2.2.5.4 Mountain glaciers
The recession of mountain glaciers was used in IPCC (1990) to provide qualitative
support to the rise in global temperatures since the late 19th century. Work
on glacier recession has considerable potential to support or qualify the instrumental
record of temperature change and to cast further light on regional or worldwide
temperature changes before the instrumental era. Two types of data from glaciers
contain climatic information: (i) mass balance observations and (ii) data on
the geometry of glaciers, notably glacier length. More comprehensive information
is now becoming available and worldwide glacier inventories have been updated
(e.g., IAHS (ICSI)/UNEP/UNESCO, 1999). Note that changes in the Greenland and
Antarctic ice sheets are discussed in Chapter 11.
We first discuss mass balance observations. The specific mass balance is defined
as the net annual gain or loss of mass at the glacier surface, per unit area
of the surface. The mass balance averaged over an entire glacier is denoted
by Bm. Systematic investigations of glacier mass balance started after 1945,
so these records are shorter than the instrumental climate records normally
available in the vicinity. In contrast to frequently made statements, Bm is
not necessarily a more precise indicator of climate change than is glacier length.
Time-series of Bm do contain year-to-year variability reflecting short-term
fluctuations in meteorological quantities but of concern on longer time-scales
is the effect of changing glacier geometry. A steadily retreating glacier will
get thinner and the mass balance will become more negative because of a slowly
increasing surface air temperature due to a lowering surface that is not reflected
in a large-scale temperature signal. Climatic interpretation of long-term trends
in of mass balance data requires the use of coupled mass balance-ice flow models
to separate the climatic and geometric parts of the signal. Such studies have
only just begun. However, mass balance observations are needed for estimating
the contribution of glacier melt to sea level rise, so are discussed further
in Chapter 11.
A wealth of information exists on the geometry of valley glaciers. Glacier
records are very useful for studies of Holocene climate variability (e.g., Haeberli
et al., 1998; and Section 2.4). Written documents going
back to the 16th century exist that describe catastrophic floods caused by the
bursting of glacier-dammed lakes or arable land and farms destroyed by advancing
glaciers, e.g., in 18th century Norway (Østrem et al., 1977). A large
amount of information is available from sketches, etchings, paintings and old
photographs of glaciers, though many show the same glaciers (Holzhauser and
Zumbühl, 1996). About fifty glaciers have two or more useful pictures from
distinctly different times. In many cases geomorphologic evidence in the form
of terminal moraines and trimlines can be used as reliable complementary information
to construct the history of a glacier over the last few centuries. Systematic
mapping of glaciers started only 100 years ago and has been limited to a few
glaciers. The most comprehensive data are of length variations. Glacier length
records complement the instrumental meteorological record because (i) some extend
further back in time; (ii) some records are from remote regions where few meteorological
observations exist; (iii) on average, glaciers exist at a significantly higher
altitude than meteorological stations.

Figure 2.18: A collection of twenty glacier length records from different
parts of the world. Curves have been translated along the vertical axis
to make them fit in one frame. The geographical distribution of the data
is also shown, though a single triangle may represent more than one glacier.
Data are from the World Glacier Monitoring Service (http://www.geo.unizh.ch/wgms/)
with some additions from various unpublished sources. |
The last point is of particular interest in the light of the discrepancy between
recent tropical glacier length reduction and lack of warming in the lower troposphere
since 1979 indicated by satellites and radiosondes in the tropics (Section
2.2.3). Long-term monitoring of glacier extent provides abundant evidence
that tropical glaciers are receding at an increasing rate in all tropical mountain
areas. This applies to the tropical Andes (Brecher and Thompson, 1993; Hastenrath
and Ames, 1995; Ames, 1998), Mount Kenya and Kilimanjaro (Hastenrath and Kruss,
1992; Hastenrath and Greischar, 1997) and to the glaciers in Irian Jaya (Peterson
and Peterson, 1994).
