The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 124

that comprise the Pleistocene. Second, there is the Holocene: this is just the current
interglacial. Confusingly the Pleistocene (2.558-0.0115 mya) is sometimes referred
to as an ice age in its own right, which conjures up a picture of a single cold spell for
the planet, as opposed to oscillations between glacial and interglacial times. (Indeed,
some parts of past interglacials were slightly warmer than today.) Similarly, the end
of the Pleistocene is sometimes described as the end of the ice age when in fact it is
just the end of the last glacial. Notwithstanding human greenhouse considerations,
our Holocene interglacial will eventually end with a return to glacial conditions. This
confusion in terminology is simply a result of the historical way in which we have
uncovered the past. It reflects the way our understanding has changed since the term
ice age was first coined by the Swiss naturalist Louis Agassiz in 1837 (after whom
the large last glacial maximum [LGM] lake was named; see later). This ambiguity of
nomenclature is just something of which the student of palaeoclimatology has to be
aware. (Such anomalies are also common in other areas of science, such as the use
of the term fertility; see Chapter 7.)
Recently, aside from the Holocene and Pleistocene, a new term, the Anthropocene,
has begun to be used. This denotes the time from which humans had a discernable
impact on the (global) biosphere. Even here there is some confusion. Some say
that this has only happened since the Industrial Revolution, but others, notably Bill
Ruddiman of the University of Virginia, interestingly - but controversially - say
that humans have been affecting the global climate for some 8000 years (Ruddiman,
2003) and that the spread of agriculture 5000 years ago released methane (Ruddiman
and Thomson, 2001) 3 . Irrespective of the exact time when humans began to have a
discernable effect on the biosphere - specifically global cycling systems (such as for
carbon, nitrogen and phosphorus) and physically (such as climate) - humans today
are certainly imparting such an impact. Today, globally some 15% of ice-free land
is dominated by human use (such as urban areas and roads) whereas 55% is part-
dominated by human use (such as agriculturally dominated landscapes and canalised
rivers). This human influence on land is so much so that it is estimated that just under
a third of terrestrial primary productivity (the amount of energy, carbon or biomass)
fixed by photosynthesis is influenced by humans (Imhoff et al., 2004), so that saying
that we are in the Anthropocene is not inappropriate. However, notwithstanding the
vagaries of terminology, the onset of 'major' glaciation in both hemispheres can
be said to mark the onset of the current Quaternary ice age of the past 2 million
years.
The previous gradual cooling that had been taking place over a few million years
before the onset of significant northern hemisphere glaciation was not entirely smooth
(the Earth system was going through a critical transition and passing a climate
threshold) but consisted of temperature oscillations with a period of a few tens of
thousands of years. After the onset of the northern hemisphere glaciation these oscil-
lations quickly (within a hundred thousand years) became more marked and at that

3

However, a back-of-the-envelope calculation suggests that human agriculture 5000 years ago was
unlikely to have had an impact on the atmosphere because the global human population and hence
the likely area farmed at the time would have been too small. We will return to the question of the
Anthropocene and when it started in section 4.6.3.