree also in the Greenland ice cores (Fig. 5). Third, the emission inventories suggest that BC emissions dropped significantly since the 1960s and reached for OECD Europe pre-industrial levels by 1980, whereas BC ice-core concentrations remained clearly above their pre-industrial values until the very recent past. Median concentrations from 1980 onwards are still 3-fold at CG03 and 1.5-fold at FH02 compared to pre-industrial AD 1741–1850 levels, respectively. Table 3 summarizes BC emission estimates based on inventories and mean ice-core rBC concentrations centred at AD 1850, 1915, 1975, and 2000 for CG03, FH02 and a Greenland ice-core stack, respectively.
To test the plausibility of an ice-albedo effect to force (or at least
contribute) to the glacier length reductions occurring during the 19th
century we here examine the exact timing of industrial BC deposition at Colle
Gnifetti and Fiescherhorn. For the latter, we interpret the sharp increase in
EC (Fig. 3) in the sample dated to the years AD 1875 to 1879 (Jenk et al.,
2006). Assuming conservatively that the increase occurred at the start age of
this discrete sample and considering a dating uncertainty of
The four high-resolution glacier length records indicate that in AD 1875
these glaciers had already completed the majority of their total cumulative
length reductions (i.e. maximum to minimum front position) of the second half
of the 19th century. Bossons had experienced 100 %, Oberer and Unterer
Grindelwald 83 % and 74 %, respectively, and Mer de Glace 79 %,
of their cumulative length losses (Table 4; Fig. 6e), with differences likely
explained by the different size and topography of the individual glaciers
(Lüthi, 2014). Consequently, the stacked record of all four glacier
terminus position curves reveals that the highest annual mean glacier length
reduction rates of
For the first time, we are able to examine a continuous, well-dated record of
BC from the pre-industrial into the most recent present at sub-annual
resolution (Fig. 2). Highly reproducible rBC measurements mirror the
low-resolution EC record obtained from the nearby Fiescherhorn ice core
(Fig. 3). We interpret this as evidence that these ice cores detect a common
signal of the atmospheric BC burden since the pre-industrial from
anthropogenic emissions of BC by industrial and transport related activities.
Other source tracers co-analysed with BC allow attribution of changes of the
main emission sources to the observed trends (Fig. 4). During the
pre-industrial (AD 1741–1850) CG03 rBC concentrations were low with episodic
spikes co-registered with ammonium attributed to anthropogenic or natural
biomass burning sources. Only later in the 19th century did concentrations of
rBC and other industrial pollutants (e.g. Bi, Pb,
Alpine glacier advances and volcanic eruption dates and resulting stratospheric aerosol properties (see Fig. 8).
Cumulative glacier length changes for the four glaciers Bossons, Mer de Glace, Oberer (O-) Grindelwald and Unterer (U-) Grindelwald with black dots marking years with observations (Nussbaumer and Zumbühl, 2012), tree-ring reconstructed Alpine summer (JJA) temperatures (Büntgen et al., 2011), minima in solar activity (Usoskin, 2017), and volcanic aerosol forcing (Revell et al., 2017; Toohey and Sigl, 2017) from AD 1500 to 1950. Grey shading marks time periods with increased volcanic aerosol forcing.
The new combined evidence strongly contradicts the previous key assumption of a synchroneity between glacier retreat in the European Alps and BC increase in the 19th century in apparent support of the hypothesis that industrial BC emissions have forced accelerated glacier melt through a snow-albedo feedback (Painter et al., 2013). 82 % [52 %] of the glacier length reductions had already occurred at the best [earliest] estimated time of emergence of industrial BC deposition (Fig. 7). The discrepancy in the temporal relation between our results and those of Painter et al. (2013) are in part explained by the low resolution in their deployed glacier length records in the 19th century (i.e. Rhône, Argentière), that tend to smooth the actual terminus position curves between AD 1850 and 1900 and also by the limited quality of the BC records available at that time (see Sects. 