the last couple of decades, much work has been done to try to determine
earth's average temperature over a long period of time. One of
the ways this is done is by drilling ice samples in Greenland and
determining from the layers of ice (much like tree rings) what
gases are present and what the average temperature was that year.
this record over Greenland appears to give valid readings only
up until the last inter-glacial period. Luckily enough, the drill
records for the Antarctic have recently revealed records over the
last 400 thousand years.
this chart of temperatures over a 400 Thousand year time period
recently gained from ice cores in the Antarctic. Please read
from left to right. It starts at current temperatures and goes
back in time. It shows the recent spike that marks the end of the last
glacial period. Notice the cold periods extend for about 100k years
and there are
4 warm spikes approx. every 100k years that last for an average
of 10k years.
correspond with readings taken in Greenland over the period of
time that Greenland can be measured and are viewed as reflective
of overall climate on Earth.
description of global warming will have to address the temperature's
cyclical nature over the last 400k years.