The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 116

The Oligocene to the Quaternary:climate
andbiology

4

We are currently in the middle of an ice age! This ice age is known as the Quaternary
ice age, and it began roughly 2 million years ago (mya) (I say 'roughly' because
how much ice do you need on the planet to say that it is an ice age?). We might
not think we are in an ice age and this is because we are in a warm part, called an
interglacial. As we shall see, there have been a number of glacials and interglacials
in our Quaternary ice age. However, this ice age did not just start by itself but arose
out of a number of factors that became relevant earlier, in the Oligocene (34-23 mya)
and Miocene (23-5.3 mya) epochs, well before the beginning of our ice age and the
Pliocene and Pleistocene glaciations (Zachos et al., 2001). These glaciations actually
had their beginnings some 5.3 mya. To understand how our Quaternary ice age came
about we will need to briefly re-cap part of the previous chapter and note some other
material to provide a more biological perspective while leaving out the extinction
events.

4.1 TheOligocene(33.9-23.03mya)

Between 35 and 15 mya the Earth's temperature was roughly 3-4 ◦ C warmer than
today and atmospheric carbon dioxide concentrations were twice as high. However,
climate forcing factors were coming into play that were to cool the planet. Carbon
dioxide levels were falling and, as noted in the previous chapter, this fall could only
have been furthered by the new C 4 plants (even though their period of major expansion
was not to take place until 8 mya: see below).
A major forcer of global climate came about by the collision of the Indian and
Asian tectonic plates. These plates first made contact with each other around 55
mya (coincidentally, roughly around the time of the Eocene climatic event) and they
continued to compact. This resulted in the subsequent uplift of what was to become
the Tibetan Plateau. This gradually rose and is still rising today.
The plateau increasingly affected global climate in three ways. First, it forced
the re-organisation of atmospheric circulation, with the creation of the East Asian
monsoon and associated climatic patterns. Second, the uplift caused ice to form in the
mountains and notwithstanding this there were other albedo effects (due to altitude
and changing nature of the land's surface), so reflecting more of the Sun's energy. This
is one of the main reasons why Milankovitch curves are frequently calculated on the
solar energy received in northern latitudes between 28 ◦ and 40 ◦ N. Third, it increased
erosion, so increasing the amounts of silicate, and hence calcium silicates that could

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