Shrinking glaciers are oft-cited examples of the effects of anthropogenic climate change, providing dramatic imagery in different parts of the world. However, this has mostly been based on global aggregates of glacier extent. Differing opinions also exist about the best way to measure glacial change all over the world. A recent study by Roe et al., published in Nature Geoscience, confirms that climate change has contributed to the shortening of numerous glaciers around the world, but the study is not immune to controversy surroundings the methods used.
Using a combination of meteorological data and observations of glacier length, Roe et al. studied the influence of climate on 37 glaciers between 1880 and 2010. The glaciers were selected based on the continuity of length observations and the need for a wide geographical distribution.
Glacier mass-balance records are a more direct measure of the effect of climate than glacier length as they measure the difference between the accumulation and ablation (sublimation or melting) of glacier ice. However, most mass-balance records do not extend for more than two decades, contributing to the previous lack of confirmation of the effect of climate change on individual glaciers around the world.
The use of observations of glacier length helped to overcome this obstacle, but challenges were still encountered in obtaining long, continuous data sets, particularly for regions such as Asia and South America. In conversation with GlacierHub, Roe shared that many factors can affect the availability of continuous data sets. “For example, the collapse of the Soviet Union led to many glacier observation programs being abandoned,” he stated.
An additional challenge arose from the variation in conditions experienced by each glacier. “Every glacier is a unique product of its local climate and landscape,” Roe shared, citing the example of maritime glaciers, which typically experience a large degree of wintertime accumulation variability. “This can mask the signal of a warming that, so far, has mainly impacted the summertime mass balance,” he added.
Nevertheless, Roe et al. found that there was at least a 99% chance that a change in climate was needed to account for the retreat of 21 of the glaciers studied. “Even for the least statistically significant (Rabots Glacier in Sweden), there was still an 89% chance that its retreat required a climate change,” Roe said.
As glaciers tend to have decadal responses to changes in climate, their retreat since 1880 is likely to be a result of twentieth-century temperature trends. They also act as amplifiers of local climate trends, providing strong signal-to-noise ratios that serve as strong evidence for the effects of anthropogenic climate change. For example, one of the glaciers included in the study, Hintereisferner in the Austrian Alps, retreated 2,800m since 1880, with a standard deviation (a measure of the deviation of values from the mean) of 130m. This value is small compared to the amount of retreat, providing a strong signal of change.
“We hope that these results will lead to a stronger scientific consensus about the cause of glacier retreat. The last round of the Intergovernmental Panel on Climate Change was quite timid, concluding only that it was ‘likely’ that a ‘substantial’ part of glacier retreat was due to human-caused climate change,” Roe added. IPCC nomenclature would make it “very likely” (≥90%) that all but one of the glaciers in this study have retreated because of climate change, allowing for stronger conclusions to be drawn.
Excitement about the results of this study was shared by Joerg Schaefer, professor at the Lamont-Doherty Earth Observatory: “Under Roe’s lead, the really smart glacier people find ways to explain this strange observation that glaciers are highly individual beasts if you look at short time scales (years and decades), but behave like a flock of well-behaved sheep when you look at longer (centennial and millennial time-scales),” Schaefer said in an interview with GlacierHub. “This will help us a lot down the road to better predict rates of glacier change for the next century.”
In contrast, Mauri Pelto, professor of environmental science at Nichols College who has been involved in the North Cascade Glacier Climate Project for 34 of years, expressed that the paper was interesting but not the first confirmation of glaciers being impacted by anthropogenic climate change. “This does not mean it is not worth writing about,” said Pelto, “but it needs to be placed in the context of the other key studies that were both earlier, and, I believe, stronger.”
For example, the authors looked at fewer glaciers than Oerlemans et al. (2005) while modelling each in more detail. Pelto notes that they also used far less data than Zemp et al. (2015) in making an even more compelling statement on the status of glaciers. Finally, the authors are not the first to conduct an attribution study: note Marzeion et al. (2014). While their statistical method is quite robust, their modelling approach that generates data does not have an impressive verification record, according to Pelto.
“Other recent studies better represent the certainty of glacier change being driven by climate,” Pelto concluded.
These opinions indicate that glacier retreat continues to attract attention and stimulate active debate, pointing to the importance of glaciers and climate change. The approach used in this study relies on glacier length, a less precise measure than mass-balance. However, its value lies in the ability to consider long meteorological and glacier length records for a number of glaciers, contributing to an important and growing body of knowledge about the effects of anthropogenic climate change on glaciers all over the world.