In our discussions of continental drift, we focussed on processes taking place over the last 400 or so million years. We now shift focus to more recent events, major glaciations, and their effects on biogeographic pattern, over the last 2 millions years; the so-called Pleistocene glaciations.
Of course, glacial events and consequences have occurred over much longer time periods than just the Pleistocene (Gondwanaland experienced repeated major glacial episodes from 500 300 mya [see page 174 in Brown and Lomolino]), but we know much more about the recent events in the Pleistocene and we are living currently in an interglacial period.
(ii) Extent of Glaciation
What is a glacier? A glacier is a perennial mass of ice that moves (or is capable of moving) across land. Glaciers form when conditions are such that the the build-up of ice during cold, snowy periods exceeds the melting of ice during warmer periods. It takes many years for the mass of ice to accumulate to the extent that it is capable of movement. Movement occurs owing to differences in elevation (i.e., gravity) and the sheer force of such a huge mass of ice compressing on lower layers (the minimum thickness appears to be ~ 50 metres).
During the Pleistocene, there were up to 20 glacial advances (and associated retreats). These glacial sheets consisted of massive ice formations up to 3-4 km thick and covered up to one-third of the earths land mass. The most recent glacial advance (the "Wisconsinan" from about 75,000 18,000 ya) was most extensive in the Northern Hemisphere and in North America. Up to 80% of the northern ice mass was in North America (fig. below) compared to Eurasia. The ice sheets in North America extended to about 45 degrees N latitude (although their effects reached much farther south). The recent ice age was less extensive in the Southern Hemisphere and was mostly restricted to high elevation areas in South America (the Andean Cordillera), Africa (mountains of East Africa), southeastern Australia, and the New Zealand Alps.
Extent of Wisconsinan glaciation in different areas:
Glacial maximum and retreat during the Wisconsinan Glaciation:
Watch the glaciers retreat in North America before your very eyes. Note that the light blue areas at the margins of the retreating ice sheets are proglacial lakes. Note their shifting sizes and interconnections; clear examples of shifting habitat sizes and isolation/connection both for aquatic and terrestrial animals.
If you are ever in Whitehorse, (Yukon), the Beringia Interpretive Centre is well worth a visit!
(iii) Causes of glaciation
Pre-Pleistocene glaciations were thought to be caused by climatic cooling driven by continental coalescence (generating huge continental climate areas), drift into far southern latitudes (near the south pole), variation in solar output, and/or volcanic activity which resulted in particulate matter in the atmosphere which could reduce solar input to the earths surface.
Pleistocene glaciations, however, occurred when the continental positions were more or less stable and over a time period of stable solar output. Pleistocene glaciations are thought to have been driven, in large part, by reduction in the interception and absorbtion of solar radiation owing, ultimately, to variation in the orbital path of the earth around the sun.
Milankovitch Cycles are named after a Serbian astronomer who first discovered these sources of variation in the earths orbit. Collectively, these cycles describe regular (periodic) changes in the earths orbital path owing to three aspects of the orbit.
Eccentricity: This describes how elliptical the earths orbit is around the sun (its NOT circular). Eccentricity is measured in percentage deviations (usually from 0-5%) and has a period of about 100,000 years (i.e. to go from the dashed orbit to the solid orbit and back to the dashed orbit in (A) below). Perihelion describes the point when the earth is closest to the sun in its orbit, aphelion describes the situation when the earth is most distant from the sun (see figure below).
Currently, the earth experiences perihelion in January so winters, in the Northern Hemisphere, are relatively mild. Therefore, approximately 50,000 years ago, when the eccentricity was described by the dashed orbit in the figure below, the earth was closest to the sun in July and this corresponds to a general warming period within the last glaciation.
Obliquity: This is the "tilt" of the earth relative to a vertical axis. It varies from about 24.5 degrees (maximum tilt) to 22.1 degrees (minimum tilt). The tilt goes through one cycle every 41,000 years. Maximum tilt when the earths perihelion is in July means summers (in the Northern Hemisphere) are hotter than average. Minimum tilt, however, when the perihelion is in January generates cooler than average summers in the Northern Hemisphere (and warmer than average winters). Note that the eccentricity and the obliquity interact to influence the contrast and intensity of seasons through time.
Precession: This refers to the orientation of the earths north pole which wanders relative to celestial bodies, in particular the "north star" (see C below). This change is independent from any changes in obliquity of the earth. The orientation of the north pole to the north "stars" changes from Ursa minor to Vega once every 11,000 years.
All three of these processes clearly would influence the extent of solar radiation striking (and being absorbed) by the earth at any particular time. For instance, assuming maximum eccentricity (i.e. a strongly elliptical orbit as in the solid line in (A) above), the earth would be farthest from the sun in July (aphelion). If the obliquity was low (i.e. minimum tilt) this would result in an even cooler summer. Cooler winters, in the Northern Hemisphere, would occur under the conditions of maximum tilt when the orbit was more circular (dashed orbit) as under these conditions the earth is farthest from the sun in winter. The coolest conditions would occur when precession was maximal, tilt was maximal, and the orbit was strongly elliptical (the Northern Hemisphere would be at tilted away from the sun when the earth was farthest from the sun in the solid line orbit (i.e. at the far left of the figure in (A)).
It is thought that summer temperatures are the critical factor in whether ice sheets retreat or grow. Cooler than average summers may cause only limited ice sheet retreat so that each winter allows a net increase in ice sheet coverage and the beginning of glacial advance. For instance, if ice and snow accumulate in the winters and cool summers result in only a partial reduction in ice sheet size from melting, net growth of ice sheets will occur over and annual period and help to initiate ice sheet growth and movement. Obliquity and precession together may account for a change in solar radiation input to the earths surface of up to 15%. By contrast, eccentricity variation results in less than a 0.5% change in solar energy input to the earth.
These three factors are thought to drive the cyclical temperature changes causing the various Pleistocene glaciations (see below).
In addition, "feedback mechanisms can contribute to the rate of cooling and warming. As ice sheets grow and eliminate plant and animal life over large areas, natural production of "greenhouse" gases declines (e.g. C02, CH4). As well, ice sheets tend to increase the earths "albedo" resulting in greater reflection of incoming solar rays back out through the atmosphere. Conversely, when the ice sheets begin to retreat, reflectance of the earths surface decreases and biogenic processes increase production of greenhouse gases resulting in rapid warming and glacial recession.
Note the tight correspondence among global temperature and natural changes in "greenhouse gases" over the last 150,000 years in the figure below.