1 and 3.2). As shown in Fig. 8, retreat rates of the terminus positions from high-resolution glacier observations were much stronger between AD 1850 and 1875 than they were between AD 1875 and 1900. Moreover, when industrial BC emissions reached their overall maximum values in the 1910–20s, indicated by ice-core BC concentrations exceeding 5 times their pre-industrial values, Alpine glaciers showed no indications of further retreat, but were instead advancing again (Figs. 7, 8). As our rBC measurement technique is less sensitive to “brown carbon” and mixed component aerosols and larger compounds outside the detectable size range (up to mass-equivalent diameter of 800 nm) such as produced by burning low quality coal or inefficient coal combustion (Sun et al., 2017), this record alone cannot rule out a potential role for other light-absorbing aerosols. However, these compounds are measured by the method applied for the EC record from FH02 which shows very good agreement with the rBC record from Colle Gnifetti (Sect. 3.2). This suggests that factors other than changes in surface snow albedo, such as temperature and seasonal precipitation distribution (Steiner et al., 2008; Zumbühl et al., 2008), may have dominated mass balance and glacier length variability of these European glaciers until at least AD 1875. The previously made claim (Painter et al., 2013; Vincent et al., 2005) that precipitation and temperature variability alone were insufficient to explain the observed glacier length variability has been recently challenged by Lüthi (2014) and Solomina et al. (2016), who demonstrated that on a regional to global scale summer temperatures are the most important parameters determining glacier mass balance variability. Previously underestimated due to an “early instrumental warm bias” (Böhm et al., 2010; Frank et al., 2007), new surface air temperature (SAT) reconstructions based on early instrumental records, historical documentary proxy evidence, and tree-rings now all give evidence that the early 19th century was exceptionally cold in Central Europe in a long-term context (Brohan et al., 2012; Büntgen et al., 2011; Luterbacher et al., 2016) (Figs. 9, S11). A strong negative radiative forcing resulting from at least five large tropical eruptions between 1809 and 1835 (Sigl et al., 2015; Toohey and Sigl, 2017), in tandem with the Dalton solar minimum (Jungclaus et al., 2017; Usoskin, 2013) appeared to have forced the glaciers to strongly advance until the 1850s, in some cases probably far outside their range of typical long-term natural variability (Fig. 8). Similarly, later glacier advances (e.g. in the 1890s and 1910s) followed other major volcanic eruptions including Krakatau (1883) and Katmai (1912) (Table 5).
A strong role of volcanic forcing is supported by a consistent strong coherence of glacier expansions following clusters of volcanic eruptions throughout the past 2000 years (Le Roy et al., 2015; Solomina et al., 2016). The stratospheric aerosol burden for the time window AD 1600–1840 was 40 % larger than during the entire Common Era (Sigl and Toohey, 2017) with volcanic eruptions frequently forcing cold spells and glacier advances (e.g. in 1600s, 1640s, 1820s, 1840s) in the Alps (Fig. 9) and elsewhere (Solomina et al., 2016). Increased summer precipitation during cool post-volcanic summers may have additionally contributed to a more positive mass balance (Raible et al., 2016; Wegmann et al., 2014) plausibly enforced by a positive albedo feedback loop resulting from increased snow cover in the Alps. The glaciers' initial and more or less synchronous retreat from the maximum terminus positions starting at 1860 may be a delayed rebound back to their positions they had before the radiative perturbed time period AD 1600–1840 (Fig. 9), and an additional decrease of snow albedo from the deposition of BC is considered not to be needed to explain these observations (Lüthi, 2014). The specific extent to which early anthropogenic warming (Abram et al., 2016), changes in atmospheric modes (Swingedouw et al., 2017) including the Atlantic Multidecadal Oscillation (AMO; Huss et al., 2010), or snow-albedo feedbacks from increasing light-absorbing aerosol deposition towards the end of the 19th century may have contributed to the overall glacier length variability in the European Alps throughout the 19th century remains difficult to determine. Confidently attributing and quantifying the contribution of natural and anthropogenic forcing to observed glacier changes will require reconciliation of early instrumental and proxy climate data (Böhm et al., 2010; Frank et al., 2007), and the use of models to decompose the relative contribution of volcanic eruptions, light-absorbing impurities such as BC or other compounds (e.g. brown carbon, mineral dust), and other potential natural or anthropogenic contributions (Goosse et al., 2018; Zekollari et al., 2014).
