Fast pace of glacier melt in the 1940s: lower aerosol pollution

In Switzerland, the increase in snow in wintertime and the glacier melt in summertime have been measured at measurement points at around 3,000 metres above sea level -- on the Clariden Firn, the Great Aletsch glacier and the Silvretta glacier -- without interruption for almost 100 years. As part of his doctoral work, Matthias Huss used this unique range of measurements to examine how climate change in the last century affected the glaciers. The work was carried out under the supervision of Martin Funk, professor and head of the Department for Glaciology at the Laboratory for Hydraulics, Hydrology and Glaciology ('VAW') at ETH Zurich, who is also co-author of the study.

Solar radiation as the decisive factor

In its work, the research team took into account the solar radiation measured on the Earth's surface in Davos since 1934. Studies over the past two decades have shown that solar radiation varies substantially due to aerosols and clouds, and this is assumed to influence climate fluctuations. Recent years have seen the emergence of the terms 'global dimming' and 'global brightening' to describe these phenomena of reduced and increased solar radiation respectively. These two effects are currently the subject of more and more scientific research, in particular by ETH Zurich, as experts feel that they should be taken into account in the climate models.

The new study, published in the journal 'Geophysical Research Letters', confirms this requirement. This is because, taking into account the data recorded for the level of solar radiation, the scientists made a surprising discovery: in the 1940s and in the summer of 1947 especially, the glaciers lost the most ice since measurements commenced in 1914. This is in spite of the fact that temperatures were lower than in the past two decades. "The surprising thing is that this paradox can be explained relatively easily with radiation," says Huss, who was recently appointed to the post of senior lecturer at the Department of Geosciences at the University of Fribourg in Switzerland.

On the basis of their calculations, the researchers have concluded that the high level of short-wave radiation in the summer months is responsible for the fast pace of glacier melt. In the 1940s, the level was 8% higher than the long-term average and 18 Watts per square metres above the levels of the past ten years. Calculated over the entire decade of the 1940s, this resulted in 4% more snow and ice melt compared with the past ten years.

Furthermore, the below-average melt rates at the measurement points during periods in which the glacier snouts were even advancing correlate with a phase of global dimming, between the 1950s and the 1980s.

Less snow fall and longer melt periods

The researchers arrived at their findings by calculating the daily melt rates with the aid of climate data and a temperature index model, based on the half-yearly measurements on the glaciers since 1914. These results were then compared with the long-term measurements of solar radiation in Davos.

Huss points out that the strong glacier melt in the 1940s puts into question the assumption that the rate of glacier decline in recent years "has never been seen before." "Nevertheless," says the glaciologist, "this should not lead people to conclude that the current period of global warming is not really as big of a problem for the glaciers as previously assumed." This is because it is not only the pace at which the Alpine glaciers are currently melting that is unusual, but the fact that this sharp decline has been unabated for 25 years now.

Another aspect to consider -- and this is evidenced by the researchers' findings -- is that temperature-based opposing mechanisms came into play around 30 years ago. These have led to a 12% decrease in the amount of precipitation that falls as snow as a percentage of total precipitation, accompanied by an increase of around one month in the length of the melt period ever since this time. Scientists warn that these effects could soon be matched by the lower level of solar radiation we have today compared with the 1940s.