qClimate in the past 500 million years have alternated between long
periods of warm climate and short periods of cold climate.
qDuring the last 500 million years, major continent-size ice sheets
existed on Earth during three icehouse ear: (1) a brief interval near 430 Myr ago, (2) a
much longer interval from 325 to 240 Myr ago, and (3) the current icehouse era of
the last 35 million year.