The world changed again beginning at about 2.5 Ma. Figure 9.1b illustrates the onset of the Pleistocene glacial cycles at this time, a turbulent period in Earth history that continues to this day. The planet has experienced more than 40 glacial-interglacial episodes during this period, with ice sheets intermittently flooding large expanses of the Northern Hemisphere continents. As seen in Figure 9.1b, these cycles increased in magnitude over the past million years, indicating that glaciations have become more severe. There has also been a change in the periodicity, with the mid-Pleistocene transition at about 1.2 to 0.9 Ma marking a shift from 40-kyr to 100-kyr glacial cycles. This is approximate; the duration of individual glacial and interglacial episodes varies.
The last glacial cycle began about 116,000 years ago (116 ka), with the inception of ice sheets in northern Canada and Scandinavia. Ice spread fitfully over North America, Europe, and parts of Asia, South America, and New Zealand for about 100,000 years, while the Greenland and Antarctic ice sheets also grew, expanding out to the edge of the continental shelf. The ice sheets reached their maximum extent 21,000 years ago, a moment known as the last glacial maximum. The ice sheets began to recede at this time, largely disappearing from the landscape by 8 ka, with the notable exceptions of Greenland and Antarctica. Vestiges of the Laurentide ice sheet also remain in the Canadian Arctic Archipelago.
The last glacial cycle is commonly called the "Ice Age," although this overlooks the fact that Earth has experienced numerous, repeated glaciations. We have not seen the last of the great ice sheets, although we are currently enjoying an interglacial respite. Barring too much human interference with the climate system, the current interglacial period is certain to come to a close, perhaps 20,000 years from now. Wallace Stegner captures this nicely in his musings about the Canadian climate:
The ice sheet that left its tracks all over the region has not gone for good, but only withdrawn. Something in the air, even in August, says it will be back.
The Quaternary glacial-interglacial cycles personify cryospheric sensitivity to climate change and the dynamic, central role that the cryosphere can play in Earth's climate. Although continental ice sheets leave the strongest imprint on the landscape, and indeed in the ocean, ice sheet advance during glacial periods was accompanied by large-scale expansions of all aspects of the cryosphere; snow, freshwater ice, permafrost, and sea ice all extended to lower latitudes and contributed to the cryosphere-cli-mate feedbacks that supported both the advance and retreat of the ice sheets, once under way.
Textbooks have been dedicated to discussions of the Pleistocene glaciations. Here, I provide a brief sketch of some important aspects of glacial cycles, with an emphasis on cryosphere-climate feedbacks. Michael Bender's text Paleoclimate examines glacial cycles in greater detail.
High-latitude orbital forcing has been demonstrated to pace the glacial-interglacial cycles. This is not a matter of changes in solar output, but of geographic and seasonal variations in the distribution of insolation over the planet. Gravitational influences from the Moon and other planets (primarily Jupiter and Saturn) introduce systematic, cyclic variations in the Earth-Sun orbit, including changes in (i) the eccentricity of Earth's orbital path around the Sun, (ii) Earth's axial tilt angle, relative to the normal to the plane of the ecliptic, also known as the obliquity, and (iii) the direction of Earth's tilt angle, relative to the celestial sphere. The latter effect, also known as precession, matters because it determines the season when Earth marks its closest approach to the Sun, a time known as perihelion. Orbital conditions change on time scales of 104 to 105 years: specifically, ~100- and 413-kyr cycles for eccentricity, 41-kyr cycles for the tilt angle, and ~19- and 23-kyr cycles for precession.
Perihelion currently falls on January 3, which means that Southern Hemisphere summers are more intense than Northern Hemisphere summers. The current value of our orbit's eccentricity is 0.0167, and this ranged from 0.005 (a nearly circular orbit) to 0.058 during the Pleistocene. This has only minor effects on global annual insolation, but the seasonal effect is substantial. Earth currently receives almost 7% more solar radiation at perihelion than aphelion (1415 vs. 1323 W m-2), and the difference increases to 23% when the eccentricity is at its most extreme value. Earth's axial tilt, currently 23.54°, oscillates from 22.1° to 24.5°, further modifying season-ality at high latitudes.
Orbital variations are also known as Milankovitch cycles, named for the Serbian mathematician who calculated the effect of these variations on the seasonal radiation at high northern latitudes. Milankovitch spent many years making the necessary calculations to build on James Croll's hypothesis that glaciations are a result of changes in seasonal insolation in the Northern Hemisphere; cold summers are needed for seasonal snow to survive and transform to perennial snowfields, which eventually grow large enough to flood the landscape. The Northern Hemisphere is the key as this is where the high-latitude land masses reside, and this is where the former ice sheets made their mark.
Milankovitch did not live to see the confirmation of his theory. His Canon of Insolation of the Earth and Its Application to the Problem of the Ice Ages, published in 1941, was met with skepticism and was not translated to English until 1969. By then, deep-sea sedimentary records, understanding of oxygen isotope records in calcite shells from marine sediments, acceptance of plate tectonics, and advances in geochronology and time series analysis (e.g., Fourier transforms) all came together to make it possible to examine quantitatively and interpret the climatic history documented in ocean sediments. The periodicities of the orbital variations are strongly echoed in the marine d18O record of glacial-interglacial cycles. orbital variations are now well established as the "pacemaker of the ice ages"; ice sheets expand over the northern continents when eccentricity, tilt, and precession align to give cool summers at high latitudes, and the ice retreats when these elements are oppositely aligned, giving warm northern summers.
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