TRANSCRIPT OF
AL GORE´S PRESENTATION
In his film, Al Gore talks about the rise in atmospheric CO2
levels: "It just keeps going up. It is relentless. And now we´re
beginning to see the impact in the real world. - This is Mount
Kilimanjaro more than 30 years ago - and more recently (2000). And a
friend of mine just came back from Kilimanjaro with a picture he
took a couple of months ago. . . Within a decade there will be no more
snows of Kilimanjaro."
GLACIER RECESSION: TIME COURSE AND
TYPE OF RECESSION
The earliest estimate of the glacier
surface area on Mount Kilimanjaro refers to the year 1880, when it made
out about 20 km². Since then, it has declined continuously; in
2000 the area measured 2.52 km² and in 2007 1.85 km². From
1912 to now, the decline in
the horizontal parts of the glaciers has
been nearly linear (i.e. the same size of area disappeared each year).
Details are given in the paper by Kaser
et
al.
(2004) and Cullen
et
al.
(2006), which are cited by
Lomborg, and in Thompson
et
al
(2002) and Thompson et al
(2009)..
According to Kaser et al. and Cullen et al., glaciers around the summit
have horizontal
surfaces which end abruptly
in vertical walls to the side facing the summit and the central crater.
The position of these vertical walls is gradually shifted by
`ablation´ of the ice. This does not imply that the ice melts
away - the temperature of the ice never surpasses ÷ 1.6° C -
but instead the ice disappears by sublimation, i.e. direct transition
from solid phase to vapour phase. According to Kaser et al., once a
vertical wall has receded by
some distance, it cannot advance again, because snow does not deposit
on a vertical wall. This means that the walls are inherently unstable -
they can only recede, never advance. The implication is that once these
walls formed in the past, the glacier was doomed to dwindle. In
addition, however, there are also sloping glacier surfaces some of
which are more nearly in balance between addition of snow and removal
by sublimation.
Kaser et al. indicate that there is practically no melting of ice on
the horizontal surfaces. This contrasts sharply with Thompson et al.
who found
considerable surface melting. In their 2009 paper, they found that half
of the ice loss is due to surface melting, and only half to shrinking
from the sides. In one of the glaciers, the glacier surface had lowered
3 m in just 7 years. The top ice layers originating from the
period after 1952 have melted away. Ice cores show that similar melting
has not occurred during the preceding centuries.
EXPLANATIONS FOR GLACIER RECESSION
Each year there are two rainy/snowy seasons and two dry seasons. During
the former, snow is deposited on the glaciers. During the latter, a
relative lack of clouds means that sunlight has direct access to the
glacier surface, and the incoming radiation supplies the energy for
sublimation without any involvement of sensible heat. An instructive
explanation of this is found in this
link. The most crucial parameter
in this process is the surface albedo - the whiter the glacier surface,
the more sunlight is rejected and the less is available for the
sublimation process. Frequent snowfall gives a white surface and hence
little sublimation. Careful studies of the mass balance of a sloping
glacier on Kilimanjaro has shown that the rate of sublimation is
affected five times more strongly by a 20 % decrease in snowfall than
by a 1° C increase in air temperature. This means that changes in
both precipitation and temperature could in principle cause glacier
retreat, but changes in precipitation, if they occur, will have the
greater impact. In
addition, there could be effects of increased deposition of dust and
soot (from forest fires) on the glacier surface (this would reduce the
albedo).
Information on climate changes in the region is scanty and
contradictory. Precipitation has changed markedly at some points of
time. This is known espcially
from variation in water level in Lake Victoria which is not far away.
Precipitation was low in the first part of the 17th century, but
increased from 1850 onwards, and there were excessive rains during the
years 1876-1878. After that, precipitation dropped suddenly to low
levels from 1890 onwards. These levels then increased again to rather
high levels after 1960. According to Thompson et al. (2009) there are
no overall general trends in precipitation in East Africa, whereas
there are rising trends in some aspects of temperatures. This
paper states (p. 3994)
that "drying has not occurred over tropical eastern Africa."
If the extent of the ice cap is mainly determined by precipitation,
then it is conceivable that it had its maximal size just around 1880
when the first recordings were made. The marked decline in ice cover
from 1880 to 1912 could be explained by the time sequence of record
amounts of precipitation followed by a period with much less
precipitation. The continued decline during the next decades could also
be explained by continued low precipitation. But for the years after
1960, the situation is different. During the mainpart
of the 20th century, recordings
from weather stations in the region indicate no significant
changes in average temperature or average rainfall. See for
instance
rainfall data in Rohde & Virji (1976): Monthly weather
review 104: 307-315. But in the latter part of the 20th century,
precipitation has increased. Røhr
&
Killingtveidt
(2003): Hydrological Sciences 48(1): 65-77 indicate that some of the highest weather
stations on the south slope of Kilimanjaro show a significant change in
rainfall after 1970-1980. This is most clear in the highest station
(above 1500 m), where annual rainfall during 1984-1999 was 42% higher
than during 1954-1983. Thus, if glacier retreat was governed by
precipitation, it should have slowed down or stopped after 1984. In
addition, after c.
