Geoscience Reference
In-Depth Information
climate system) is transferred to the atmosphere,
expressed as a rise in global mean surface tempera-
ture. When considering timescales of decades
or longer, thinking must turn to climate
forcings
and attendant
feedbacks
. Forcing factors represent
imposed perturbations to the global system, and
are defined as positive when they induce an
increase in global mean surface temperature, and
negative when they induce a decrease. Forcing
factors may in turn be of natural or anthropogenic
origin. The magnitude of the global tempera-
ture response to forcing depends on the feed-
backs. Positive feedbacks amplify the temperature
change while negative feedbacks dampen the
change.
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.
•
Astronomical periodicities
. As noted in Chapter
3A.2, the earth's orbit around the sun is subject
to long-term variations, leading to changes
in the seasonal and spatial distribution of
solar radiation incident to the surface. These
are known as Milankovich forcings after
the astronomer Milutan Milankovich, whose
careful calculations of their effects built upon
the work of nineteenth-century astronomers
and geologists. There are three principal
effects: the eccentricity (or stretch) of the orbit
influencing the strength of the contrast in solar
radiation received at perihelion (closest to sun)
and aphelion (furthest from sun), with periods
of approximately 95,000 years and 410,000
years; the tilt of the earth's axis (approximately
41,000 years) influencing the strength of the
seasons; and a wobble in the earth's axis of
rotation, which causes seasonal changes in the
timing of perihelion and aphelion (
Figure
13.3
). This precession effect, with a period of
about 21,000 years, is further illustrated in
Figure 3.3
. The range of variation of these
three components and their consequences
are summarized in
Table 13.1
. Astronomical
periodicities are associated with global tem-
perature fluctuations of ±2-5°C per 10,000
years. The timing of orbital forcing is clearly
represented in glacial-interglacial fluctuations
with the last four major glacial cycles span-
ning roughly 100,000 years (or 100ka). The
astronomical theory of glacial cycles became
widely accepted in the 1970s after Hays, Imbrie
and Shackleton provided convincing evidence
from ocean core records.
1 Climate forcing
Many different types of climate forcing can be
identified. Key forcings are associated with the
following processes:
•
Plate Tectonics
. On geological timescales, plate
tectonics have resulted in great changes in
continental positions and sizes, the configura-
tion of ocean basins and (through associated
phases in volcanic activity) atmospheric com-
position. While there is little doubt that such
changes altered the globally averaged surface
albedo and greenhouse gas concentrations,
plate movements have also altered the size
and location of mountain ranges and plateaus.
As a result, the global circulation of the atmos-
phere and the pattern of ocean circulation
were modified. In 1912, Alfred Wegener pro-
posed continental drift as a major determinant
of climates and biota, but this idea remained
controversial until the motion of crustal
plates was identified in the 1960s. Alterations
in continental location have contributed sub-
stantially to major Ice Ages of the distant
past (such as the Permo-Carboniferous glaci-
ation of Gondwanaland) as well as to intervals
with extensive arid (Permo-Triassic) or humid
(coal deposits) environments during other
•
Solar variability.
The sun is a variable star. The
approximately 11-year solar (sunspot) cycle
(and 22-year magnetic field cycle) are well
known. As discussed in Chapter 2, the 11-year
sunspot cycle is associated with ±1W m
-2