While the CG03 rBC time series reproduces the general major emission
trends well from gridded BC emission inventories (Fig. 6) and it provides additional
structure that is currently not captured by the BC inventories. This includes
a stepwise increase of BC rather than linearly rising emissions; a short
reduction of emissions between the two world wars likely related to global
economic depression (Gabrieli and Barbante, 2014; Schwikowski et al., 2004)
and smaller reductions since the 1960s as opposed to the BC emission
inventories. Similar to Europe, Greenland ice-core BC records also do not
support the idea of a gradual increase in BC since AD 1850 but show a very
rapid increase of emissions around AD 1890, suggesting that the emission
inventories data before AD 1900 may be biased. This is plausible given the
small number and incomplete nature of consumption and technology-related
records contributing to these inventories during this time (Bond et al.,
2007, 2004). As many datasets (e.g. refinery outputs) during the 19th
century were only available for the USA and with extrapolation backward in
time applied often when specific data was unavailable (Bond et al., 2007) it
is not surprising that the BC emission trend in Europe (and other regions)
more or less closely follows that for North America prior to AD 1900 (Bond
et al., 2007; Y. H. Lee et al., 2013). The discrepancy between the
inventory-estimated and the much lower ice-core indicated reduction of BC in
CG03 since the 1960s is striking. This mismatch suggests that the measures taken
to reduce the release of BC into the atmosphere may not have been as
efficient as the energy-consumption data suggests. This may hint that the
emission factors (e.g. BC emitted per fossil-fuel unit burned) are
frequently reported as too low in these inventories, which in the light of
the Volkswagen emissions scandal revealed in 2015, seems at least a plausible
scenario. A comparable offset had been recently also noted between modelled
emissions and CG03 nitrate and ammonium records between 1995 and 2015
(Engardt et al., 2017) suggesting that besides rBC also
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. The comprehensive suite of elemental and chemical species co-analysed enabled BC source attribution from industrial and biomass burning emissions. The precisely dated ice core allowed precise comparison of the timing of observed acceleration of glacier retreat in the mid-19th century with that of increased deposition of black carbon on the glaciers caused by the industrialization in Europe. Closely reproducing the main structure of the Fiescherhorn EC record (Jenk et al., 2006), our study suggests that at the time when European rBC emission rates started to significantly increase (only after 1870) the majority of Alpine glaciers had already experienced more than 80 % of their total 19th century length reduction. Therefore, we argue that industrial BC emissions and subsequent deposition on Alpine glaciers are unlikely to be responsible for the rapid initial deglaciation at the end of the Little Ice Age in the Alps. We hypothesize that glacier length changes throughout the past 2000 years have been forced pre-dominantly by summer temperatures reductions induced by sulfuric acid aerosol forcing from large volcanic eruptions. In this sense, the retreat from the volcanically forced maximum glacier terminus positions starting in the 1860s can be seen as a lagged response of the cryosphere after the volcanic induced cooling had reached its maximum following a sequence of major tropical eruptions in AD 1809, 1815, 1823, 1831 and 1835. Only after AD 1870, when BC emissions started to strongly increase, snow-albedo impurity effects may have potentially contributed to the glacier length reductions.
Much of the understanding of future climate change is based on model simulations, but models used to predict future climate must be evaluated against past climate for accuracy (Hansen et al., 2007; Lamarque et al., 2013). Aerosols in climate models are mostly evaluated with observations from the past few decades, time periods during which mitigation measures for air quality control were widely in place. Ice-core records, possibly the only data sets to provide long-term historical information on aerosols, are thus critical for model evaluation, especially during time periods of widespread air pollution in industrialized countries during the 19th century. Here we present the first continuous BC record from Central Europe covering the past 270 years that has the resolution, precision, and reproducibility to serve in the future as a benchmark for climate models through dedicated model-data intercomparison (Koch et al., 2011; Y. H. Lee et al., 2013). Aerosol deposition at any single site also depends on factors such as atmospheric transport efficiency and the spatial distribution and conservation of snowfall. Incorporating more BC records from multiple sites into a stacked composite is expected to enhance the signal from the atmospheric burden over the noise caused by spatial variations in atmospheric transport and snow accumulation. Therefore, this should be considered a main focus for future research together with developing comparable records from other suitable ice-core sites in the Alps.
The CG03 ice-core data are available in the PANGAEA
repository,
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MiS conceived this study, performed BC and ion analyses, developed age-models, analysed data, and wrote the paper; NJA performed ToE analyses; JG analysed trace elements, DO helped processing ice cores and developed SP2 methodology, TMJ supervised the development of the SP2 method and led the 2015 CG ice-core drilling campaign, MaS coordinated the project, and MiS led the manuscript writing with input from all coauthors.
The authors declare that they have no conflict of interest.
This work was supported by the Swiss National Science Foundation through the research program “Paleo fires from high-alpine ice cores” (CRSII2_154450/1). It was fostered by participation in the Volcanic Impacts on Climate and Society (VICS) working group of PAGES (Past Global Changes). The authors thank Joe R. McConnell and Nathan Chellman for providing Greenland ice-core data; Matt Toohey for providing SAOD reconstructions; Samuel Nussbaumer for providing glacier length reconstructions; Fang Cao for providing the FH02 EC record; Carlo Barbante for partly funding the CG03 drilling campaign; Sabina Brütsch for ion chromatography analyses; and all members of the CG ice-core drilling expeditions in 2003, 2008 and 2015. We also would like to thank Thomas Painter and colleagues and the three anonymous reviewers for their comments and valuable input helping to improve the manuscript. Edited by: Becky Alexander Reviewed by: three anonymous referees