[ Benton shows why it was probably lava flows, not impact from meteors that caused the Permian extinction.
I don’t know why everyone isn’t reading whatever they can find on the greatest mass murder of all time — the Permian Extinction. Especially since we humans are causing a 6th mass extinction. Will ours be as big as the Permian? It will be if we burn as much fossil fuels as were contained in the Siberian traps I would guess. But if we don’t have an equal amount of fossil fuels, will fossils still do as much harm because they’re being burned orders of magnitude faster, before life has enough time to adapt?
When I read this book I was hot on the trail of trying to figure out if methane (gas) hydrates might be the murder suspect. But Benton twice says no: 1) The end-Paleocene methane burp did not lead to a major extinction event, and 2) the gas hydrates were probably not the main killer at the end of the Permian. Though he does see methane burbs as an accomplice: “the release of gas hydrates added to the misery”.
I am still trying to figure out if there is a methane apocalypse in our future. There is mounting evidence that it may not be. But not nearly enough research has been done, so I’ll continue to follow the murder mystery as the story unfolds. I’ve put arguments against a methane apocalypse happening in the future in the post “Methane hydrate apocalypse? Maybe not…”.
In addition, peak conventional oil production peaked in 2005 and so decline is likely within 10 years as unconventional fails to keep up with the exponentially increasing decline rates of the 500 Giant Oil Fields that provide over half of our oil. We are also at or near peak coal. And close to peak natural gas. That means there may not be enough fossil fuels left to reach the worst-case IPCC scenarios (RPC 8.5). Many geologists think a max of IPCC RPC 2.5 is most likely. That will be bad, but maybe not Permian extinction bad!
Alice Friedemann www.energyskeptic.com author of “When Trucks Stop Running: Energy and the Future of Transportation, 2015, Springer]
Benton, Michael J. 2003. When Life Nearly Died. The greatest mass extinction of all time. Thames & Hudson. EXCERPTS FOLLOW:
140 DIVERSITY, EXTINCTION AND MASS EXTINCTION
The astronomers enthusiastically polished their telescope lenses and pointed them skywards in search of Nemesis, Planet X or tilting galaxy edges. Geological supporters of periodicity went out to find evidence for massive impacts at mass extinction boundaries to match the physical evidence that had already been established for the KT event. But the critics of periodicity argued that each mass extinction was a one-off, and that there was no linking principle. The 26 myr. cycle discovered by Raup and Sepkoski was, they argued, a statistical artifact or the result of limited data analysis.
So what is the current view of periodicity? I think that most paleontologists and geologists have just quietly let it drop. Close analysis of the fossil data has failed to confirm periodicity Indeed, scrutiny of some of the extinction peaks in Fig. 19, such as the three in the Jurassic, has suggested that these are largely artifacts of the data collecting. Also, the searches for Nemesis and Planet X have not been successful; nor has the search for indicators of impact at the time of the other identified mass extinctions. Iridium, shocked quartz or craters have been found for only two or three of the ten postulated mass extinction peaks that are elements of the periodic cycle, but this evidence is feeble in the extreme when compared to the manifold lines of evidence for impact at the KT boundary.
Periodicity of mass extinctions was an intriguing idea, but it has been almost conclusively rejected now. But that does not mean that one should not take an overview of all the extinction events of the past in search of common factors.
Scaling and taxonomic targets
Extinction events of the past vary in magnitude, and they may usefully be sorted into major, intermediate and minor events, based on their magnitudes. The end-Permian mass extinction is in a class on its own, since it is known that 6o-65% of families disappeared at that time, and this scales up to a loss of 80-95% of species. The four intermediate mass extinctions are associated with losses of 20-30% of families, and perhaps 50% of species. The minor extinction events experienced perhaps 10% family loss and 20-30% species loss, but these cannot be called mass extinctions.
184 LIFE’S BIGGEST-CHALLENGE
amazing array of special devices – broad flanges, spikes, gas-filled balloons – that stop them from sinking; some have spiral body shapes so that they spin slowly They include many unique groups that spend their entire lives in that form, while others are the larvae of typical marine animals, such as crabs, sea urchins or corals, that will eventually metamorphose into their adult forms. The plankton form the base of all food chains in the sea. They are eaten by shrimps, fishes and other larger animals, and these in turn form the diet of larger fishes, sharks, seals and whales.
Kill the plankton, and you kill all life in the sea.
The radiolarians, delicate net-like little organisms with a light skeleton generally made of silica (silicon dioxide, the main component of sand and of flint), today feed on bacteria and plant-like plankton. Their skeleton is made up of tiny spicules, or needles, of silica, forming perforated spheres, some with spikes, just like miniature Christmas decorations. Others are like tiny string shopping bags, suspended from a single corner. All are perforated with numerous regular holes. When they die, the flinty little skeletons of radiolarians rain down on to the deep ocean floor where ‘ they accumulate slowly, at only 4 or 5 millimeters per 1000 years. Nevertheless, over millions of years, radiolarian oozes have solidified into cherts, glassy pure-silica deposits, that currently make up about 3% of the modern ocean floor.
In Late Permian deep marine rock successions, radiolarian cherts are found quite commonly in China, Japan and Canada, but then disappear completely at the end of the Permian, only to reappear in the Mid Triassic, some 8 million years later. This ‘chert gap’ is matched by the virtual annihilation of all species of radiolarians at the end of the Permian – an event that is particularly striking since otherwise the radiolarians had had a singularly uneventful history for the previous 450 million years. It takes some enormous environmental shock to kill off such minute planktonic organisms that must have been present as millions of millions of individuals all over the world. Far easier to kill off large and less numerous animals such as dinosaurs.
