Vor 8000 Jahren ging es den skandinavischen Gletschern schon einmal an den Kragen

In den Rocky Mountains in Montana haben Nationalparksmitarbeiter klammheimlich Schilder abgebaut, die den Exitus der dortigen Gletscher für 2020 vorhergesagt hatten. Das Problem: Die Gletscher sind immer noch da.

Mittlerweile ist man sich offenbar auch gar nicht mehr so sicher, ob die Gletscherschmelze im westlichen Nordamerika nicht zu einem gewichtigen Teil durch natürliche Variabilität gesteuert wird. Menounos et al. 2019 gehen damit offensiv um:

It remains uncertain whether mass change observed over the last 18 years is related to natural climate variability known to affect glacier mass balance in [Western North America] (Bitz & Battisti, 1999; Hodge et al., 1998; Moore & Demuth, 2001), stochastic variability, or whether these recent changes are related to anthropogenic climate change. […] If the last 18 years provide a suitable analogue for the next 30–50 years, future glacier change will be modulated by decadal‐scale climate variability.

Passend zum Thema ist die folgende Studie zur Gletschergeschichte Spitzbergens. Eine Gruppe um Wilhelm van der Bilt untersuchte die Gletscherveränderungen für die vergangenen 10.000 Jahre. Vor etwa 7000 Jahren waren die Gletscher im Untersuchungsgebiet offenbar vollkommen abgeschmolzen, also während der holozänen thermischen Maximums. Vor 4000 Jahren wuchsen die Gletscher dann wieder stark an, was den Beginn des Neoglazials kennzeichnet. In der Kleinen Eiszeit legten die Gletscher dann nochmal besonders kräftig zu. Hier der Abstract (van der Bilt et al. 2015):

Reconstruction of glacier variability from lake sediments reveals dynamic Holocene climate in Svalbar

The Arctic is warming faster than anywhere else on Earth. Holocene proxy time-series are increasingly used to put this amplified response in perspective by understanding Arctic climate processes beyond the instrumental period. However, available datasets are scarce, unevenly distributed and often of coarse resolution. Glaciers are sensitive recorders of climate shifts and variations in rock-flour production transfer this signal to the lacustrine sediment archives of downstream lakes. Here, we present the first full Holocene record of continuous glacier variability on Svalbard from glacier-fed Lake Hajeren. This reconstruction is based on an undisturbed lake sediment core that covers the entire Holocene and resolves variability on centennial scales owing to 26 dating points. A toolbox of physical, geochemical (XRF) and magnetic proxies in combination with multivariate statistics has allowed us to fingerprint glacier activity in addition to other processes affecting the sediment record. Evidence from variations in sediment density, validated by changes in Ti concentrations, reveal glaciers remained present in the catchment following deglaciation prior to 11,300 cal BP, culminating in a Holocene maximum between 9.6 and 9.5 ka cal BP. Correspondence with freshwater pulses from Hudson Strait suggests that Early Holocene glacier advances were driven by the melting Laurentide Ice Sheet (LIS). We find that glaciers disappeared from the catchment between 7.4 and 6.7 ka cal BP, following a late Hypsithermal. Glacier reformation around 4250 cal BP marks the onset of the Neoglacial, supporting previous findings. Between 3380 and 3230 cal BP, we find evidence for a previously unreported centennial-scale glacier advance. Both events are concurrent with well-documented episodes of North Atlantic cooling. We argue that this brief forcing created suitable conditions for glaciers to reform in the catchment against a background of gradual orbital cooling. These findings highlight the climate-sensitivity of the small glaciers studied, which rapidly responded to climate shifts. The start of prolonged Neoglacial glacier activity commenced during the Little Ice Age (LIA) around 700 cal BP, in agreement with reported advances from other glaciers on Svalbard. In conclusion, this study proposes a three-stage Holocene climate history of Svalbard, successively driven by LIS meltwater pulses, episodic Atlantic cooling and declining summer insolation.

