When glaciers grow to a certain size, they turn into ice caps. And when ice caps expand to cover larger regions they merge into ice sheets that might attain continental dimensions. In this process the oceans will deprived of such large quantities of water that global sea level was lowered more than 150 meters. Because the Antarctic ice sheet had already been in place since the Miocene or even earlier, we will not take this, the world's largest ice sheet, into account now even though it plays a profound role in the Earth's climate. With the present distribution and size of continents and oceans far the most voluminous Pleistocene ice sheets developed on the northern hemisphere. The earth's glaciation history through the past four to six million years is recovered from deep sea sediments because the magnitude of continental glacier ice can be inferred from the ratio between oxygen isotopes in benthic foraminifera shells. The relative isotopic composition of oxygen in ocean water called delta-O-18 was changed dramatically over time indicating a steady lowering of global temperatures during the last five million years. This meant increasingly colder and longer glacial episodes caused by solar forcing. Compared to the 50 million year drift from greenhouse to ice house, Pleistocene climate change operate in short time scales when local glacial episodes are separated from warm interglacials. Climate has changed regularly in four major rhythms. Long term changes are linked to changes in the Earth's orbit around the sun called the eccentricity. While intermediate changes are linked to the tilt of the axis called the obliquity, and short time changes are tied to the wobble, the precision of the axis of rotation. The combination of these parameters is called the solar forcing and seems to be the pacemaker of Pleistocene climatic change. Since the first evidence of ice rafting in the eastern North Atlantic Ocean some three million years ago, climate change has been operating in the obliquity and persistent frequency bands giving rise to the build-up and vanishing of small-sized regional ice sheets with about 41,000 year intervals. After 1 million or 0.6 million years ago, the eccentricity has dominated climate cycles and allowed the growth of large northern hemisphere ice sheets on 100,000-year basis. The steady growth of glacier ice culminated in extreme cold climate with maximum ice sheets build-up and it was accompanied by a drop in global sea level of more than 100 meters below that of the present. Ice Ages rapidly terminated when insulation peaked leading to a series of relatively short (10-15,000-year long) warm interglacials. There seems to be an intimate relation between atmospheric, continental greenhouse gases, ice volume, and climate. Thus the composition of gases trapped in air bubbles from an arctic ice cores have revealed a close synchronicity between temperature and atmospheric carbon dioxide and methane. It follows that during cold spells of the ice ages the amount of atmospheric greenhouse gases were low compared to milder ice age episodes, and that the level of greenhouse gases reached maximum values during interglacials. Whether changes in ice volume or greenhouse gas levels represents forcing or feedback to the climate system is still under debate. Even shorter climate changes are detected from oxygen isotope variations in cores drilled into the ocean floor and the large ice sheets of Antarctica and Greenland. During the last ice age, millennial scale climate oscillations were over lying climate changes tuned by solar forcing. These rapid and high amplitude climatic process of less than 10000 years in duration are intimately tied to the interaction between atmosphere and ocean in the North Atlantic region. Age-dated variations in delta-O-18 values in Greenland ice cores have revealed 24 pulses of rapid warming followed by gradual cooling with amplitudes of as much as 15 degrees centigrade in annual temperatures over Greenland. These events appear in close tandem with flunked rates and quantities of cold demanding planktic foraminerera register in the North Atlantic Deep-Sea record. Their presence in the deep sea record has revealed dramatic changes in sea surface temperatures during the last ice age, some 115-11,000 years ago. These so-called dense ice core oscillations are lined up in bundles of long-time cooling cycles which abruptly terminate in major ice rafting events. Large quantities of ice raft debris in the deep sea cores expose major release of icebergs. This signals episodes of ice sheet instability and eventually breakdown that may lead to the launch of armadas of icebergs into the North Atlantic. Under warm spells, glaciers and permafrost in Northern Europe melted and shrub tundra or even opened sub-Arctic forests reclaimed fora former glaciated regions. On the global scene, sea level rose and fell, the thermohaline circulation became weaker or stronger. Weather systems changed their path, the monsoons and trade winds intensified or weakened, and so on. It was under these changing environments that anatomically modern humans began to disperse from Africa into most parts of Eurasia and Australia successfully adapting themselves to the new living conditions. In summary, it must be said that three main factors have governed Earth's climate on the route to the modern world with its overpopulation, mass extinction, pollution and the human impact on climate and environment. The long-term changes during the cenozoic era that took place over tens of millions of years were due to plate tectonic reorganization that resulted in the opening of new oceans and building of young mountain chains. Medium climate change over the past two to three million years operate in 100,000 and 41,000 years scale following the rhythms of solar forcing governed by the Earth's orbit around the sun and the tilt and precession of the earth's axis of rotation. Thirdly, millennial scale climate oscillations due to the interaction between ice sheets, oceans, and atmosphere dramatically changed the environments around the globe especially during ice ages.
