For most of its history, the Earth has had no ice at its poles. On the greenhouse planet, carbon was CO2 in the atmosphere rather than trapped in the rocks. Forty million years ago, with continental drift moving Antarctica to the South pole, and ocean currents moving around the continent rather than bringing warm water from the Equator, ice began to form there, reflecting the Sun’s radiation back into space and cooling the planet. The last 2.7m years have had many cycles of interglacials and glaciation- the term “ice age” was coined in the early twentieth century when there were thought to have been three or four glaciation periods in history, but there have been hundreds.
Also in the early twentieth century, Milankovitch calculated the variations in the eccentricity of Earth’s orbit, its axial tilt, and the precession of the Equinoxes. The more the axis of rotation varies from 90° from the plane of the orbit, the more the seasons vary and the more regions cool in winter. These cycles can influence when the glaciers advance.
We can calculate how much ice there is at the poles from the oxygen isotope ratio. Oxygen forms two stable isotopes, O16 and O18. Water molecules with O16 in them are lighter, so more prone to evaporation. When there is ice at the poles, water falling as snow there will remain for thousands or millions of years, and the ratio of O18 to O16 in the seas will be higher. Seashells and coral are made of calcium carbonate, CaCO3, and so the ratio of the two oxygen isotopes in the water may be calculated from ocean bed fossils, whose age is indicated by their place in the geologic column.
I find that fact beautiful. The elegance of the idea, and the complex effort required to produce and analyse the evidence of changing temperatures makes me proud to be human and delighted to be alive at this time in our history. Antarctic ice cores now go back 800,000 years, each year making a new layer of snow, and preserve samples of the atmosphere in the past: we can find the varying proportions of nitrogen, oxygen and carbon dioxide. The wilful ignorance of young earth creationists and the deliberate deception of those who purport to argue their case scientifically gets more shocking- and pitiable, as they close their awareness to the wonders of human ingenuity and discovery. They are driven to more desperate quibbling as the weight of evidence increases.
As coral grows just under the surface of the sea, coral remains show that sea level has varied from a hundred metres below current levels during glaciations, and five metres above in the warmest parts of interglacials. The Earth’s temperature has varied by far more than the 4° predicted for this century; but never so quickly. Species have had time to adapt; not having that time is causing the incipient mass extinction of the anthropocene epoch.