Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 339

11.1.4 Other evidence on the role of CO 2
11.1.4.1 Temperatures over the past two millennia: the ''hockey stick''
Obviously, one of the great issues facing humankind is the question of how much
global warming is to be expected from increased emission of CO 2 in the 21st
century. The primary basis for the argument that increased CO 2 was the most
significant factor in 20th century global warming rests upon climate modeling,
which up until now has shown itself to be of uncertain veracity. In addition, there
are other circumstantial inferences that may be relevant.
If after due consideration and analysis it turns out that the expected global
temperature rise from increased CO 2 emissions in the 21st century is injurious to a
critical extent, draconian changes in world energy production and consumption
may be needed to mitigate the situation. Many alarmist organizations have
already prematurely leapt to this conclusion.
While some alarmists have claimed that current CO 2 concentrations are
unprecedented, we pointed out in Section 2.2 that hundreds of millions of years
ago the CO 2 concentration may have been as high as hundreds of times greater
than the present atmospheric level. It is widely believed that over the course of
geological history, CO 2 levels have periodically been much higher than those that
prevail today.
Turning our attention to the more recent past, a relevant issue is whether
there have been climate fluctuations during the past few thousand years of magni-
tude comparable or greater than recent global warming. Since these fluctuations
occurred prior to large-scale human impact, they probably reflect the range of
natural variation, at least during the very recent past in the Holocene. If that
range of natural variation exceeds the current trend, it would imply that the
current trend could possibly be just another natural fluctuation. On the other
hand, if the range of natural variation prior to large-scale human impact is much
smaller than the current trend, that would seem to imply that current climate
trends probably originate from industrialization and land use. The problem here is
knowing how far back in time one should look for climate variability. Climate
alarmists have emphasized the magnitude of current global-warming trends by
making elaborate and exaggerated claims that current warming far exceeds any-
thing the Earth has known for thousands (or even millions) of years—which is of
course nonsense. Figure 4.15a shows data that indicate that the previous three
interglacials were considerably warmer than the Holocene. Yet, the data do not
indicate that CO 2 levels were higher than Holocene CO 2 levels. The CO 2 -tempera-
ture connection seems to be weaker than the alarmists assume. As Figure 4.7
shows, significant temperature fluctuations may occur during an interglacial
independent of the activities of humans. There is no reason to expect temperatures
during an interglacial to remain constant and benign.
Of particular note is the temperature history of the past 1,000 to 2,000 years.
There have been many studies of past climate variations over this time period
which were based on a variety of proxies (tree rings, corals, ice cores, pollen, etc.).
Realizing that there exist many local, regional, and hemispheric proxies, with