Die bewegte natürliche Klimadynamik Spitzbergens war auch Thema von Rothe et al. 2015. Die Autoren untersuchten Gletscherveränderungen im Nordwestteil Svalbards und fanden, dass der Karlbreen-Gletscher 9200-3500 Jahre vor heute sehr viel kleiner als heute bzw. komplett abgeschmolzen war. Das waren heiße Zeiten! Abstract:

Arctic Holocene glacier fluctuations reconstructed from lake sediments at Mitrahalvøya, Spitsbergen

The Arctic region has experienced a significantly larger warming during the last decades compared to the rest of the world, and model simulations indicate a continued amplification of future global warming in the Polar Regions. A better understanding of natural climate variability in the Arctic is much needed to provide a better context for the observed warming trend. By utilising proxy data it is possible to obtain palaeoclimatic records beyond the range of instrumental observations, which increase our understanding of long-term Arctic climate change. Here, a continuous record of past changes in Equilibrium-Line Altitude (ELA) has been reconstructed for the alpine glacier Karlbreen, located on the northwest coast of Spitsbergen (79° N), based on sediment analyses from a distal glacier-fed lake. A multivariate statistical analysis suggests that the concentration of geochemical elements Ti, Si and K in the lake sediments, together with the physical parameter dry-bulk-density (DBD), reflect changes in the amount of inorganic detrital input to Kløsa, which is closely linked to the size and ELA of the upstream glacier Karlbreen. A linear regression model based on historically documented glacier extents was used to calculate continuous ELA changes back to ∼3500 cal. yr. BP. From about 9200 to 3500 cal. yr. BP, the sedimentary record indicates that Karlbreen was very small or had completely melted away. Karlbreen was probably close to its maximum Holocene extent several times during the Neoglacial, first around 1700 cal. yr. BP, then later at ∼225 and ∼135 cal. yr. BP. An ice-cored moraine system in front of Karlbreen extends well into the main basin of Kløsa, and it is difficult to explain how this moraine could have formed without disturbing the sedimentary record in the lake (e.g. through slumping events). The sedimentary record in Kløsa is continuous and undisturbed over the past 6700 years, suggesting that the outermost moraine formed prior to this time and that it most likely survived the Holocene Thermal Maximum on Svalbard.

Das selbe Bild im arktischen Norwegen, wo viele Gletscher während des holozaenen themrischen Optimums komplett verschwunden waren. Hier eine Studie von Wittmeier et al. 2015:

Reconstructing Holocene glacier activity at Langfjordjøkelen, Arctic Norway, using multi-proxy fingerprinting of distal glacier-fed lake sediments

Late Glacial and Holocene glacier fluctuations are important indicators of climate variability in the northern polar region and contain knowledge vital to understanding and predicting present and future climate changes. However, there still is a lack of robustly dated terrestrial climate records from Arctic Norway. Here, we present a high-resolution relative glacier activity record covering the past ∼10,000 cal. a BP from the northern outlet of the Langfjordjøkelen ice cap in Arctic Norway. This record is reconstructed from detailed geomorphic mapping, multi-proxy sedimentary fingerprinting and analyses of distal glacier-fed lake sediments. We used Principal Component Analysis to characterize sediments of glacial origin and trace them in a chain of downstream lakes. Of the variability in the sediment record of the uppermost Lake Jøkelvatnet, 73% can be explained by the first Principal Component axis and tied directly to upstream glacier erosion, whereas the glacial signal becomes weaker in the more distal Lakes Store Rundvatnet and Storvatnet. Magnetic susceptibility and titanium count rates were found to be the most suitable indicators of Holocene glacier activity in the distal glacier-fed lakes. The complete deglaciation of the valley of Sør-Tverrfjorddalen occurred ∼10,000 cal. a BP, followed by a reduced or absent glacier during the Holocene Thermal Optimum. The Langfjordjøkelen ice cap reformed with the onset of the Neoglacial ∼4100 cal. a BP, and the gradually increasing glacier activity culminated at the end of the Little Ice Age in the early 20th century. Over the past 2000 cal. a BP, the record reflects frequent high-amplitude glacier fluctuations. Periods of reduced glacier activity were centered around 1880, 1600, 1250 and 950 cal. a BP, while intervals of increased glacier activity occurred around 1680, 1090, 440 and 25 cal. a BP. The large-scale Holocene glacier activity of the Langfjordjøkelen ice cap is consistent with regional temperature proxy reconstructions and glacier variability across Norway. Long-term changes in the extent of the northern outlet of the Langfjordjøkelen ice cap largely followed trends in regional summer temperatures, whereas winter season atmospheric variability may have triggered decadal-scale glacial fluctuations and generally affected the amplitude of glacier events.

