The Role of Solar Activity on Holocene Glacier Length Variability in the Swiss Alps
Hormes, A., Beer, J. and Schlüchter, C., 2006. A geochronological approach to understanding the role of solar activity on Holocene glacier length variability in the Swiss Alps. Geogr. Ann., 88 A (4): 281–294.
Review by George Taylor
A team of European researchers has found that glaciers in the Swiss Alps have lengthened and receded repeatedly during the Holocene in significant correlation with changes in solar irradiance.
Anne Hormes of Ångströmlaboratory, Uppsala University, Sweden, Jürg Beer of the Department of Surface Waters, EAWAG, Dübendorf, Switzerland, and Christian Schlüchter of the Department of Quaternary and Environmental Geology, University of Bern, Bern, Switzerland were the researchers.
Abstract — We present a radiocarbon data set of 71 samples of wood and peat material that melted out or sheared out from underneath eight present day mid-latitude glaciers in the Central Swiss Alps. Results indicated that in the past several glaciers have been repeatedly less extensive than they were in the 1990s. The periods when glaciers had a smaller volume and shorter length persisted between 320 and 2500 years. This data set provides greater insight into glacier variability than previously possible, especially for the early and middle Holocene. The radiocarbon-dated periods defined with less extensive glaciers coincide with periods of reduced radioproduction, pointing to a connection between solar activity and glacier melting processes. Measured long-term series of glacier length variations show significant correlation with the total solar irradiance. Incoming solar irradiance and changing albedo can account for a direct forcing of the glacier mass balances. Long-term investigations of atmospheric processes that are in interaction with changing solar activity are needed in order to understand the feedback mechanisms with glacier mass balances.
The researchers radiocarbon dated bits of wood and peat material that emerged from underneath eight present day mid-latitude glaciers in the Central Swiss Alps. The organic matter had grown in the glacial basins when the glaciers were smaller:
A crucial question is the original growing location of the subfossil trees and peat. Did the trees and peat grow where glaciers now fill the basin? This is undoubtedly the case because the reworked peat is glacially compressed and at Lago di Musella was overridden by a later glacier advance. In geological terms we are dealing with a sedimentary basin confined by the Holocene lateral moraine complexes. Peat clasts from Unteraargletscher point to the in situ development in former proglacial areas that are now occupied by the glaciers. In particular, the steep slopes of the environment are far from suitable habitats for substantial peat growth and suggest that the peat grew in the former proglacial area itself and therefore cannot have been moved to the finding place by avalanches. The mechanical deformation of the wood samples indicates that a glacial readvance did actively override trees and peat. This is supported by palaeovegetational evidence that the tree-line was up to 100–200 m higher than at present during the Holocene (Burga 1988; Tinner et al. 1996). This higher position of potential lateral tree-fragment input to the glaciers by avalanches is insufficient to bring the samples to the basal shear zone of the ice. Given the locations of our samples, we are convinced that organic sedimentation and tree growth took place in the basins now occupied by the glaciers.
In a previous publication (Hormes et al. 2001) evidence was presented from samples of detrital wood and peat that the glacier tongues must have been several hundred meters shorter than at the time of sampling in AD 1995–2000. In the current study, radiocarbon data identified periods of glacial retreat during the following periods before present (BP): 9010–7980, 7250–6500, 6170–5950, 5290–3870, 2300–1170 years BP.
The researchers concluded that, “since glacier recessions occurred in different environments at the same time, they resulted from external forcing beyond the regional scale.” They investigated possible causal factors that might explain why the glacier mass balance changes occurred, citing as possible reasons:
• air temperature (mainly summer);
• precipitation (mainly winter);
• air humidity;
• incoming short-wave radiation;
• albedo; and
• cloudiness
But the correlation that leapt out at them from the data had more to do with variations in solar irradiance:
Recent studies have shown a striking consistency between solar activity signals in peat bogs (Mauquoy et al. 2004), marine sediment records in the North Atlantic (Bond et al. 2001), speleothems in Oman (Fleitmann et al. 2003), and lake sediments from central Europe (Magny 1993) and Alaska (Hu et al. 2003). In the beginning of our investigation we did not consider that changes in solar activity might have a significant influence on glacier mass balances and therefore on glacier length reduction. However, as a result of the focused study on the radiocarbon curve and the good correspondence between the radiocarbon plateaux and our data on glacier length reduction, it became apparent that solar activity might have a greater impact on glaciers than previously assumed.
Changes in solar activity during the Holocene were estimated using “cosmogenic radionuclides” found in tree rings and ice cores. Solor radiation shields the earth from cosmic rays. Low solar radiation means more cosmic rays get through, and more (radioactive) carbon14 is created from (non-radioactive) carbon 12. High solar radiation leads to less 14C being generated during those time periods:
The data set on glacier length reduction is compared here with the intensity of the solar modulation. This signal, which is related to the open magnetic field of the sun, can be derived from cosmogenic radionuclides such as 14C in tree-rings and 10Be in ice cores (Vonmoos et al. 2006). During periods of high solar activity the open magnetic field carried by the solar wind is more intense, leading to a reduction of the cosmic ray flux and therefore in the production rate of cosmogenic radionuclides (Stuiver and Quay 1980; Lal 1988).
Comparing the glacial length modulation (data set developed with glacial detritus) with solar irradiance modulation (data set from cosmogenic radionuclides) revealed a striking correspondence:
Every period of low 14C production, which indicates high solar activity and an increase in solar irradiance, is followed by a reduction of glacier lengths in the Central Alps (Fig. 3). Note that whenever the solar modulation parameter F (Vonmoos et al. 2006), which is based on the 14C production rate (Stuiver and Braziunas 1988) and the geomagnetic field record (Yang et al. 2000), drops below a value of around 650, it is followed by a minimal extension of the length of the investigated glaciers…
It is most obvious that during the early and late Holocene glacier lengths were reduced during strong increases in solar activity.
Strong evidence developed by the researchers indicates that the primary reason for glacier variability in the Holocene is solar variability. By comparing glacier length with variability of the solar modulation (which is related to the open magnetic field of the sun), they concluded that every such period of high solar activity was followed by a reduction of glacier lengths in the Central Alps.
For the instrumental period (since the late 19th century), strong correlations exist between Total Solar Irradiance (TSI) and measured glacier length, so the solar-glacier relationship still exists. These long-term series showed significant correlation coefficients, with r2 = 0.79, 0.67 and 0.49, between total solar irradiance and glacial length The relationship was statistically strongest when glacial length changes were adjusted to a 9 to 14 year time lag behind solar irradiance changes.
The researchers concluded:
The present analysis of reductions in the length of Holocene glaciers indicates that variability in solar activity might influence glacier mass balances far more than has been estimated previously. A better understanding is needed of the interactions between solar activity, radiation changes and glacier mass balances. Monitoring of glaciers equipped with radiation measurement systems could help to answer the question concerning the extent to which incoming radiation and albedo are responsible for recent reductions in glacier length. The atmospheric processes that are influenced by changing solar activity also need a deeper understanding. The correlations presented indicate how important long-term measurement series will be in order to understand the physics of these interactions.
Reference
Hormes, A., Müller, B. and Schlüchter, C., 2001: The Alps with little ice: evidence for eight Holocene phases of reduced glacier extent in the Central Swiss Alps. Holocene, 11: 255–265.