1990 there have been clear surface temperature increases in east Africa
(link). Thompson
et
al. (2009) write: "A 25-year temperature and
precipitation history recorded in the Amboseli Basin, a few kilometers
from the northern base of Mount Kilimanjaro, reveals a warming rend in
both maximum and minimum temperatures and large interannual variability
in precipitation but no long-term trend."Therefore, the situation now is that
precipitation has changed so as to favour ice retention, whereas
temperature has changed so as to favour ice melting. This suggests that
temperature rather than precipitation plays a role for what we are
seeing now.
WEATHER IN THE PAST
Judged from the situation in Lake Victoria, the period 1800-1850 had
very low precipitation, maybe slightly lower than during 1900-1960. If
the extent of the ice cap were governed solely by precipitation, it
should have been very small before 1850. But this seems not to have
been the case. There has been more ice after 1700 than before
that year (judged from the fact that the small glacier inside the
crater has existed only for 300 years, according to the ice core
study).
However, a series of facts indicate that there was a 300 year long
period of extreme drought around 4,000 years BP. In the ice cores studied by Thompson et
al., this period appears as a marked dust layer. Some of the ice cores
are no older than this, indicating that where they were drilled, there
was no ice 4,000 years BP. That is, the ice cap then was even smaller
than now, at least to the side facing the crater. On the other hand,
the fact that the largest ice field continues further back, right down
to the
end of the last glaciation, indicates that even during a period much
drier than now, this ice cap persisted. This suggests
that low precipitation can not, or not fully, explain that the ice cap
is disappearing now. Furthermore,
the
upper layers of the northern ice field contain evidence of surface
melting that does not appear anywhere else in the ice core representing
an 11,700 year long history (Thompson et al. 2009). Thus, what is
happening now is unprecedented.
CHANGED WEATHER PATTERNS
Precipitation
depends on conditions in the Indian Ocean. In certain years, the usual
distribution of temperatures in this ocean (warmest to the east) is
reverted (warmest to the west). In such years, precipitation in east
Africa is higher than usual. There is information that this climate
pattern prevailed much more often during the period 1820 - 1880 than
before and after this period (link). This is based on proxy data such as
historical accounts of the water level of Lake Victoria, and of winds
and currents in the Indian Ocean. It is believed that precipitation was
higher during that period, and that it decreased suddenly by about 20 %
in 1880. This could explain that the glaciers on east African
mountains, especially Kilimanjaro, receded from 1880 onwards.
Is the changed temperature
distribution in the
Indian Ocean a reflection of the ongoing climate change? In an attempt
to answer this question, Mote and Kaser (link) write: "It is possible . . that there is
an indirect connection between the accumulation of greenhouse gases and
Kilimanjaro´s disappearing ice. There is strong evidence of an
association over the past 200 years or so between Indian Ocean surface
temperatures and the . . precipitation patterns that either feed or
starve the ice on Kilimanjaro. . Any contribution of rising greenhouse
gases to this circulation pattern necessarily emerged only in the last
few decades; hence it is responsible for at most a fraction of the
recent decline in ice and a much smaller fraction of the total decline."
TROPOSPHERE AIR TEMPERATURES
As explained in this
link, partially based on this
reference, air temperatures in the
mid troposphere in the altitude of the tip of Mount Kilimanjaro (c.
5,000
m) are fairly uniform all the way around the equator. These
temperatures have risen by an average of 0.2°C per decade during
the period 1960-1979 and afterwards, although the rise is not seen in
every part
of this period. With a fairly uniform trend around
the globe, it is tempting to assume that the simultaneous
shrinking of glaciers everywhere in the tropics is caused by one single
factor, the air temperature at this altitude. However, those that
favour the hypothesis that changes on Kilimanjaro are not due to rising
temperatures, present other data for the mid troposphere that do not
show a rising temperature (link). The data treatment of the
satellite-based temperature measurements is complicated, and different
interpretations may have led to the different estimates of temperature
trends.
FINAL REMARK
In conclusion, it must be said that the issue is complicated and not
settled. There remain two explanations of what is happening. These need
not to be competing alternatives, but may supplement each other. It is
likely that changes in precipitation are the main cause for glacier
retreat in the decades after 1880, whereas surface melting due to
rising temperatures seems to explain about half of the glacier decline
happening today.
What is described here refers only to Mount Kilimanjaro. Here the
glaciers are situated so high above the mean 0° isothermal surface
that melting due to heat transfer plays a small role. Other
tropical glaciers situated at lower altitudes and in more wet climates
are more affected by
changes in temperature, especially glaciers
in the Andes (Favier et al. (2005): J. geophysical research
109:D18105, link).
A list of informative papers on the subject from the tropical
glaciology group in Austria and Switzerland is available at this
web
site.