Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 29

and not uniformly over all regions. Most of the warming occurred in the higher
northern latitudes. Warming has not been in lockstep with rising CO 2 levels.
Climatologists have also attempted to estimate T G using proxies over
geological time periods as long as hundreds of millions of years. Proxies are
indirect indicators of past temperature based on some natural process that
occurred in the past that was dependent on temperature. Many proxies have been
proposed and utilized. The proxies that are of the greatest value in estimating
global temperatures over tens or hundred of millions of years are oxygen isotope
ratios in benthic ocean sediments in which the 18 O concentration is an inverse
measure of T G . While the conversion of the direct signal d 18 OtoT G is only
approximate, the d 18 O measurements appear to be reliable and we have rough
estimates of how T G varied over the past 500 million years, even though absolute
values are less certain.
Believing that every effect has a cause or causes, climatologists have searched
for possible causes of these long-term climate changes and inevitably, after
eliminating all other candidates, they have assumed that variability of CO 2 con-
centration was the major factor that caused long-term climate changes over many
millions of years:
''The major transitions between climatic icehouse and greenhouse conditions
are ultimately most probably driven by the deep Earth processes of plate
tectonics, as a function of the long-term balance between CO 2 degassing at
spreading centers and the conversion of atmospheric CO 2 to mineral carbon
through long-term silicate weathering and oceanic carbonate formation''
(NAS, 2011).
The argument goes (more or less): ''If it wasn't CO 2 , what else could it have
been?'' This argument has some merit. We can estimate from first principles the
heating effect that rising CO 2 will produce in the atmosphere. If that was the only
thing that occurred—that is, only the CO 2 concentration changed and no second-
ary effects took place—we would be able to predict the effect of changing CO 2
with some precision. The problem is that other effects do take place as a conse-
quence of the climate changes induced by changing CO 2 , such as changes in
humidity, cloudiness, winds, ocean currents, land cover, ice sheets and glaciers,
etc., and these secondary changes may be of greater magnitude than the original
stimulus of CO 2 change, and they are very dicult to predict.
While the current principal interest is in the climate change induced in the 21st
century by increasing the CO 2 concentration from 280 to 560 ppm, multiple efforts
by many climatologists cannot seem to overcome the uncertainties inherent in per-
forming this analysis, and their models still lack credibility and consistency. It has
therefore occurred to several climatologists that perhaps by studying the past (tens
of thousands of years ago to hundreds of millions of years ago) and finding rela-
tionships between CO 2 and climate during those periods, they might be able to
obtain real world data on how climate and CO 2 are connected. This real world
data will presumably have built into it all the secondary processes that take place.