Geoscience Reference
In-Depth Information
We also show a sharply rising line of dots at the upper right in Figure
28; these are the temperature projections for the next two centuries from
the Yale DICE model with uncontrolled climate change (these estimates
are representative of other models). Our temperature projections with
unchecked CO
2
emissions would push global temperature far beyond
the upper end of the ice core record.
It would be necessary to go back much further in geological time
and biological history to fi nd temperatures as high as those projected
for the coming centuries. While the proxy record is necessarily ap-
proximate, it appears that the earth reached maximum temperatures of
4-8°C above today if we go back 500 million years. CO
2
concentrations
were as much as eight times current levels in the Jurassic period, and
even higher in earlier periods. These higher levels are not surprising
because today's fossil fuels are the result of decaying vegetation from
these earlier periods of much higher CO
2
concentrations.
Many years ago, I suggested the paleoclimatic temperature extremes
could serve as an appropriate target. The reasoning was as follows: “As
a fi rst approximation, it seems reasonable to argue that the climatic
effects of carbon dioxide should be kept within the normal range of long-
term climatic variation. According to most sources the range of variation
between distinct climatic regimes is in the order of
5°C, and at the
present time the global climate is at the high end of this range. If there
were global temperatures more than 2 or 3° above the current average
temperature, this would take the climate outside of the range of obser-
vations which have been made over the last several hundred thousand
years.”
10
Setting targets with reference to historical trends was adopted by
the infl uential German Advisory Council on Global Change back in
1995. The council suggested that climate policy should be set with ref-
erence to a “tolerable temperature window.” This window would look
at the range of fl uctuation for the earth's mean temperature in the last
few hundred thousand years. It estimated that the planet today is near
the upper end of the range and proposed somewhat arbitrarily extend-
ing the historical range by
1
⁄
2
°C at either end. With this window, the
±