Die Zeit 8000-4000 Jahre vor heute war für die Gletscher in Norwegen nicht besonders angenehm. Denn die meisten Gletscher existierten damals gar nicht, wie Nesje et al. 2008 bereits berichteten. Wenn wir heute das Schrumpfen der Gletscher beklagen vergleichen wir mit dem Zustand der Kleinen Eiszeit vor 300 Jahren, als die meisten Gletscher ihre größte Ausdehnung der letzten 10.000 Jahre erreichten. Abstract von Nesje et al. 2008:

Norwegian mountain glaciers in the past, present and future

Documentation of glacier changes is a key element for reconstruction of past climate variability and early detection of global climate change. In this paper, records of Holocene glacier variations in different regions in Norway have been synthesised. During the period from approximately 8000 to 4000 cal. yr BP, most glaciers in Norway were completely melted away at least once due to high summer temperatures and/or reduced winter precipitation. Lichenometrically and historically dated moraines at Jostedalsbreen, in Jotunheimen, at Hardangerjøkulen, and at Folgefonna were used to extend records of glacier length variations back to their maximum position during the ‘Little Ice Age’. The timing of the maximum ‘Little Ice Age’ glacial advance in different parts of southern Norway varied considerably, ranging from the early 18th century to the late 19th century. Cumulative glacier length variations of glaciers in southern Norway show an overall retreat from ∼ AD 1750 to the 1930s–40s. Thereafter, most Norwegian glaciers retreated significantly. Short maritime outlet glaciers with a short response time (< 10–15 yr) started to advance in the mid-1950s, whereas long outlet glaciers with longer frontal time lag (> 15–20 yr) continued their retreat to the 1980s. In the 1990s, however, several of the maritime glaciers started to advance as a response to higher winter accumulation during the first part of the 1990s. Since 2000 most of the observed glaciers have retreated remarkably fast (annual frontal retreat > 100 m) mainly due to high summer temperatures. The last glacier inventory in Norway published in 1988 shows that there were 1627 glaciers covering a total area of 2609 km2 with an estimated volume of 164 km3. Modern climate–glacier relationships from mass balance data in Scandinavia have been used to present possible effects on the Norwegian glaciers of climate scenarios between 1961–1990 and 2070–2100 presented by the ‘RegClim’ project. This long-term weather ‘forecast’ for western Norway indicates a rise in the summer temperature of 2.3 °C and an increase in the winter precipitation of 16% by the end of the 21st century. This climate scenario may, if it occurs, cause the equilibrium-linealtitude (ELA) to rise 260 ± 50 m. As a result, about 98% of the Norwegian glaciers are likely to disappear and the glacier area may be reduced ∼ 34% by AD 2100.

Die natürliche Dynamik und vorindustrielle Klimageschichte wird in der heutigen Debatte leichtfertig außer acht gelassen. Selbst in den letzten Jahrzehnten gab es in den skandinavischen Gletschern Schmelz- und Zuwachsphasen die sich abwechselten. Holmlund & Holmlund 2019 erinnerten am Beispiel des schwedischen Storglaciären an einen Gletschervorschub in den 1990er Jahren.