2. Over the last 400,000 years the Earth's
climate has been unstable, with very significant temperature
changes, going from a warm climate to an ice age in as rapidly
as a few decades. These rapid changes suggest that climate
may be quite sensitive to internal or external climate forcings
As can be seen from the blue curve, temperatures have been
less variable during the last 10 000 years. Based on the
incomplete evidence available, it is unlikely that global
mean temperatures have varied by more than 1°C in a century
during this period.
The information presented on this graph indicates a strong
correlation between carbon dioxide content in the atmosphere
and temperature. A possible scenario: anthropogenic emissions
of GHGs could bring the climate to a state where it reverts
to the highly unstable climate of the pre-ice age period.
Rather than a linear evolution, the climate follows a non-linear path with sudden and dramatic surprises when GHG levels reach an as-yet unknown trigger point.