Another element of the plankton today are the foraminifera. Foraminifera look like tiny spiral or coiled snails, with a shell made from calcium carbonate (calcite) or glued sand grains, enclosing their single-celled soft parts. The shell may form a tall spiral, a flat coil or disc, or a mass of small globules, in each case being divided into a number of internal
chambers. In the Permian all foraminifera were sea-bed dwellers, feeding themselves on the rain of organic matter and plant-like plankton that sank from the surface. The dominant foraminiferan group in the Permian were the fusulines. Their shells were made from many tiny crystals of calcite. Some of them reached as much as 10 centimeters in length -most unusual for such a single-celled animal. The fusulines flourished in the Permian, evolving fast and giving rise to over 2000 species. Indeed, they evolved so fast, and achieved such diversity, that they are used as important guide fossils for dating Permian marine rocks. Then they all disappeared.
Until recently, the die-off of the fusulines was thought to have lasted for most of the second half of the Permian, some 20 million years or more. It was said to have been a gradual die-off, perhaps linked to long-term climatic deterioration. But new work has shown that the early studies had been too limited, and had relied on a literal reading of the record. The problem was backward smearing, the Signor-Lipps effect, as noted above (Chapter 7).
A recent study of the Permo-Triassic boundary in Austria by Michael Rampino and Andre Adier, both of New York University, has shown on the basis of very detailed collecting that most fusuline species did indeed go extinct right at the Permo-Triassic boundary. Other fusuline species dis-appeared from the rock record as much as 16 meters below the boundary, which could be taken to imply a long-term die-off. However, these species were rare forms, known only from small numbers of specimens. Rampino and Adier argued that these were misleading datum points: the disappearances do not indicate extinctions. If these rare forms had been commoner, they would probably have been sampled up to the Permo-Triassic boundary as well. As mentioned before, with a patchy fossil record, it is unlikely that palaeontologists will ever find the very last member of a particular species to have lived on Earth. If a hundred species died out in an instant, the fossil record might still paint a picture of long-term gradual disappearances. Rare forms are especially liable to this phenomenon of false early disappearance, the Signor-Lipps effect.
Not all foraminifera died out during the end-Permian crisis. The generally large fusulines had completely crashed out of sight. But survivors included mainly smaller forms and some flattened species that burrowed in the sea-floor sediments feeding on detritus. A characteristic feature of these, and other survivors, is that they may have been adapted to living in conditions of low oxygen. Perhaps this is a clue to the nature of the environmental stresses at the end of the Permian.
Reefs were common in Permian shallow tropical waters. For example, huge reefs developed over much of west Texas and New Mexico. During the Mid Permian, as the tropical seas became deeper, reefs built up around the edge of the ancient Delaware Basin, reaching a height of some 600 meters. Just like modern coral reefs, the living corals and other animals, were in the upper parts of the reef, keeping in close touch with the sea surface which allowed the associated plant-like organisms to photosynthesize. Deeper parts of the reef were formed from overgrown, dead coral skeletons, as well as shells and other reef rubble.
Reef life then was hugely diverse, with hundreds of species living in close proximity. The framework of the reef was built from sponges, corals and bryozoans, animals that secrete a stony skeleton in which they live. Living on the dead corals were various clinging molluscs and worms. Creeping among the coral fronds were snail-like molluscs, sea urchins, starfish and shrimps. And swimming above were jet-propelled nautiloids and ammonoids (relatives of squid and octopus), swimming arthropods and fishes of various kinds. Just as today, Late Permian reefs were diversity hotspots – locations of unusual species richness.The seas have retreated now of course, but the west Texas landscape has not changed much in the past 100 million years. The visitor today can essentially stand on the Late Permian sea bed and look around at the towering Guadalupe Mountains, made up of reef limestones, termed the Capitan Limestone Formation. The vast size of the reef, hundreds of meters thick and 400 kilometers long, is immediately clear, and the richness of Mid Permian tropical reef life is evident. Such large and richly diverse reefs are also known from China right to the end of the Permian.
Reefs were entirely wiped out by the end-Permian event. Like the ‘chert gap’, there is a ‘reef gap’ lasting for some 7 or 8 million years in the Early Triassic. Many sponge groups survived apparently unaffected through the end-Permian crisis, but others, especially those associated with the tropical-belt reefs, were decimated.
187 LIFE’S BIGGEST CHALLENGE
The corals were even harder hit. Throughout the preceding 200 million years, limestone deposits of tropical zones are absolutely teeming with the skeletons of rugose and tabulate corals. Any novice fossil collector will have accumulated dozens of specimens of these corals – the tiny ice cream cones of small, solitary rugose corals and the fist-sized, rounded tabulate coral colonies composed of dozens of regular, honeycomb-like or star-shaped chambers. Some are even shaped like a bursting sun – hence the name of the coral, Heliolites, the ‘sun rock’. Solitary corals built them-selves tubular houses from calcite which they laid down round and round their soft bodies as protection from predation. Their tubular houses range in size from a few millimeters long to the vast meter-long cones of Caninia in the Carboniferous. Mostly they were fixed upright on to rocks or other hard materials on the sea bed.
Colonies of rugose or tabulate corals are among the most beautiful of fossils. Colonies arose when one progenitor coral animal cemented its skeleton down to the seafloor, and then set about building its stony dwelling chamber. By endlessly splitting, the original coral animal formed numerous identical clones of itself, each of which constructed a little chamber. There is economy in such an arrangement, since each subsequent coral animal has to build only a few side walls and can use the pre-existing parts of the colony. In the end, most colonies formed bulbous, cabbage-shaped structures or broad plates on gradually expanding trumpet-like stalks. Each coral animal kept itself free of the others, and fed by capturing food particles on sticky tentacles. If danger threatened, the coral animals withdrew deep into their stony houses.
