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present in the Yucatan Peninsula of that day. Just when the dust,
smoke, and soot had dissipated and conditions might have returned
to near normal, this gas cloud produced a greenhouse effect that
lasted for a thousand years or more. Those creatures that had mirac-
ulously survived all that came before, now faced a millennium of
greenhouse temperatures. (Recent modeling by Pope and Ocampo,
however, indicates that the greenhouse effect might not have been
this strong.)
Obliterating shock waves, stupendous earthquakes, enormous
tsunami, a rain of fire, smoke, soot, darkness, a global deep freeze,
worldwide acid rain, ozone loss, greenhouse warming—it seems a
miracle that anything could have survived, and yet, remember our
thought experiment on just how difficult it is to exterminate an
entire species. Over 99.99% of individuals can die and enough
breeding pairs might be left alive to allow the species to survive. But
certainly no one can claim that the impact of a 10-km meteorite in
the Yucatan Peninsula 65 million years ago lacked the power to
cause the K-T mass extinction.
We know that a 100-million-megaton impact happened at K-T
time; we know that it must have had some combination of the
effects just described. What we do not know is just how the many
lethal possibilities would have interacted with each other and with
living organisms. These questions will occupy impact modelers, geo-
chemists, paleontologists, and others, for years. Meanwhile, some
paleontologists, though now objectively required to admit that im-
pact happened, remain unwilling to grant that it had anything to do
with extinction until "precise biological/ecological mechanisms are
proposed that uniquely account for observed taxic patterns and the
stratigraphic timing of K-T extinction and survivorship." 6 6 The clear
implication is that the burden of proof still rests entirely with the
pro-impactors: They must explain how the impact effects killed cer-
tain species and spared others. But the existence of the Chicxulub
crater shifts the burden. Since we know that impact occurred, those
who deny that it caused the mass extinction have just as much of an
obligation to explain how species escaped as those who support the
link between impact and extinction do to explain how they did not.
T HE S ECOND
H URDLE
Looking back at the evidence described in this and earlier chapters,
we can see that the predictions of the impact half of the Alvarez the-
ory hold up well. To cap it off, geologists have located the impact
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