Geoscience Reference
In-Depth Information
greenhouse gases make an important contribution. This
diagram should be viewed in relation to
Figure 9.19
,
where the probable range of the time scales of change is
shown for a variety of potential causative factors.
Consequently it is almost impossible to tell how long any
trend we are able to identify will persist. We can only guess
at what even the immediate future holds.
0
Past Future
5
10
15
20
25
It is clear that in the medium to long term, over the time
scale of tens to thousands of years, our climate varies, not
randomly but systematically. Broad, consistent fluctua-
tions occur, giving periods of relative warmth and periods
of coldness, years of aridity and years of wetness. The
reasons for the fluctuations are not fully understood;
variations in Earth's orbit and rotation, changes in solar
output, internal adjustments to the vegetation, topog-
raphy and atmosphere, all may be contributory factors.
The time scale of possible causative factors is shown in
Figure 9.19
,
but we must not forget that most of the
factors are interactive; we cannot isolate a single process
30
35
-150
-100
-50
0
50
100
AP
BP
Time (ka)
the last 120,000 years and the next 80,000. The sudden
decrease in ice volume at present is due to the enhanced
greenhouse effect.
Source: After Goodess et al. (1992)
10
9
10
8
10
7
10
6
10
5
10
4
10
3
10
2
10
1
1
Solar variability
Earth orbital parameter variations
Pole wandering, continental drift
Continental uplift, mountain building, sea-level changes
Mass and composition of atmosphere (except CO
2
, H
2
O,O
3
)
Volcanic activity
production of stratospheric aerosols
Atmosphere
ocean
cryosphere
biosphere
lithosphere autovariation
Atmosphere
ocean
cryosphere autovariation
Atmosphere-ocean autovariation
Atmosphere autovariation
?
Human activities
(land-use,gas, aerosol, heat pollution, etc)
Int
erval between ice ages
Duration
of ice age
s
Glacial interglacial fluctuations of present ice age
Duration of recent interglacials
Major fluctuations of present interglacial
Fluctuations of past thousand years
Inter-annual variability
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9
10
8
10
7
10
6
10
5
10
4
10
3
10
2
10
1
1
Probable range of time scale involved (years, power of ten)
Figure 9.19
Potential causative factors in climatic change and the probable range of the time-scale of change attributable to
each.
