Geoscience Reference
In-Depth Information
1 External forcing
Tectonic processes
. On geological timescales, there
have been great changes in continental positions and
sizes, and in the configuration of ocean basins as a result
of crustal processes (known as plate tectonics). These
movements have also altered the size and location of
mountain ranges and plateaus. As a result, the global
circulation of the atmosphere and the pattern of ocean
circulation have also been modified. Changes in conti-
nental location have contributed substantially to major
ice age episodes (such as the Permo-Carboniferous
glaciation of Gondwanaland) as well as to intervals with
extensive arid (Permo-Triassic) or humid (coal deposits)
environments during other geological periods. Over
the past few million years, the uplift of the Tibetan
Plateau and the Himalayan ranges has caused the onset,
or intensification, of desert conditions in western China
and Central Asia.
Solar variability
. The sun is a variable star, and it is
known that early in the earth's history (during the
Archean three billion years ago) solar irradiance was
about 80 per cent of the modern value. Paradoxically,
however, the effect of this 'faint early sun' was offset,
most likely, by a concentration of carbon dioxide that
was perhaps 100 times higher than now, and perhaps
also by the effects of a largely water-covered earth. The
approximately eleven-year solar cycle (and twenty-two-
year magnetic field cycle) is well known. As discussed
in Chapter 3, the sunspot cycle causes a ±1 W m
-2
fluctuation in solar irradiance and much larger effects
on ultraviolet radiation. Intervals when sunspot and
solar flare activity were much reduced (especially the
Maunder minimum of
AD
1645 to 1715) may have had
cumulative effects leading to temperature decreases
of about 0.5°C. Solar variability seems to have played
an important role in decadal-scale variations of global
temperature until the late twentieth century, when
anthropogenic effects became dominant.
Astronomical periodicities
. As noted in Chapter 3A.2,
the earth's orbit around the sun is subject to long-term
variations. There are three principal effects on incoming
solar radiation: the eccentricity (or stretch) of the orbit,
with periods of approximately 95,000 years and 410,000
Figure 13.3
The astronomical (orbital)
effects on the solar irradiance and their
timescales over the past 500,000 years.
(A) and (B) Eccentricity or orbital stretch.
(C) and (D) Obliquity or axial tilt. (E) and
(F) Precession or axial path wobble.
Sources
: Partly after Broecker and Van
Donk (1970), and Henderson-Sellers and
McGuffie (1987). (B), (D) and (F) From
Review of Geophysics and Space Physics
8, 1970, by permission of the American
Geophysical Union.
