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They indicated that further studies by them would be aimed in this direction.
Dawson (1992) provided references to a number of major volcanic eruptions
of significance. In addition to Toba, he mentions four major volcanic eruptions
in Mexico and Guatemala; the three Mexican eruptions had ages of 100, 65, and
35 kybp . The age of the Guatemalan eruption was inferred to be about
84-90 kybp . There were Antarctic eruptions between 30 and 16 kybp . Major
North Atlantic ash deposits (possibly from Iceland) have been identified with an
age around 58 kybp .
A study of sulfates in the Greenland ice cores provided data on volcanic
eruptions (VEI
4) over the past 9,000 years (Zielinski et al., 1994). During this
period, 69 major volcanic events were identified. Nevertheless, it remains unclear
whether this volcanic activity had more than temporary impacts on climate.
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8.5 GREENHOUSE GASES
It is dicult to support the proposition that changes in greenhouse gases cause ice
age-interglacial cycles, because one would then have to explain why the changes in
greenhouse gas concentrations occur in the first place. As discussed in Section 4.5
(see references therein), the general consensus is that changes in greenhouse gas
concentrations are primarily effects (rather than causes) of climate change in
regard to ice age cycles. However, it is possible that changes in greenhouse gas
concentrations provide positive feedback to climate trends established from other
causes. While principal attention has been addressed to CO 2 —and, to a lesser
extent, CH 4 —water vapor is the most potent greenhouse gas and the presence of
a lowered global temperature and huge ice sheets acting as cold traps would likely
have lowered global atmospheric water vapor content, thus amplifying any cooling
trend once started. Conversely, the exponential dependence of water vapor press-
ure on temperature could lead to rapid increases in water vapor content during
warming trends, thus providing positive feedback to warming trends.
Clearly, changes in greenhouse gas concentrations between glacial and
interglacial conditions are significant. However,
it seems unlikely that
these
changes can be construed as innate causes of such cycles.
8.6 ROLE OF THE OCEANS
In many aspects of climate change, the scientific community often coalesces on a
consensus view based on limited data. The thirst for accepted models seems to
outrun the accumulation of data and true understanding. As a result, hypotheses
tend to become rigidified into accepted explanations and acquire the weight of
fact, while herd behavior induces many scientists to go along with the flow like
weather vanes adapting to prevailing winds (see Section 11.1.2). Such is the case
regarding the role of oceans in climate change; not that there is no role but,
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