"With shrubs we now, hopefully, are able to reconstruct the summer mass balance of larger ice shields such as in Greenland or Svalbard beyond instrumental records," Allan Buras of the University of Greifswald told environmentalresearchweb. Such data could ultimately help to calibrate and improve models of glacier behaviour.

The idea for the study came to Buras while he was eating lunch during fieldwork in southern Norway. Looking at the landscape around him, he surmised that the main driver of shrub growth in the area was probably summer temperatures. And it was likely that the 20th century retreat of the Hardangerjökulen ice cap he could see in the distance was also related to summer temperatures. So it struck him that it would be worth examining whether shrubs could help reconstruct historical glacier retreats.

Together with colleagues Buras measured growth rings in the stems of 24 samples of willow shrub found roughly 3 km from the ice cap. The team compared these results to data from the Norwegian Meteorological Office's Finsevatnet climate station as well as to mass-balance data for the ice cap from 1963–2010 in the scientific literature. The result? Wider growth rings, which were associated with warmer temperatures in the peak growth times of July and August, were linked to a more negative glacier mass balance, i.e. increased melting.

The oldest shrub the team tested was 77 years of age, the second oldest 44; the average age was 28. Since the researchers found that a sample size of more than five shrubs led to better results, they were only able to make reliable approximations of glacier summer mass balance for the last 36 years, from 1975 to 2010. Although Hardangerjökulen has glacier monitoring data available beyond this, back to 1963, many glaciers do not have records this extensive. And shrubs such as juniper, which can be as much as 200 years old, may be suitable for the same analysis.

"We would be happy if our approach is tested with already available shrub chronologies in other regions and maybe even within a network analysis such as the Shrub Hub, an international community of scientists, to reconstruct the glacier melt of large areas like Greenland, Svalbard and Norway," said Buras. Having sampled shrubs in Greenland this summer, the team hopes to get additional funding to conduct surveys in other areas.

The team reported the study in Environmental Research Letters (ERL).