The rugose and tabulate corals, which had been the mainstay of reef formation worldwide for 200 million years of the Palaeozoic, all died out at the end of the Permian. It seems that they had undergone a long-term decline before the very end. First to go were the massive colonial forms, and at the end it was the turn of the colonies made from less intimately intergrown tubes and the solitary forms. The early losses of some coral groups seem to relate to changing habitats. For example, the warm tropical seas that had covered Texas and New Mexico had withdrawn in the Late Permian. This was not part of the crisis, merely a change in sea levels and continental positions. Where coral reefs are found in the latest Permian, however, such as in South China, the corals survived right to the end.
190 LIFE’S BIGGEST CHALLENGE
Of course, had this not happened – if, for example, the tiny numbers of species that squeezed through the bottleneck from the Permian to the Triassic had actually all died out – the effects would have been negative. But then it would have been a total wipeout, pure and simple.
First-year geology students always complain about having to learn the groups of fossils. One of the key facts they have to grasp is the difference between brachiopods and bivalves. These are two distinct groups of shelled animals which have different ancestors, but which look superficially similar. A brachiopod consists of two shell halves, more properly called valves, that enclose and protect the animal inside. The shell is fixed to the seabed by a tough thread that emerges from the tip of one of the valves. The two valves are joined along the hinge line, and they may be opened by muscular activity to allow food particles to be sucked in and waste material expelled.
It is easy to tell a brachiopod from a bivalve: brachiopod valves are different in dimensions, while those of bivalves are identical mirror-images. One valve of the brachiopod is larger than the other, and it is often shaped like a Roman oil lamp – teardrop-shaped, with the extension at the hinge-end often perforated by a large circular hole for passage of the attachment thread (just like the hole for the wick in the Roman oil lamp). The other valve is circular and smaller. Bivalve valves, on the other hand, generally fit exactly over each other, being identical in size.
The brachiopods and the molluscs of the Permian were hit hard by the mass extinction. Molluscs, such as clams, oysters, mussels, whelks, octopus and squid, dominate the seafloor today, while brachiopods are relatively rare, being found only in rather deep waters and confined to certain parts of the world. However, the situation was the reverse in the Palaeozoic, and the end-Permian crisis perhaps has a large part to play in engineering the switchover.
To human eyes the brachiopods may seem pretty limited in their potential – all they really do is sit on the seabed opening and closing their valves. They feed by sucking water, plus food particles, into their shells, passing it over a looped filtering organ, the lophophore, and blowing water out the other side. However, the Permian was a time of astonishing innovation in the group. The cone-shaped rich thofenids copied the corals, cementing themselves to a rock or another shell with the tip of the cone and standing upright in tight clusters to form mini-reefs. The smaller valve had become simply a small lid, like that of a pedal bin, which could be opened to allow feeding. The fat, and often large, brachiopods also flourished in the Permian, when remarkable new spiny forms appeared. The spines were delicate tubular structures sprouting wildly all over the base valve, and these extraordinary brachiopods must have used them to anchor themselves in soft, muddy seafloors. So these two successful groups had conquered new habitats and modes of life, and who knows where the brachiopods might have gone but for the mass extinction.
The brachiopods were devastated by the end-Permian event. At the level of superfamilies, 10 out of 26 disappeared, which doesn’t seem too bad (it equates to a loss of 38%). However, at the level of families, jo out of^-j died out (91% loss). It has been estimated that some pj% of genera of brachiopods were hit by the extinction, and that equates to about 99% of species. So all but a tiny handful of this hugely diverse and abundant group bit the slime.
In contrast to this collapse of the brachiopods, some of the molluscs weathered the end-Permian crisis much better. There are three main groups of molluscs: the bivalves (‘two valves’), gastropods (‘stomach foot’) and cephalopods (‘head foot’). The origins of the last two of these names are rather startling. Gastropods do indeed have a stomach in their foot, but their foot is actually^ almost their whole body. Technically, the soft slimy portion of a snail or whelk that creeps over the ground is the foot, but obvi-ously the whole body of the animal, from eye stalks at the front to anus at the back, is enclosed in the foot. Cephalopods include the octopus and the squid, as well as the fossil ammonoids with their coiled shells. The ‘head’ is the front portion with its huge eyes and ring of massive tentacles that haul food towards the mouth. The ‘foot’ consists of the tentacles and a siphon that can squirt water or black ink for rapid jet propulsion and confusion of an enemy. So, technically, the head and foot are closely associated, and the ‘body’ of the ammonoid, or of the octopus, is a bag-like structure containing the stomach and guts.
Most families of bivalves passed through the event relatively unscathed, with only three out of 40 disappearing. Bivalves in the Late Permian were rarer elements of the seabed faunas than were the
249 ON THE RIVER SAKMARA
As we raced around the South Urals we saw site after site, each of which had produced some skeletons of amphibians and reptiles. We found the mass of new information hard to take in, and asked if anyone had actually done a census of the rise and fall of the different animal groups through time. Indeed, Valentin told us, he had compiled a huge catalogue of all the sites. When we returned to Saratov with him after the expedition, an interesting journey of 700 kilometers in the front of a huge Gaz 66 truck, Valentin showed us his card index. In it, 400 or so sites are listed, each with a determination of its geological age and a list of the fossils that had been found there.
A big job for the future will be to work with our Russian colleagues to translate the card index, and plot the information it contains about the rise and fall of reptiles across the Permo-Triassic boundary. A preliminary scan of the information suggests that the pattern is just the same as in South Africa – a catastrophic drop in diversity, followed by a long and slow recovery of ecosystems.
The final solution
So what caused this, the biggest of all mass extinctions? The nature of what happened is now much clearer than it had been, say, in 1990. Careful dating has shown that the event took place 251 million years ago, and that species losses were anything from 90 to 95%. This was no local phenomenon, since it has been detected in rocks from China to Spitsbergen, from Greenland to South Africa, from Russia to Australia. In every case, whether looking at events on land or in the sea, the rate of species loss seems to have been similarly huge. There were no safe refuges, nowhere to hide.
There is also no evidence for selectivity, except that the survivors tended to be widespread species. But with such a tiny survival rate – 10% or less of species made it through – it is clear that plants and animals were being wiped out with almost no regard to their adaptations. Certainly on land the large animals all disappeared, but this could be explained as part of a chance process. If there are 100 species, of which 10 are large, a 95% loss of species is likely to kill all the large animals. Add to that the probability that, as today, large animals are rarer than small animals (i.e. smaller population sizes), then it is easy to see why all the rhinos and elephants might disappear, but a few rats and squirrels might survive. It would be wrong, though, to say that this proves that individual rats and squirrels are better adapted to survive crises. It is perhaps only their greater abundance that protects them as a species
We have some hints of the environmental changes too. In the sea, the rocks show an increase in anoxia. Many of the surviving marine creatures seem to have been peculiarly adapted to living in such conditions of low oxygen There are hints too of low productivity, meaning a lack of organic matter in food chains, so many of the surviving species were presumably able to survive on very little food.
On land, as the recent studies in South Africa and Russia have shown, the end of the Permian is marked by a sudden change in sedimentation, with megafans composed of huge boulders. In neither case can this be explained by a dramatic increase in rainfall. Indeed, the evidence suggests increased aridity, so the dramatically heightened levels of erosion and runoff can best be explained by a sudden loss of vegetation and soils, perhaps worldwide. Soils show that climates also became warmer.
So what caused the crash? The event must have been sudden. It must have reduced oxygen levels, increased temperatures and reduced rainfall, all on a worldwide scale. It must have had the ability to push all of life virtually to the brink. The tentacles of the killing agent reached into shallow seas and into the deepest oceans. On land, they penetrated lowland basins and mountainous regions, rivers and lakes. What kind of crisis could have been so profound that it killed reptiles on land and brachiopods and corals on the sea floor?
It cleared the Earth of vegetation, even if for a short time.
This is more profound than any of the puny threats that humans have devised so far, whether nuclear bombs or mass forest clearance. A global rise in temperature of half a degree in a century as a result of industrial pollution and global warming? That fades into insignificance beside the crisis 251 million years ago.
272 WHAT CAUSED THE BIGGEST CATASTROPHE OF ALL TIME?
‘Negative feedback’ means that a process is countered by the opposite, or negative, process, so regulating the effects of the process and maintaining a steady state. ‘Positive feedback’, on the other hand, means that the process is enhanced by more of the same, with further positive processes operating in the same direction.
So with the atmosphere. Excess carbon dioxide is mopped up by plants (during photosynthesis plants absorb carbon dioxide and produce oxygen) and through weathering. Carbon dioxide is stripped out of the atmosphere by rain water, forming weak carbonic acid, which then dissolves limestones on the ground. The carbon combines with the weathering products of the limestone, and the oxygen is given off as carbon dioxide. If you drip an acid on to limestone, the limestone will fizz – this is the carbon dioxide bubbling off.
By 2001, a trendy new carbon source had been identified. And this was one that was fast. Gas hydrates are crystalline solids composed of a cage of water molecules trapping gas inside. The water cages can trap various gas molecules, including carbon dioxide and hydrogen sulphide, but the commonest gas hydrates trap methane, a gas composed of carbon and hydrogen. Gas hydrates form at water depths greater than 100 meters, and particularly in polar regions. Because of the high pressures at such depths, the gas hydrates are amazing gas concentrators; if 1 million liters of methane hydrate is brought to the surface suddenly, 160 million liters of gas can be released.
Since the 1970s, when gas hydrates were discovered, they have been identified deep in sediments around the margins of most continents, and particularly around the poles. The huge frozen masses of ice and compressed gas fill pore spaces within the sea-floor sediments, and occupy vast fields that can be detected by means of geophysical soundings.
Worldwide, it is estimated that gas hydrates contain at least 10,000 billion tonnes of carbon, about twice the amount of carbon held in all fossil fuels on earth. If some perturbation hits one of these gas hydrate bodies, and the gas is released, huge volumes of carbon dioxide or methane would bubble up through the ocean and explode from the surface, temporarily displacing the normal atmosphere above.
Could this be a killer? On 3 December 1872, the ship Mane Celeste was found adrift off the Azores. There was no sign of life on board, either above or below decks. There were no clues to explain why the crew had disappeared. Indeed everything appeared to be quite normal. In the crew’s quarters, clothing lay folded neatly on bunks and washing hung on lines; in the galley, breakfast had been prepared and some of it had been served. Could the crew have been killed by the release of a massive bubble of methane hydrate? Millions of cubic meters of methane or carbon dioxide erupting from below would have a devastating effect, but would then be swallowed up into the atmosphere, leaving no trace. Why were all the crew missing? This will never be known – perhaps the pulse from below and the stagnation of the atmosphere made them all run on deck and jump overboard in search of fresh air.
What if numerous gas hydrate bodies, all round the world, were to have been released at the same time? Evidence has now been found for such a mass gas escape, a so-called methane burp, 22 million years ago, at the end of the Paleocene. At that time there was a pulse of global warming up to 7° C over approximately 10,000 years, as shown by oxygen isotopes and the record of fossil plants. It has been suggested that this pulse of warming was caused by the release of 2000 billion tonnes of methane hydrate into the atmosphere.
The pulse of warming was brief, and conditions rapidly returned to normal. Gerry Dickens of the University of Michigan and colleagues suggests that this is good evidence that the warming was caused by gas hydrates. The rapid warming led to the death of many species, the excess organic matter from dead plants and animals was washed into the sea, and carbon dioxide levels in the atmosphere were quickly reduced by the incorporation of the organic matter into oceanic deposits and by increased weathering following the loss of plant cover.
The end-Paleocene methane burp did not lead to a major extinction event.
The effects were worldwide, and many species died out, but the Earth returned to normal soon enough, and most species recovered. Could such a model be enough to kill almost all life?
Explaining the carbon isotope spike
The gas hydrates were probably not the main killer at the end of the Permian.
But they may help to explain the massive negative shift in the carbon isotope curve, which dropped by 4 or 5 parts per thousand. This perhaps does not sound much of a shift towards the lighter isotope of carbon, but it actually represents a global shift in the entire carbon budget – the introduction of billions of tonnes of light carbon into the oceans.
What are the possibilities? The influx of isotopically light carbon could have come from the collapse of productivity that happened at the Permo-Triassic boundary, and the entry of huge amounts of rotting wood and animal carcasses into the sea. But, Paul Wignall has calculated, this would not be enough. It is estimated that the entire biomass of life on Earth today contains 830 billion tonnes of carbon. If all life is killed instantaneously, that amount of organic carbon could be washed into the sea and buried. But there are already billions of tonnes of inorganic, heavier carbon in the ocean-atmosphere system, so the addition of 830 billion tonnes would make very little difference to the ratio.
Even the Siberian Traps eruptions could not have supplied enough isotopically light carbon. If the volume of basalt produced was a million cubic kilometers, that would have produced 10,000 billion tonnes of carbon, which would have been a mixture of carbon. So, again, it was not enough to cause the carbon isotope shift. Even with the best figures, the Siberian Traps eruptions could have produced only 20% of the shift that actually happened.
Geologists have embraced gas hydrates with fervor – almost with a sigh of relief. When the calculations are done, nothing else has enough light carbon, nor can act fast enough. The carbon in gas hydrates is isotopically very light. The release of only 10% of the estimated 10,000 billion tonnes of carbon contained in gas hydrates today would be sufficient to cause the shift …the secret is the very light composition of carbon. Although the Siberian Traps may have released the same mass of carbon, its isotopic weight was much heavier, and could not have produced the observed negative spike.
The killing model Paul Wignall has put everything together into a single flow chart. The key crisis seems to have been the eruption of the Siberian Traps. Worldwide devastation was caused by the production of different gases during the eruptions, and these gases were presumably pumped into the atmosphere sporadically during the entire span of the eruptions. Perhaps a single major eruption could have been absorbed by the Earth, and the short-term disturbance of the atmosphere-ocean system corrected by normal feedback processes. But repeated eruptions may have been too much, and may have led inexorably to total collapse of all normal interactions between the physical world and life.
Four gases from the eruptions may have been to blame. Carbon dioxide had the longest-term effects, leading immediately to global warming and anoxia, which persisted for hundreds of thousands of years. Each pulse of eruption may have reprimed the effects, and prevented any normal feed-back systems from kicking into operation. The release of gas hydrates added to the misery.
Sulphur dioxide was also produced. This gas has a shorter residence time in the atmosphere, but the cooling effects of the sulphates may have caused a snap glaciation in some parts of the world, with associated falls in sea level as marine water was frozen into ice. Whether there was such a glaciation, and how long it lasted, cannot be said at present. And, as we saw above, there is limited geological evidence for freezing, but if it was short-term, as the theory suggests, one would not necessarily expect to find evidence preserved in the rocks.
Chlorine gas may also have been produced. In conjunction with the sulfates and the carbon dioxide these would produce acid rain. When combined with water, these gases form hydrochloric acid, sulfuric acid and carbonic acid, and hydrofluoric acid may also have been released. If such a delightful cocktail of acids were to rain out of the sky, normal plant life would have been devastated, just as today acid rain kills forests. With normal plants dramatically reduced, animal life on land would go too. Perhaps this postulated acid burst wiped out much of life on land and led to the fungal spike, the mushrooms and molds being the first land life to be able to recover.
Acid rain also, of course, increases the rate of normal weathering on land, and the loss of plants would make it worse as soils were stripped off. Retallack and colleagues certainly detected this in the record of soils across the Permo-Triassic boundary, and increased rates of runoff of sediment into the sea are indicated also by a shift in strontium ratios. A dramatic increase in the ratio of strontium-87 to strontium-86 across the Permo-Triassic boundary suggests that huge amounts of terrestrial material were entering the sea via rivers.
The whole end-Permian crisis may have been made even worse by a runaway greenhouse effect. Normally, the atmosphere-ocean system will correct imbalances, and return carbon and oxygen levels to normal. This is a negative feedback process. If carbon dioxide levels increase, burial of organic matter, weathering or proliferation of forests will eat up the excess gas. However, a runaway greenhouse is a positive feedback system. An increase in carbon dioxide, for example, is not countered by processes that mitigate the effect. On the contrary, it triggers processes that add yet more carbon dioxide to the atmosphere.
The end-Permian runaway greenhouse may have been simple. Release of carbon dioxide from the eruption of the Siberian Traps led to a rise in global temperatures of 6°C or so. Cool polar regions became warm and frozen tundra became unfrozen. The melting might have penetrated to the frozen gas hydrate reservoirs located around the polar oceans, and massive volumes of methane may have burst to the surface of the oceans in huge bubbles. This further input of carbon into the atmosphere caused more warming, which could have melted further gas hydrate reservoirs. So the process went on, running faster and faster. The natural systems that normally reduce carbon dioxide levels could not operate, and eventually the system spiraled out of control, with the biggest crash in the history of life.
