  A review of the recent refereed literature fails to confirm quantitatively
that carbon dioxide (CO2) radiative forcing was the prime mover in the changes
in temperature, ice-sheet volume, and related climatic variables in the glacial
and interglacial periods of the past 650,000 years, even under the "fast
response" framework where the convenient if artificial distinction between
forcing and feedback is assumed. Atmospheric CO2 variations generally follow
changes in temperature and other climatic variables rather than preceding them.
Likewise, there is no confirmation of the often-posited significant supporting
role of methane (CH4) forcing, which despite its faster atmospheric response
time is simply too small, amounting to less than 0.2 W/m2 from a change of 400
ppb. We cannot quantitatively validate the numerous qualitative suggestions
that the CO2 and CH4 forcings that occurred in response to the Milankovich
orbital cycles accounted for more than half of the amplitude of the changes in
the glacial/interglacial cycles of global temperature, sea level, and ice
volume. Consequently, we infer that natural climatic variability notably the
persistence of insolation forcing at key seasons and geographical locations,
taken with closely-related thermal, hydrological, and cryospheric changes (such
as the water vapor, cloud, and ice-albedo feedbacks) suffices in se to explain
the proxy-derived, global and regional, climatic and environmental
phase-transitions in the paleoclimate. If so, it may be appropriate to place
anthropogenic greenhouse-gas emissions in context by separating their
medium-term climatic impacts from those of a host of natural forcings and
feedbacks that may, as in paleoclimatological times, prove just as significant.
