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mixing and a possible causal relationship between climate and surface-/deep-water
mixing.
The high-latitude changes in ocean circulation and the onset of significant north-
ern hemisphere glaciation were not only interrelated; the high-latitude cooling (hence
the polar ice) also resulted in the subtropical regions beginning to cool. There is a
clear and obvious synchronicity between the two and so we know that the cooling
experienced then was a planet-wide event. The tropics themselves, though, did not
see any fundamental changes, although undoubtedly there were changes in tropical
biome extent: there was sufficient environmental similarity to warmer times in some
parts of the tropics to provide a refuge for many species. This lends support to the
view that the rapid climate transition about 2.75 mya must have primarily involved
extra-tropical processes such as ocean (thermohaline) circulation, ice albedo, biogeo-
chemical processes and so forth that came together in an ice-sheet-determined and/or
some combination of tectonic and greenhouse threshold.
As the planet continued to cool a new threshold was reached around 2 mya with
new modern tropical circulations. These included the development of what is known
as the Walker Circulation and cool subtropical temperatures.
The term Walker Circulation was first introduced in 1969 by Professor Jacob
Bjerknes for two circulation cells in the equatorial atmosphere, one over the Pacific
Ocean and one over the Indian Ocean. Schematically these are longitudinal cells
where, on one side of the ocean, convection and the associated release of latent
heat in the air above lifts isobaric surfaces upward within the upper troposphere
and creates a high-pressure region there (Gilbert Walker was an early 20th-century
British climatologist who studied air circulations over the Pacific Ocean). This modern
atmospheric circulation, combined with the previous re-organisation of the northern
hemisphere's ocean circulation, again served to cool the Earth further.
Following the climate threshold, from about 0.75 mya, the glacial-interglacial
cycles became more pronounced and more clearly took place with an increase in
periodicity from 41 000 years (reflecting the Milankovitch obliquity cycle) to approx-
imately 100 000 years (reflecting a 93 000-year eccentricity). This 100 000-year peri-
odicity and degree of glacial-interglacial temperature swing has persisted to the
present day (Ravelo et al., 2004); our present day, of course, is a warm interglacial.
The switch between the 41 000-year global climate cycle to 100 000 years appears
to be more driven by a change in ocean circulation than the gradual overall decline in
atmospheric carbon dioxide across glacial-interglacial cycles. Oxygen isotope ana-
lysis (see Chapter 2) of planktonic Foraminifera reveals changes in Pacific circulation
at this time (de Garidel-Thoron et al., 2005). This is not to dismiss atmospheric carbon
dioxide as a driver of climate change, but other factors also play their part and may
be the dominant (as opposed to sole) factors initiating a change in the Earth's climate
system at some threshold point.
One might summarise the Quaternary and Pliocene trend in global climate as one
that had three characteristics: first, overall long-term cooling; second, Milankovitch-
driven oscillations between warm and cold; and third, an increase at a transition point
in both the period of climate oscillations and their degree.
In addition to isotopic analysis of biological remains in sediments, the fossil
record and other techniques (involving biology and/or geology) used to discern
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