An example of long-term climate variations:
dinosaur fossils have been found in rocks which
are known to have been near the Earth's polar regions
at the time of the dinosaurs.
There is also evidence of tropical swamps at the
same high latitudes.
Today's polar climates
likely could not have supported
such an ecosystem, so the Earth's poles
must have been substantially warmer at that
time.
We have also talked about the snowball Earth:
An increase in snow and ice on the planet (for
whatever reason) causes the albedo -- reflectivity --
of the planet to increase, which decreases the temperature
further. This is called positive feedback.
Eventually the Earth becomes a snowball.
The escape route is via plate tectonics and
volcanos. Increased atmospheric CO2 warms
the planet back up.
Here
is a cartoon of one snowball Earth "freeze-fry" episode
from
Paul Hoffman's web page.
The missing piece here is this: Why does the Earth warm up
into the "fry" stage? The answer is the greenhouse effect.