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We were surprised to read yesterday that ‘about
5,500 glaciers could disappear or drastically retreat by the end of the
century…’ – The Guardian. While it may seem obvious that climate change will
ultimately lead to the demise of Himalayan glaciers, 2100 seems awful
soon!
The headlines:
•
Up to 99% of Everest's glaciers could begone by 2100: Global warming could devastate the Himalayas, scientists warn –
Daily Mail 27 May 2015
•
Most glaciers in Mount Everest area willdisappear with climate change – study – The Guardian Wednesday 27 May 2015
•
‘About 5,500
glaciers could disappear or drastically retreat by the end of the century with
severe impacts on farming and hydropower…’ - The Guardian Wednesday 27 May 2015
•
Climate change could shrink MountEverest’s glaciers by 70 percent, study finds – The Washington Post - May 27
2015
The research published in The Cryosphere is a modelling study showing the sensitivity of
glaciers in the Everest region to climate change. It represents a step forward in
the application of such glacier models and incorporation of the current
process-based understanding of Himalayan glaciers, but as stated by the authors
is not without uncertainty. Although providing a first look at the possible
sensitivity to climate change of glaciers in the Everest region, this paper
also shows that glaciologists still have some way to go, both in terms of building
models that represent the processes that cause these mountain glaciers to
change, and in collecting field data to calibrate and validate their models.
Large uncertainties in
glacier modelling:
Some major limitations with this paper are
discussed during this process and indeed in the paper itself, but this uncertainty
does not necessarily translate into the media headlines. Similarly the modelling
process cannot account for key aspects of the current understanding of the
behaviour of large debris-covered glaciers (see below), such as the ones
shown to disappear by 2100 in a widely publicised GIF animation. Models will always include uncertainties, since they are by design a
simplification of a reality which cannot be represented in its entirety.
Greater simplifications are often justified for regional studies.
The model results presented in the figure below
from the paper by Shea and coauthors shows the majority of ice thickness loss
(red) occurring at low elevation on glaciers in the Everest region. Compare
this with the second figure from Bolch et al. (2011), which shows the change in
glacier surface height (indicating change in ice thickness) measured from
satellite observations over a similar period to the model results (1970–2007).
Modelled
mean annual mass budget, 1961–2007. From http://www.icimod.org/?q=18340
The ablation (melt) pattern shown above is known
to occur on clean-ice glaciers since they response predominately to temperature
(more melt at lower elevations), but thick rock debris cover towards the
terminus of these glaciers insulates the ice below, meaning the majority of
melt occurs higher up in the mid reaches of the ablation zone of these low
gradient glaciers where the debris-cover is thinner (e.g. Bolch et al., 2011; Benn
et al., 2012).
The model calibration
(1961 - 2007) overestimates the magnitude and spatial distribution glacier
ablation (melt). For example,
between 1970 - 2007, Bolch et al. (2011) show between 25 - 50 m of total
elevation (ice thickness) loss across the Khumbu Glacier, or ~0.67 - 1.35 m per
year. This loss is essentially zero at the terminus (see figure below), whereas
a figure reported in this recent study (shown above) shows a maximum modelled
elevation (ice thickness) loss of ~ 1.4 m per year, which is over twice the rate
of surface lowering measured from satellite observations.
 |
Elevation
differences from Bolch et al. (2011)
|
While the model represents
valuable progress in the field of glacier modelling, it is published as a
sensitivity study with large accompanying uncertainties, and therefore should
be treated as such.
Improved modelling studies
are just around the corner: The authors acknowledge that ‘at this stage, due to computing
time, a lack of knowledge about debris-covered dynamics or data for
validation, we cannot address in details some of the reviewers’ comments but
we are confident that it will be possible in the future’. These studies
are therefore important to highlight model sensitivities and where more field
data is required to validate and improve existing for future research. Making
predictions of glacier change in Himalaya requires both improvements in existing
glacier models to capture more of the processes important to the behaviour of
mountain glaciers, and data from these glaciers to tell us how they are
changing over time. After all, a model is only as strong as the data that it is
validated with.
Limited field data:
Perhaps most importantly, the starting
ice thickness (and hence volume) simulated by the model of the glaciers in the
Everest region does not correlate well with what is presented in the literature.
Such depth measurements are scarce but are available in some places. For
example Gades et al. (2000) and Moribayashi (1978) show the ice thickness on
the Khumbu glacier near the base of the icefall is over 400 m. However, the modelled
ice volume at the same location is only 200 m as shown on the animation.
It is therefore likely that the glaciers in this region have a notably larger
ice volume than is represented in this model and would therefore take notably
longer to disappear.
There are other factors which can lead to rapid glacier recession
such as glacial lake formation, and also factors that prolong the lifespan of
glacier ice considerably, such as the surface layers of rock debris found on
many large Himalayan glaciers that insulate them from warming air temperatures.
These will not be addressed here since they are not incorporated into the
modelling study, but would probably result in different predictions of glacier
response to climate change.
Where next?
Interestingly in the response to reviewers comments dated 29 January 2015. The authors explicitly state that ‘…our
results should not be considered as projections or predictions of future
glacier change’, rather the study was a tool to explore possible glacier sensitivities
to climate change.
Overall the work of Shea et al., 2015 provides an
important study into how the regions glaciers could respond to climatic
warming. However, the headlines should perhaps treat more cautiously the
results presented. There are certainly many things we don’t yet know about
how glaciers in the Himalaya will change during the 21st Century!
References:
Benn, D.I. Bolch, T.
Hands, K. Gulley, J. Luckman, A. Nicholson, L.I. Quincey, D. Thompson, S.
Toumi, R. and Wiseman, S. 2012. Response of debris-covered glaciers in the
Mount Everest region to recent warming, and implications for outburst flood
hazards. Earth-Science Reviews. 114(1–2), pp.156-174.
Bolch, T. Pieczonka, T.
and Benn, D.I. 2011. Multi-decadal mass loss of glaciers in the Everest area
(Nepal Himalaya) derived from stereo imagery. The Cryosphere. 5(2),
pp.349-358.
Gades, A. Conway, H. and
Nereson, N. 2000. Radio echo-sounding through supraglacial debris on Lirung and
Khumbu Glaciers, Nepal Himalayas.
Moribayashi, S. 1978.
Transverse Profiles of Khumbu Glacier Obtained by Gravity Observation
Quincey, D.J. Luckman, A.
and Benn, D. 2009. Quantification of Everest region glacier velocities between
1992 and 2002, using satellite radar interferometry and feature tracking.
Journal of Glaciology. 55(192), pp.596-606.
