http://www.commondreams.org/views03/0701-06.htm
Published on Tuesday,
July 1, 2003 by the Guardian/UK
Shadow of Extinction
by George Monbiot
It is old news, I admit. Two hundred and fifty-one million years
old,
to be precise. But the story of what happened then, which has
now been
told for the first time, demands our urgent attention. Its implications
are more profound than anything taking place in Iraq, or Washington,
or even (and I am sorry to burst your bubble) Wimbledon. Unless
we understand what happened, and act upon that intelligence,
prehistory may very soon repeat itself, not as tragedy, but
as catastrophe. The events that brought the Permian period
(between 286m and 251m years ago) to an end could not be clearly
determined until the mapping of the key geological sequences
had been completed. Until recently, palaeontologists had assumed
that the changes that took place then were gradual and piecemeal.
But three years ago a precise date for the end of the period
was established, which enabled geologists to draw direct comparisons
between the rocks laid down at that time in different parts of
the world.
Having done so, they
made a shattering discovery. In China,
South Africa, Australia, Greenland, Russia and Svalbard, the
rocks record an almost identical sequence of events, taking
place not gradually, but relatively instantaneously. They show
that a cataclysm caused by natural processes almost brought life
on earth to an end. They also suggest that a set of human
activities that threatens to replicate those processes could
exert the same effect, within the lifetimes of some of those
who are on earth today.
As the professor of
paleontology Michael Benton records in his
new book, When Life Nearly Died, the marine sediments deposited
at the end of the Permian period record two sudden changes.
The first is that the red or green or gray rock laid down in
the presence of oxygen is suddenly replaced by black muds of
the kind deposited when oxygen is absent. At the same time,
an instant shift in the ratio of the isotopes (alternative
forms) of carbon within the rocks suggests a spectacular change
in the concentration of atmospheric gases.
On land, another dramatic
transition has been dated to precisely
the same time. In Russia and South Africa, gently deposited
mudstones and limestones suddenly give way to massive dumps
of pebbles and boulders. But the geological changes are minor
in comparison with what happened to the animals and plants.
The Permian was one
of the most biologically diverse periods
in the earth's history. Herbivorous reptiles the size of rhinos
were hunted through forests of tree ferns and flowering trees
by saber-toothed predators. At sea, massive coral reefs accumulated,
among which lived great sharks, fish of all kinds and hundreds
of
species of shell creatures.
Then suddenly there
is almost nothing. The fossil record very
nearly stops dead. The reefs die instantly, and do not reappear
on earth for 10 million years. All the large and medium-sized
sharks disappear, most of the shell species, and even the great
majority of the toughest and most numerous organisms in the sea,
the plankton. Among many classes of marine animals, the only
survivors were those adapted to the near-absence of oxygen.
On land, the shift
was even more severe. Plant life was almost
eliminated from the earth's surface. The four-footed animals,
the category to which humans belong, were nearly exterminated:
so far only two fossil reptile species have been found anywhere
on earth that survived the end of the Permian. The world's
surface came to be dominated by just one of these, an animal
a bit like a pig. It became ubiquitous because nothing else
was left to compete with it or to prey upon it.
Altogether, Benton
shows, some 90% of the earth's species
appear to have been wiped out: this represents by far the
gravest of the mass extinctions. The world's "productivity"
(the total mass of biological matter) collapsed.
Ecosystems recovered
very slowly. No coral reefs have been
found anywhere on earth in the rocks laid down over the
following 10 million years. One hundred and fifty million
years elapsed before the world once again became as biodiverse
as in the Permian.
So what happened? Some
scientists have argued that the mass
extinction was caused by a meteorite. But the evidence they
put forward has been undermined by further studies. There is
a more persuasive case for a different explanation. For many
years, geologists have been aware that at some point during
or after the Permian there was a series of gigantic volcanic
eruptions in Siberia. The lava was dated properly for the first
time in the early 1990s. We now know that the principal explosions
took place 251 million years ago, precisely at the point at
which life was almost extinguished.
The volcanoes produced
two gases: sulphur dioxide and carbon
dioxide. The sulphur and other effusions caused acid rain,
but would have bled from the atmosphere quite quickly. The
carbon dioxide, on the other hand, would have persisted.
By enhancing the greenhouse effect, it appears to have warmed
the world sufficiently to have destabilized the super concentrated
frozen gas called methane hydrate, locked in sediments around
the polar seas. The release of methane into the atmosphere
explains the sudden shift in carbon isotopes.
Methane is an even
more powerful greenhouse gas than carbon
dioxide. The result of its release was runaway global warming:
a rise in temperature led to changes that raised the temperature
further, and so on. The warming appears, alongside the acid rain,
to have killed the plants. Starvation then killed the animals.
Global warming also
seems to explain the geological changes.
If the temperature of the surface waters near the poles increases,
the circulation of marine currents slows down, which means that
the ocean floor is deprived of oxygen. As the plants on land died,
their roots would cease to hold together the soil and loose rock,
with the result that erosion rates would have greatly increased.
So how much warming
took place? A sharp change in the ratio
of the isotopes of oxygen permits us to reply with some precision:
6C. Benton does not make the obvious point, but another author,
the climate change specialist Mark Lynas, does. Six degrees is
the upper estimate produced by the UN's scientific body, the
intergovernmental panel on climate change (IPCC), for global
warming by 2100. A conference of some of the world's leading
atmospheric scientists in Berlin last month concluded that the
IPCC's model may have underestimated the problem: the upper
limit, they now suggest, should range between 7 and 10 degrees.
Neither model takes into account the possibility of a partial
melting of the methane hydrate still present in vast quantities
around the fringes of the polar seas.
Suddenly, the events
of a quarter of a billion years ago begin
to look very topical indeed. One of the possible endings of
the human story has already been told. Our principal political
effort must now be to ensure that it does not become set in stone.
George Monbiot's book
'The Age of Consent:
a Manifesto for a New World Order' is published
by Flamingo. His website is www.monbiot.com
Guardian Unlimited
© Guardian Newspapers Limited 2003
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