Australia: The Land Where Time Began

A biography of the Australian continent 

Centennial Retreat of Glaciers - Categorical Evidence of Climate Change

Some of the most iconic imagery for communicating to the public that anthropogenic climate change is actually happening is the near-global retreat of glaciers over the last century. However, it is surprising that there has not been a quantitative foundation for the attributing of the retreat to climate change, except in the global aggregate. Uncertainties in the numerical modelling and the short length of records of glacier mass-balance, is responsible for this gap between public perception and scientific basis. In this paper Roe et al. present a method for assessing change of individual glaciers that is based on signal-to-noise ratio, which is a robust metric that is not sensitive to uncertainties in glacier dynamics. Roe et al. have demonstrated that retreat of individual glaciers that have been observed represent some of the highest signal-to-noise ratios of climate change that have been documented. Therefore the retreat of local glaciers on a centennial scale does indeed constitute categorical evidence of climate change.

In the lives of nearby communities (Nussbaumer * Zumbühl, 2012) alpine glaciers are consequential and captivating elements of the Earth System. The history of several hundred glaciers is well known as a result of research that has been carried out since the 19^th century. Glacier mass-balance^* is a more direct measure of climate (Braithwaite & Zhang, 1999; Medwedeff & Roe, NA) than the length of glaciers, but there are only a few mass-balance records extending more than 2 decades.

Near-global glacier retreat of glacier fronts on a scale of centuries appears to be improbable without some climate change that is coordinating the process. The formal assessment of the role climate change has played  in the retreat of glaciers has, however, been limited to the numerical modelling of 3 individual glaciers, each with only a single set of model parameters (Oerlemans, 2000); and to a comparison of the global aggregate mass loss in integrations of global climate models, forced and unforced (Marzeion, 2014).

Each glacier is a product of its local climate and landscape, so by itself is a blunt statistical instrument. Only a few independent degrees of freedom in a centennial record is implied by characteristic glacier length response times of several decades (Leclercq & Oerlemans, 2012), with the result that it has a poor statistical resolving power to evaluate a trend. Partly due to these factors, the most recent assessment of the Intergovernmental Panel on Climate Change (IPCC) concluded that it was only likely that a ‘substantial’ part of the retreat of a glacier is due to anthropogenic climate change, which was a much weaker attribution than for other metrics of climate change (Bindoff et al., 2013). In this paper Roe et al. introduce a method of combining observations of glaciers with the local meteorological trends that are resolved better, which facilitates strong conclusions. The centennial scale retreats of 37 glaciers that were dispersed widely have each necessarily required a climate change. The climate changes that are required are centennial-scale trends that are distributed globally,

*Area averages accumulation minus ablation, ≡b (m per yr)

Sources & Further reading

1. Roe, G. H., M. B. Baker and F. Herla (2017). "Centennial glacier retreat as categorical evidence of regional climate change." Nature Geosci 10(2): 95-99.