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Australia: The Land Where Time Began |
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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 19th 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)
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |