- 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.
-
- http://www.guardian.co.uk
-
- Shadow Of Extinction
-
- By George Monbiot The Guardian - UK 7-1-3
-
- 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 new book: 'The Age of Consent: a Manifesto
for a New World Order'
-
- His website is: http://www.monbiot.com
-
- Source: http://www.guardian.co.uk/comment/story/0,3604,988380,00.html
-
- http://www.guardian.co.uk
-
- Related Websites
-
- http://Global-Warming.net
http://Acid-Rain.net
http://Extinct.net
http://GreenhouseEffect.net
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