Over the last 50 million years, small variations in the movements of continents have changed the Earth's climate from being a greenhouse to an icehouse. ‘Global climate cooling’ explains how this happened, charting the history of the world's climate over the last 100 million years. What caused the big freeze and why did it happen when it did? Tectonic forcing alone cannot explain the rapid intensification of Northern Hemisphere glaciation. Studies using ocean sediments suggests that there were three main steps in the transition to the Great Ice Ages. The evidence is based on when rock fragments ripped from the continent by ice were deposited in the adjacent ocean basin by icebergs.