Historical Look at Temperature Changes
Throughout time, the average temperature of the global climate has oscillated between warm and cold as compared to today’s average temperature (plotted as 0 δ18O on the graph axis below).
Notice how the blue line spikes up on the chart. The global average temperature was hot during these times. There was probably little or no ice cover. When it spikes down, it was colder, and there may have been much ice cover.
Notice how the black line goes up and down less extremely than the blue line. This is an average of temperature over a longer time period than the blue line so it smooths out the spikes.
Notice the blue bars near the bottom of the chart designating glacial periods. The average temperature was very cold during these times with a lot of ice cover around the globe