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1940s and 1950s followed by a moderate mass loss until the end of the 1970s and
a subsequent acceleration that has lasted until now.''
They also emphasized ''the shortcomings of the available dataset'' and ''the
relatively small set of current long-term observations with a strong bias towards
the Northern Hemisphere and Europe.''
Braithwaite (2009) echoed Zemp et al. in emphasizing the sparsity of the
mountain glacier dataset. He also pointed out that the 30 glaciers that have 30
years of data ''are biased towards wetter conditions than are typical for global
glacier cover.'' They went on to say: ''The mass-balance variations for these
glaciers are therefore not representative of the global glacier cover'' and ''these 30
glaciers must be showing larger mass-balance changes than the global average.''
They concluded:
may
therefore be too large. There is already a discrepancy between observed sea-level
rise and estimated contributions from the different sources, and the discrepancy
will increase if the contribution from glaciers is really overestimated.''
''Current estimates of the glacial contribution to sea-level rise
...
Haberli and Hoelzle
12
reviewed data on several mountain glaciers. Of
particular interest is the Aletsch Glacier in Switzerland. These authors present
pictures of this great glacier taken in 1856 and 2001, showing the great retreat of
the glacier. The 20th century history of Aletsch Glacier retreat shows that (i) the
retreat began before the buildup of CO
2
concentrations and (ii) there have been
alternating advances and retreats for two millennia. On the other hand, the retreat
seems to have accelerated in the late 20th century as CO
2
built up.
Nature Magazine commented on glacier retreat in its June 10, 2010 issue. The
issue's editorial was based on a recent Swiss study (Huss et al., 2010)
13
that
involved over 10,000 in situ observations of 30 Swiss mountain glaciers. The
results suggest that Swiss glacier retreat was due to a combination of factors:
(1) changes in precipitation patterns, (2) cyclic changes in North Atlantic sea
surface temperatures (60-year cycle?), and (3) increased air temperatures. Since
Nature is highly biased toward the extreme alarmist view, it is not surprising
that the editorial ended with some dire predictions for the remainder of the
21st century. Huss et al. concluded ''Strong glacier retreat is very likely in the 21st
century!'' Extrapolating forward from the 20th century, this seems to be a fairly
safe prediction. The next several decades should provide a test of this prediction.
Holzhauser et al. (2005) reported on glacier and lake level variations over the
last 3,500 years. Their results clearly show the MWP and the LIA. However, the
retreat of glaciers in the most recent period appears to be sharper than previous
retreats and reflects global warming in the 20th century. This may be linked to
13
M. Huss, G. Jouvet, and M. Funk, C2SMClimate Change Scenario Workshop, ETH Zurich,
March 2, 2010.
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