Much of our current information of the climate data for the last six million years has come from mainly core samples taken from the deep ocean floor. An interesting graphic from the Precambrian to Today appears at the webpage;
ICE HOUSE or HOT HOUSE?
However, regarding the earlier debate in this thread, I think an article "
Earth’s Atmosphere" that appears in the "
Encyclopaedia of Astronomy and Astrophysics" (2001), which summaries the important period of climate activity - and the causes of it as;
"These long-term variations modify the spatial and seasonal sequences of solar energy deposition at the surface of our planet, resulting in variations of Earth’s climate. For example, at the end of the last glacial period, about 10 000 years ago, the inclination of the Earth’s rotational axis relative to the ecliptic plane was
about 24.5 degrees (at present 23.5 degrees) and the closest approach of the orbiting Earth to the Sun occurred in July (at present January). This past condition resulted in about 8% more solar radiation reaching the northern hemisphere in the summer and about 8% less in winter time. Although the astronomical effects are weak by themselves, they are relayed by strongly nonlinear feedback effects which result in large-amplitude climatic variations. Besides, the climate is also thought to exhibit large variations by itself, without any changes to astronomical effects."
"There are also three significant Milankovitch cycles: the eccentricity of the Earth’s orbit varies from more elliptical to more circular with a period of the order of 100 000 years; the obliquity of the Earth’s axis oscillates between two extremes with a period of 41 000 years; and the spin axis precesses with a 23 000 year cycle."
From the same source, is the article entitled "
Solar–Terrestrial Connection: Long-Term and Short-Term Climate Variability", which is very very useful.
One point also should be raised, that is not in this rather interesting discussion thread, is volcanism and the junk tossed up in to the atmosphere can accelerate climate change as well. If I remember my geology, there was a volcanic activity about 20 to 23 million years ago west as south of Brisbane. Known as the
Mount Warning Shield Volcano, this volcanic mountain rose 2 km., the highest point anywhere on the Australian continent in the last 250 to 500 million years. The size extended 100 kilometres, spreading from Byron Bay, Mount Tamborine and Lismore. The caldera is known as the
Tweed Volcano, whose size is 40 km. across and about one kilometre deep. It remains one of the biggest volcano we know of!
Such volcanic activity would have catastrophic influence on the southern climate, especially as the latitude of Australia was closer to just south of where Tasmania is today.