The 800,000-year Antarctic
ice core record shows
the coldest periods
during major glaciations
generally coincide with
the lowest levels of CO2
in the atmosphere.
The Milankovitch cycles,
based on planetary geometry,
are a good explanation
for the 100,000 year
ice age cycle.
But they don't explain
any increase or decrease
in atmospheric CO2 levels.
Milankovitch cycles could
explain a fluctuation in global
temperature due to changes
in solar radiation, which in turn
could cause either CO2 outgassing
from, or absorption into, the oceans.
Both sets of ice core data
from Antarctica show
changes in temperature
usually precede changes
in CO2 levels, suggesting
temperature change is
the cause of change
in the CO2 level.
It's difficult to imagine
how CO2 could have
increased from a
pre-industrial 280 ppm,
to 410 ppm today, without
man made CO2 emissions
from burning fossil fuels.
Many scientists
think this increase
in atmospheric CO2
is the dominant cause
of the slight warming
since 1950
( +0.6 C. in 68 years ).
Since the Little Ice Age
peaked (coldest) around 1700,
the climate has been warming
intermittently for 300+ years.
It is possible that this warming
is a continuation of a much longer
period of warming that had begun
long before human-caused
CO2 emissions could be a factor.