Now we'll take a closer look at what's been going on in the past five million years. The earlier portion of this time is known as the Pliocene epoch, and the latter portion, beginning around 1.8 million years ago, is the Pleistocene. The choice of the Pliocene-Pleistocene boundary is based on
Map projection and shading as for Fig.
an obsolete notion of the time when Northern Hemisphere ice sheets attained nearly their modern extent. At the level of geological periods, the Neogene-Quaternary more closely approximates this distinction. The Quaternary period extends all the way to the present, but the Pleistocene is terminated somewhat arbitrarily at the end of the last major ice sheet retreat around 10,000 years ago. The remainder of time up to the present is the Holocene epoch. This is a very human-centric division, since the time we are experiencing now is a fairly ordinary Pleistocene-type interglacial period - or at least it was up until the commencement of the industrial revolution. A more rational division would extend the Pleistocene up to about the year 1700, whereafter we would enter something with a name like "Anthropocene" (for reasons to be discussed in Section 1.12).
The Pliocene and Pleistocene are a time of establishment of the great Northern Hemisphere ice sheets. During this time, the ice sheets settled into a rhythm of expansion and retreat - the rhythm of the coming and going of ice ages. The notion of an "ice age" is distinct from that of the "icehouse climate state" introduced earlier. The latter term refers to a span of time (usually several million years) during which there is permanent ice at one or both poles. Within the time embraced by an icehouse climate state, ice volume is not constant, but fluctuates episodically with variations in ice volume that can be on the order of a factor of two (judging from the Pleistocene). Individual episodes of large ice volume within an icehouse climate are referred to as ice ages, with the warmer periods in between referred to as interglacials, though the ice doesn't come close to disappearing completely. In the Pleistocene, the fluctuation in ice volume is dominated by changes in Northern Hemisphere ice sheets, but as ice ages come and go, the entire globe becomes colder and warmer.
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