Geoscience Reference
In-Depth Information
strikes about every 100 million years. This conclusion, based
only on known craters, is completely independent of the one
based on astronomy, yet it gives the identical result.
Thus the Alvarez impact theory described an event that is rare
but that does occasionally take place and when it does, must pro-
duce large-scale effects. Even though in the early 1980s geologists
were still coming to understand the role of impact in the history of
the solar system, the Alvarez theory was within the range of what
was known and observable. It clearly passed the credibility test and
needed to be taken seriously.
The theory itself consisted of two parts: first, that a meteorite
struck the earth 65 million years ago, and second, that the effects
thus produced were so severe that they led to the K-T mass extinc-
tion. Unfortunately for their theory, but fortunately for
Homo sapi-
ens,
it is not easy to test the second part, for no large meteorite has
struck in the minute fraction of geologic time recorded by human
history.
One approach to the problem of verifying the theory's second
claim is through computer modeling. In 1983, influenced by the
Alvarez theory, a group of scientists that included the late Carl
Sagan used computer models to show how a nuclear war in which
fewer than half of the combined number of warheads then avail-
able to the United States and the Soviet Union were exploded
would throw enough dust, smoke, and soot into the atmosphere to
block sunlight for several months, particularly in the Northern
Hemisphere. This might set in motion the same sequence of events
as predicted by the Alvarezes (lowering temperatures by tens of
degrees, halting photosynthesis, destroying plant life, and disrupting
the food chain). The ozone layer might also be affected, allowing
the sun's ultraviolet radiation to penetrate and cause further dam-
age. Their paper, which appeared in
Science,
concluded that nuclear
war would have so few survivors, if any, that it would produce
another great extinction—this time, possibly of
Homo sapiens.
12
The threat of nuclear winter caught the attention of the world and
may have been influential in halting the growth of nuclear weapons
and ending the Cold War.
Though it would not include deadly radioactive fallout, cosmic
winter would be far worse than nuclear winter. The impact of a
10-km meteorite would release a vastly greater amount of energy
than Krakatoa, which caused the death of 35,000 people. It would
do far more damage than the atomic bomb that was dropped on
Hiroshima, which had the energy equivalent of about 13
kilotons
