Jökull - 01.01.2005, Síða 7

Global Warming: Take Action or Wait?
So, in a sense, it’s a standoff. Those who refuse
to accept the predictions made by computer simula-
tions can use the temperature record as an excuse for
inaction. Let us wait until there is clear evidence that
greenhouse gases have produced a significant warm-
ing before we take expensive action. Those who ac-
cept the models as our most reliable guide to the fu-
ture would counter that unless we create a plan as to
how we might curtail the CO2 buildup and unless we
begin to implement this plan, we have no chance to
prevent CO2 from climbing well above 560 parts per
million. For in order to stem the CO2 rise, we will
have to make a complete revision in our entire energy
infrastructure.
THE PALEOCLIMATE RECORD
Fortunately, there is another source of information
which in my estimation tips the balance strongly to-
ward the side of those who demand that action be
initiated. It is the paleoclimate record stored in ice,
in sediments and in stalagmites. In order to correlate
the messages preserved in these diverse archives, stu-
dents of past climates have developed precise meth-
ods for absolute dating. Only in this way has it been
possible to correlate records preserved in different re-
gions of the planet. The task is akin to that facing
historians who depend on calendars to relate events
which took place in different countries. In addition
to time clocks, these archives must contain proxies
which record temperature, or rain fall, or glacial ex-
tent, or atmospheric CO2. An amazing array of both
clocks and proxies have been developed over the last
50 years (Broecker, 2002). Taken together, they pro-
vide us with a clear picture of how the climate of our
planet has evolved over the last half million years. To
me, this reconstruction shouts out loud and clear, that
rather than being immune to nudges, our climate sys-
tem greatly over-responds to them. By comparison
with the nudges responsible for these large climate
changes of the past, those associated with even a dou-
bling of atmospheric CO2 will constitute not just a
nudge but rather a sizable push.
Space is not available to adequately explain the
manner in which the climate records for the last half
million years has been reconstructed. Rather, I will
briefly recap three types of climate change the Earth
has experienced. Each of the three can be shown to
have been in response to a known nudge.
Example 1. During the last half million years four
major glaciations have occurred. Each involved the
growth of large ice sheets in North America and in
Europe. That in North America covered all of what is
now Canada and penetrated into the United States as
far south as the Ohio River and Staten Island. As a re-
sult of this growth of ice cover, sea level stood about
120 meters lower than now. Bubbles of air trapped
in Antarctic ice tell us that the CO2 content of the at-
mosphere dipped to as low as 190 ppm during each
episode of glaciation. Based on the models we use to
predict the future, this CO2 drop by itself could ac-
count for a glacial cooling of several degrees.
Each of these four cycles had the same basic
shape; a 100 thousand-year-duration decline in tem-
perature and increase in ice extent culminated in a
warming which suddenly terminated the glaciation
and returned the Earth to its interglacial condition. For
the last 10,000 years, i.e., the time during which our
civilization developed, the Earth has been in such an
interglacial state (see Figure 5).
Figure 5. Records of atmospheric CO2 content and
Antarctic air temperature as recorded in the Vostok
ice core (Petit et al., 1999). – Hitastig og styrkur kol-
tvísýrings í andrúmslofti Suðurskautslandsins s.l. 420
þúsund ár, samkvæmt mælingum á ískjörnum.
JÖKULL No. 55, 2005 7