Geoscience Reference
In-Depth Information
higher temperatures, which melt ice and snow, decrease
the surface albedo, allowing more absorption of
incoming solar radiation and lead to increased surface
heating and further warming.
Negative feedback mechanisms appear to be much
less important in the face of short-term radiative forcing
and it appears that they can only reduce the rate of
warming but cannot, of themselves, cause global cool-
ing. Natural and anthropogenic aerosols are thought
to have a net cooling effect but the magnitude is
highly variable in time and space. Cloud cover effects
are particularly complex, producing both positive and
negative feedback. Negative feedback may operate
when increased global heating leads to greater evap-
oration and greater amounts of high-altitude cloud
cover, which reflect more incoming solar radiation and
thus lessen the rise in global temperature.
identified in the 1960s. Uplift of the western Cordilleras
of North America and the Tibetan Plateau by plate
movements during the Tertiary period (50 to 52 Ma)
caused regional aridity to develop in the respective
continental interiors. However, geographical factors
are only part of the explanation of climate variations.
The mid-Cretaceous period, about 100 Ma, was
notably warm in high latitudes. This warmth may be
attributable to atmospheric concentrations of carbon
dioxide three to seven times those at present, augmented
by the effects of alterations in land-sea distribution and
ocean heat transport.
Within the major ice ages there are recurrent
oscillations between glacial and interglacial conditions
(see Box 13.2). Eight such cycles of global ice volume
are recorded by land and ocean sediments during the
last 0.8 to 0.9 Ma, each averaging ~100 ka, with only
10 per cent of each cycle as warm as the twentieth
century (Figure 13.5D and E). Prior to 0.9 Ma, ice
volume records have a dominant 41 ka periodicity,
while ocean records of calcium carbonate indicate
fluctuations of 400 ka. These periodicities represent
the forcing of global climate by changes in incoming
solar radiation associated with the earth's orbital
variations (see Chapter 3A). Building on the work of
nineteenth-century astronomers and geologists, calcu-
lations published by Milutin Milankovich in 1920
established an astronomical theory of glacial cycles.
This became widely accepted only in the 1970s because
the inadequate dating of glacial records, as well as the
complex nature of radiation-climate relationships and
the time lags involved in ice sheet buildup and decay
allowed considerable scope for alternative explanations.
The precession signature (19 and 23 ka) is most appar-
ent in low-latitude records, whereas that of obliquity
(41 ka) is represented in high latitudes. However, the
100-ka orbital eccentricity signal is generally dominant
overall. This is surprising because the effect of eccen-
tricity on incoming solar radiation is modest, implying
some non-linear interaction in the climate system.
Several models of ice sheet growth and decay, incorpo-
rating bedrock isostatic response with orbital radiative
forcing, also generate ice oscillations with periods
of about 100 ka, implying that internal dynamics are
important.
Only four or five glaciations are identified on land
due to the absence of continuous sedimentary records,
but it is likely that in each of these glacial intervals large
ice sheets covered northern North America and northern
C THE CLIMATIC RECORD
1 The geological record
To understand the significance of climatic trends over
the past century, these trends need to be viewed against
the background of past conditions. On geological time-
scales, global climate periodically undergoes major
shifts between a generally warm, ice-free state and an
ice age state with continental ice sheets. There have been
at least seven major ice ages through geological time.
The first occurred 2500 million years ago (Ma) in the
Archean period, followed by three more between 900
and 600 Ma, in the Proterozoic. There were two ice ages
in the Paleozoic era (the Ordovician, 500 to 430 Ma;
and the Permo-Carboniferous, 345 to 225 Ma). The late
Cenozoic glaciation began about 15 Ma in Antarctica
and about three million years ago in northern high
latitudes.
Major ice ages are determined by a combination
of external and internal forcing (solar luminosity,
continental location, tectonics and atmospheric CO
2
concentration). The ice sheets of the Ordovician
and Permo-Carboniferous periods formed in high
southern latitudes on the former mega-continent of
Gondwanaland, whereas bipolar glaciation developed
in the Pliocene-Pleistocene epoch. Alfred Wegener
proposed continental drift as a major determinant of
climates and biota in 1912, but this idea remained
controversial until the motion of crustal plates was
