Geoscience Reference
In-Depth Information
Table 13.1
Orbital forcings and characteristics.
Element
Index range
Present value
Average periodicity
Obliquity of Ecliptic (
ε
)
22-24.5°
23.4°
41ka
(Tilt of axis of rotation)
Effects equal in both hemispheres, effect intensifies
poleward (for caloric seasons)
Low
ε
High
ε
Weak seasonality,
Strong seasonality, more
steep poleward
summer radiation at poles,
radiation gradient
weaker radiation gradient
Precession of Equinox (
ω
)
0.05 to -0.05
0.0164
19, 23ka
(Wobble of axis of rotation)
Changing earth-sun distance alters seasonal cycle
structure; complex effect, modulated by eccentricity
of orbit
Eccentricity of Orbit (e)
0.005 to 0.0607
0.0167
410, 95ka
Gives 0.02% variation in annual incoming radiation;
modifies amplitude of precession cycle changing seasonal
duration and intensity; effects opposite in each hemisphere;
greatest in low latitudes
years; the tilt of the earth's axis (approximately 41,000
years); and a wobble in the earth's axis of rotation,
which causes changes in the timing of perihelion (Figure
13.3). This precessional effect, with a period of about
21,000 years, is illustrated in Figure 3.3. The range of
variation of these three components and their conse-
quences are summarized in Table 13.1.
cussed in Chapter 2. It was pointed out that there is a
large 'natural' greenhouse caused by atmospheric water
vapour, distinct from human-induced changes in other
greenhouse trace gases over the past few centuries.
Glacial-interglacial changes in terrestrial vegetation
and in the oceanic uptake of trace gases, as a result of
changes in the thermohaline circulation of the global
ocean (see Chapter 7D), resulted in major fluctuations
in atmospheric carbon dioxide (±50 ppm) and methane
(±150 ppb). Negative (positive) excursions are asso-
ciated with cold (warm) intervals, as illustrated in Figure
2.6. The changes in greenhouse gases (CO
2
and CH
4
)
and global temperatures are virtually coincident during
both glacial and interglacial transitions, so that there is
no clear causative agent. Both the long-term and rapid
changes in atmospheric CO
2
seen in polar ice cores seem
to result from the combined effects of ocean and land
biological activity and ocean circulation shifts.
Volcanic eruptions
. Major explosive eruptions inject
dust and sulphur dioxide aerosols into the stratosphere,
where they may circle the earth for several years causing
brilliant sunsets (see Figure 2.11). Equatorial eruption
plumes spread into both hemispheres, whereas plumes
from eruptions in mid- to high latitudes are confined
to that hemisphere. Records of such eruptions are
preserved in the Antarctic and Greenland ice sheets for
at least the last 150,000 years. Observational evidence
from the last 100 years demonstrates that major erup-
tions cause a hemisphere/global cooling of 0.5 to 1.0°C
in the year following the event, but there is strong
regional variability.
Rates of change
. Obviously, changes in climate result-
ing from changes in the earth's geography through
geological processes (e.g. the position and size of ocean
basins, continents and mountain ranges) are only
perceptible on timescales of millions of years. Such
Atmospheric composition
. The effect of greenhouse
gases on the energy budget and temperatures is dis-