The view from the burrow
What did all of this look like at the time? Imagine the scene in the Karoo Basin in Dicynodon Zone times, which we encountered in Chapter 9. Dicynodon itself, the medium-sized plant-eater that was most abundant at the time, may have been able to make burrows in which it could escape from the normal rigors of the tropical-monsoonal climate in which it lived. As the crisis approached, Dicynodon would have scuttled along the river bank and plunged into his cool burrow, expecting that it would all pass in a day or so and he could crawl out again.
The first basalt eruptions began thousands of years before, and far away, in Siberia, and continue, sporadically. None of the noise of the explosions would be heard in Africa, nor would Dicynodon have seen any of the erupting lava, ash or gas. But air temperatures might bounce up a little. Locally, around the eruption site, there might be a snap freeze caused by the emission of sulphur dioxide, but that would be a short-term phenomenon, soon overwhelmed by the warming effects of the carbon dioxide emission. The first eruptions pass pretty well unnoticed.
Then, a year or so later, there is a larger eruption. A further snap freeze is replaced by a greater rise in air temperature. This time Dicynodon feels it. It is the dry season, the time between the annual monsoonal rains, and it hurts. Life is balanced on a fine margin between survival and death during the dry season in any case, as we saw in Chapter 9. Even a 1 degree rise in temperature can kill off more plants than normal, and then more herbivorous animals fail to survive through to the rainy season. The blast of heat sends Dicynodon into his hole.
This time, the eruption initiates some further processes. The cocktail of gases ejected into the atmosphere rises high into the stratosphere and encircles the globe. The gases and fine dust distort the normal appearance of the heavens – sunrises and sunsets look weird, with splashes of red, yellow and purple, and this is seen all round the world. A few days later, the perturbation triggers catastrophic acid rain. The chlorine, fluorine, sulphur dioxide and carbon dioxide emitted from the volcano combine with rain water in the high clouds to produce a cocktail of hydrochloric, hydrofluoric, sulphuric and carbonic acids. For millions of square kilometers around the eruption site, the acid rain burns off most of the plants. First to go are the trees and larger plants. Even as far away as South Africa, the effects can be seen. Plants lie dead where once they grew. They rot and decompose. Dicynodon creeps about, looking for some palatable morsels, but finds very little – just some mushrooms, mosses, ferns and club-mosses nestling in damp crevices around the river banks.
Then comes a distant rumbling, unheard in South Africa, but the coup de grace nonetheless. Since the eruptions began, some 10,000 years earlier, the atmosphere and sea surface have warmed by two or three degrees and the frozen northern polar region begins to melt around the fringes. The Polar Regions in the Permian were much smaller than they are today, with limited ice caps. But frozen tundra extends hundreds of kilometers away from the poles, and the ocean margins are frozen too. A great icy mass of gas hydrate, locked in the sediments at the margin of the polar sea, is warmed by a degree or two, and it suddenly gives way. First a few bubbles, then many, and finally a huge expansion of gas – 160 times the original volume. What was once frozen and at high pressure, becomes gas at normal temperatures and pressures, and a vast volume of methane and carbon dioxide bursts upwards through the oceans and shoots out into the atmosphere, raising spouts of sea water hundreds of meters into the sky.
The addition of millions of cubic kilometers of carbon dioxide into the atmosphere, even though it is happening near the North Pole, affects the whole Earth. In the course of a few days, Dicynodon, feebly searching for scraps among the stinking decay of plants in southern Africa, and already feeling a rise in temperature of two or three degrees, is now hit by a further devastating blow. Carbon dioxide is driving the normal levels of oxygen downwards – he is gasping for air. And, day-by-day, the asphyxiation becomes worse.
After a week, heavier rains come. The monsoon has begun. The rain is still acidic, although much of the acid has now been washed out of the system. And the rain carries away all the stinking vegetation down the slopes, into the rapidly filling wadis. Jostling tree trunks, branches and mats of leaves rush down to the sea, where they are dumped a few kilometers offshore, at the end of the estuarine tracts. But most of the soil is washed away too. Without the binding roots of the plants, the soil is vulnerable. After a few days, there is almost no earth left, just bare rock, with pockets of soil clinging on in hollows where mosses, ferns and club-mosses have survived. The countless billions of tonnes of organic carbon locked into the plants and the soil across the whole Karoo have been stripped into the sea. Almost nothing remains. With the soil went the worms, spiders, centipedes, flies, beetles and everything that could not hang on to the rocks and the rushing torrents of water.
Carbon dioxide levels in the atmosphere are still higher than normal. And there are no negative feedback processes. Normally, the carbon dioxide would be removed from the atmosphere by photosynthesis, and the monsoonal rains would have been followed by a dramatic greening of the land. Dried-up trees would spring into life, producing leaves from their gnarled branches. The bare earth would miraculously sprout low ferns and seedferns, as dormant seeds broke into life. But the soil has gone, the land is just naked rock. Nothing like this had happened since the Precambrian, some 300 million years before, when life on land had not yet evolved.
Dicynodon follows the water downhill to the sea, half-starved now, having gone without food for more than a week. Mushrooms, which seem to be all that can flourish, are the only thing available, and they are far from his preferred diet. His instincts suggest there will be food where the water is. But he is wrong. Everything is topsy-turvy in this apocalyptic world. Just as the plants on land were killed by the acid rain, so too the seaweeds that fringed the shores. The increased carbon dioxide levels in the atmosphere have penetrated the top dozens of meters of the sea, and the plankton has been decimated. Within a week, all the shoals of fishes that used to rely on the plankton as their staple diet have starved too; then on up through the food chain: the sharks and larger fishes that fed on the planktivorous smaller fishes die too. The whole sea is poisoned. Rising temperatures have imposed anoxia.
