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Shadow of extinction
By George Monbiot
July 1— 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, paleontologists 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 ten 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 percent 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 ten 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: 6ºC. 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 seven and ten 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.
Source: Guardian (UK)
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