Geologic Climate Cycles
In this chapter we are going to zoom out even further in time. Over millions of years, the climate of the Earth changes in different sorts of ways, with different sorts of patterns, than we have seen so far. The climate of the last 35 million years includes ice sheets: large and permanently frozen, holding significant amounts of water. Before this time, for millions of years, there were no ice sheets at all.
A time period such as today, with permanent ice sheets somewhere on Earth, will be referred to as a “great ice age.” Within our current great ice age, the ice sheets periodically grow and melt back, and global temperature rises and falls, in what are called “glacial cycles.” The Earth is currently in an interglacial interval of a great ice age.
Figure 9 shows climate’s descent from the hothouse world of tens of millions of years ago into the ice–age conditions of the present–day. Ice sheets existed in times indicated by the bars at the bottom of the graph. According to this reconstruction (from Zachos et al., Science, 2001), ice sheets in the Northern hemisphere date back less than 10 million years.
Antarctica is an ideal continent to put an ice sheet, because it is centered on the pole and surrounded by ocean all around. The opening up of the Drake Passage between South America and