Dicynodon, and all the other animals – the closely related smaller and larger dicynodonts, the bulky, knobbly plant-eating pareiasaurs, the small scuttling procolophonids, therocephalians, millerettids, and the large, sabre-toothed gorgonopsians – are close to death. They wander about on a blank, rocky landscape. They have difficulty in breathing, and their backs are burnt at midday by the hotter-than-normal sun.
Then comes the third eruption. It is not by itself a particularly huge eruption. But it leads to a further cycle of acid rain. Temperatures rise again by a further fraction of a degree. Another vast bubble of methane and carbon dioxide is released in the far north from a frozen gas hydrate deposit. Dicynodon is now living through a runaway greenhouse effect. Nothing can turn back the devastating rise both in carbon dioxide and in temperature. He curls up and dies, along with nearly every other living thing on land.
In the sea, the vast influx of organic matter from the land – all the dead plants, animal carcasses and soil – have carpeted the seabed in a stinking, black slime. The decaying organic matter consumes oxygen and gives off hydrogen sulphide. Seafloor life – all the reefs and their denizens, as well as the creeping worms and arthropods, and the burrowing mollusks and shrimps – die. Their carcasses are incorporated into the fetid, black, anoxic bottom slime. Stripped of oxygen from the atmosphere above, and with the black mud below, the oceans go into a spiral of anoxia: oxygen levels fall, step-by-step, until almost nothing survives. Only a few worms, brachiopods and mollusks that can exist at depth in oxygen-poor conditions manage to live through these harsh conditions.
The eruptions continue, at random intervals, to pulse basalt lavas over Siberia. Hundreds of meters of fresh rock accumulate. Sometimes eruptions are separated by days or weeks; at other times, there may be a standstill for a few thousand years, and the rare surviving plants and animals manage to re-establish themselves for a short time, before a further cycle of devastation begins. Some of the eruptions are small and have limited global effects, of course, but others are large enough to lead to the global effects just described. Some day, with ever-more precise study, it may be possible to tease apart some of the detail of the separate phases of the eruption of the Siberian Traps, and whether the killing happened all at the start of the eruption cycle, or whether the process was drawn out over half a million years.
Is this what really happened?
All the pieces of the cataclysm 151 million years ago have been put in place. Research in the past ten years has led to an astonishing, earthbound scenario for almost complete devastation of the Earth and of its inhabitants. The killing model makes sense in terms of what is now understood about eruptions and their effects on the atmosphere, about oxygen and carbon cycling in the earth-life system, about the composition of the oceans, and about gas hydrates. But much of this is very new work, and it might have to be modified in the future.
For many, there is a lingering desire for something more apocalyptic, more instantaneous, some deus ex machina such as a huge extraterrestrial impact. Surely, they argue, if the KT event, when 50% of species disappeared, required a vast meteorite, we need something even more catastrophic to kill off 90% or more of species? But the evidence for impact at the Permo-Triassic boundary is limited at the moment. That might all change of course any day, if the helium/fullerene story is confirmed, if a crater of suitable age is discovered, or if large amounts of shocked quartz and iridium turn up in rock successions in different parts of the world.
However, I’ll bet on the Siberian Traps coupled with gas hydrates for the moment.
What was so special about the Siberian Traps, and the whole end- Permian scene, that could have allowed the crisis to develop? The Siberian Traps were not the biggest flood basalts of all time. Much larger flood basalt volumes were erupted over the Central Atlantic, Java, the Caribbean-Colombian area and the Brito-Arctic province, 200, 120, 90 and 60 million years ago respectively. But these four were associated with generic extinction rates of only 20-30%, notably at the higher end of the range, but hardly devastating, and certainly not on a scale with the end-Permian loss of genera.
Maybe it was simply a coincidence of factors. The end-Permian was the only time when the continents were fully assembled into a single supercontinent and when there was a large flood basalt eruption episode. During the later large-scale basalt eruptions, the continents had drifted apart and maybe life had diversified sufficiently in the different continents and oceans to be able to resist a range of severe climatic changes. One almost certainly has to add the coincidence of a massive methane burp, although there is no independent evidence yet for this (apart from the difficulty of otherwise explaining the remarkably large and rapid negative carbon isotope shift). Thorough testing of all the hypotheses by Robert Berner of Yale University has shown that massive methane release has to be the main cause of the dramatic carbon shift, with lesser contributions from volcanic degassing and mass mortality. This isn’t proof, but he ran all the possible causes against his well-established climatic models.
Much more has yet to be found out about the end-Permian crisis. Geologists and palaeontologists are far from understanding step-by-step just what happened. But, as we have seen, ideas have sharpened and focused remarkably since 1995, and will doubtless continue to do so. One thing is clear, however. The biggest mass extinction of all time did happen 251 million years ago, and even if we cannot yet fully explain why, it is important to look at the consequences of cutting life down to 10% or less of its normal diversity. There are lessons to be learnt.
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In 1992, Al Gore, then the Vice-President of the United States of America, wrote,
… it occurred to me that … we are causing 100 extinctions each day – and many scientists believe we are …’
This is a startling figure, and the prediction resulting from the calculation quoted by Gore is that all of life will be extinct in 400 to 800 years. Do we believe this? What is the scientific basis for such dramatic predictions? Or should we settle back comfortably with the extreme Bible-belt Americans who think that everything on Earth was created by God for the benefit of humanity, and therefore that anything done by human beings is by definition good?