Relating mass balance fluctuations to meteorological conditions is more complicated
for tropical glaciers than for mid- and high latitude glaciers, and it has not
been demonstrated that temperature is the most important factor. Nevertheless,
the fast glacier recession in the tropics seems at first sight to be consistent
with an increase in tropical freezing heights of 100 m over the period 1970
to 1986 as reported by Diaz and Graham (1996), corresponding to an increase
of 0.5°C at tropical high mountain levels, which they also link to increases
in tropical SST since the mid-1970s (Figure 2.10).
However, although Gaffen et al. (2000) found a similar increase over 1960 to
1997, they found a lowering of freezing level over 1979 to 1997 which, at least
superficially, is not consistent with glacier recession.
Figure 2.18 shows a representative selection of
glacier length records from different parts of the world and updates the diagram
in IPCC (1990). It is clear from Figure 2.18 that
glacier retreat on the century time-scale is worldwide. The available data suggest
that this retreat generally started later at high latitudes but in low and mid-latitudes
the retreat generally started in the mid-19th century.
On the global scale, air temperature is considered by most glaciologists to
be the most important factor reflecting glacier retreat. This is based on calculations
with mass balance models (Greuell and Oerlemans, 1987; Oerlemans, 1992; Fleming
et al., 1997; Jóhannesson, 1997). For a typical mid-latitude glacier,
a 30% decrease in cloudiness or a 25% decrease in precipitation would have the
same effect as a 1°C temperature rise. Such changes in cloudiness or precipitation
can occur locally or even regionally on a decadal time-scale associated with
changes in circulation, but global trends of this size on a century time-scale
are very unlikely. As mentioned in the SAR, Oerlemans (1994) concluded that
a warming rate of 0.66 ± 0.20°C per century at the mean glacier altitude
could explain the linear part of the observed retreat of 48 widely distributed
glaciers.
Glaciers are generally not in equilibrium with the prevailing climatic conditions
and a more refined analysis should deal with the different response times of
glaciers which involves modelling (Oerlemans et al., 1998). It will take some
time before a large number of glaciers are modelled. Nevertheless, work done
so far indicates that the response times of glacier lengths shown in Figure
2.18 are in the 10 to 70 year range. Therefore the timing of the onset of
glacier retreat implies that a significant global warming is likely to have
started not later than the mid-19th century. This conflicts with the Jones et
al. (2001) global land instrumental temperature data (Figure
2.1), and the combined hemispheric and global land and marine data (Figure
2.7), where clear warming is not seen until the beginning of the 20th century.
This conclusion also conflicts with some (but not all) of the palaeo-temperature
reconstructions in Figure 2.21, Section
2.3 , where clear warming, e.g., in the Mann et al. (1999) Northern Hemisphere
series, starts at about the same time as in the Jones et al. (2001) data. These
discrepancies are currently unexplained.
For the last two to three decades, far more records have been available than
are shown in Figure 2.18. Many are documented at
the World Glacier Monitoring Service in Zürich, Switzerland (e.g., IAHS
(ICSI)/UNEP/UNESCO, 1998) The general picture is one of widespread retreat,
notably in Alaska, Franz-Josef Land, Asia, the Alps, Indonesia and Africa, and
tropical and sub-tropical regions of South America. In a few regions a considerable
number of glaciers are currently advancing (e.g., Western Norway, New Zealand).
In Norway this is very likely to be due to increases in precipitation owing
to the positive phase of the North Atlantic Oscillation (Section
2.6), and in the Southern Alps of New Zealandand due to wetter conditions
with little warming since about 1980. Finally, indications in the European Alps
that current glacier recession is reaching levels not seen for perhaps a few
thousand years comes from the exposure of radiocarbon-dated ancient remains
in high glacial saddles. Here there is no significant ice flow and melting is
assumed to have taken place in situ for the first time in millennia (e.g., the
finding of the 5,000-year-old Oetzal �ice man�).
|