Probably both positions are gross caricatures, and it would be sensible to be cautious. Al Gore based his statement on the reputable calculations of Paul and Anne Ehrlich in 1990 that perhaps 70-150 species are becoming extinct each day. Scaling this up to current estimates of total global diversity leads to the alarming prediction of how long life will last on the Earth. Perhaps the daily extinction estimate is too high, but such calculations always lead to startling conclusions about the total time to extinction of all life.
If human activities are truly causing such devastation, then we are certainly witnessing the sixth mass extinction (the other five being the geologically documented ‘big five’). In this case, a close understanding of the events of the past will shed light on what is happening now, and what may happen in the future. In particular, people often ask how long does it take for life to recover after the devastation of a mass extinction
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The ‘reef gap’ following the end-Permian extinction was one of the most profound pieces of evidence of major environmental crisis. The rich tropical reefs of the Late Permian had all gone, and nothing faintly resembling a reef was seen for 10 million years after the event. This can hardly be a result of poor study or collecting. Not a single coral specimen, a bryozoan or any other reef animal has been found. What were once huge structures, often tens or hundreds of kilometers across, and dominating many coastal strips, had gone entirely. When the first tentative reefs reassembled themselves in the Mid Triassic, they were composed of a motley selection of Permian survivors, a few species of bryozoans, stony algae and sponges.
There are various kinds of reefs. We think of structural reefs as typical – great walls of coral skeletons, often built up over millennia, and some-times tens of kilometers long. But the Mid Triassic reefs were modest affairs termed ‘patch reefs’, that is, low amalgamations of reef-like creatures forming a little cluster on the sea bed. The scleractinian corals were there, close relatives of the corals that abound in tropical seas today; they were diverse, but rare. It took another 10 million years before these corals had become relatively common, in the Late Triassic, and before reefs grew in size and complexity again. But they were still much smaller than in their heyday in the Late Permian.
Reefs show it, ammonoids show it and bivalves show it. The reptiles and plants too. One of the surprising recent discoveries about the Early Triassic recovery of life in the sea is that there was this apparent gap of 10 million years before recovery really began, in the Mid Triassic. Evolution was in effect suspended. Simplistically, one might have expected the recovery to begin at once. After all, the lands and seas had been stripped of life. Normally, plants and animals live in tight patterns of ecological harmony, kept in balance by day-to-day interactions such as competition and predation. If one species is pulled out of the system, the others will soon move in and take over. So why did life not begin to expand and diversify at once?
There are three suggestions. The first is that the delay is apparent, not real. For whatever reason, palaeontologists have simply failed to collect fossils in rocks of Early Triassic age, and there was no delay. Life was burgeoning and bursting to evolve, but the fossils either were not preserved, or they have been missed. This is always a hard argument to refute, since all the palaeontologist can say, perhaps rather plaintively or tetchily, is that he or she has looked damned hard, and there really is nothing there. Tens of thousands of person hours have been spent by competent palaeontologists, who seem to manage to find fossils in abundance elsewhere, poring through Early Triassic rocks. What do they find? Lystrosaurus and Claraia, and nothing else. The longer and harder they look, and as they continue to find nothing, one has perhaps finally to believe that the gap is real.
So if the gap is real, what was the problem? The two suggestions are that either conditions were so harsh that nothing could live, or that the end-Permian crisis had been so profound that it knocked out all normal ecological and evolutionary processes. Or maybe both factors were in operation.
There is little doubt that the earliest Triassic world was grim. Oceans worldwide were at a standstill, with anoxic waters widespread and deposition of black shales common. Pyrite was forming in these shales, and other chemical anomalies indicate that normal processes had ceased. The low-oxygen conditions of the deep sea floor suggest that normal oceanic circulation had stopped, or slowed down. Normally, there is mixing of deep, cold sea-bottom waters with the warmer surface waters. The process moves at a stately pace, and full mixing may take decades, but it happens.
If normal mixing stops, the nutrient cycling processes would be destroyed. Upwelling, cold, ocean-bottom waters cycle organic nutrients along certain coastal margins. The most famous example of this is along the west coast of South America, where the nutrient feast from the deep encourages huge shoals of fish (and huge herds of fishermen, human and avian). If organic matter were lying undisturbed on the sea bed – and it was, as witnessed by the black carbon-rich shales that were being laid down in the deep oceans and basins – there could have been no nutrient cycling by upwelling.
The oceanic anoxia expanded into shallow waters too. So shallow seas, normally teeming with life, also suffered low-oxygen conditions. This would have devastated all the groups that normally rely on abundant oxygen and nutrients, and while these horrific conditions continued they could not re-establish themselves. It has been estimated that anoxic conditions persisted in shallow sea waters for several hundred thousand, or perhaps a million years. The silent, anoxic ocean floor persisted for much longer, probably the full 10 million years of the cessation of evolution.
Conditions were no better on land. The low-oxygen conditions in the oceans affected the atmosphere as well, and evidence comes from the ancient soils laid down during this interval. The famous ‘coal gap’ of the Early and early Mid Triassic was a time of sparse vegetation. The extinction crisis had stripped the land of plant life, and erosion became rapid. The Early Triassic soils were sparse and showed a low diversity of plants that were specialized in surviving in hot, acidic conditions. After 10 million years, finally, in the Mid Triassic, the first thin coal seams are found. Coal indicates relatively lush, usually tropical-type vegetation. It was only in the Late Triassic, some 20 or 25- million years after the mass extinction, that thicker coal seams are found, something like those that were formed in the Late Permian.
The second proposal, that life was held in check by a breakdown in the normal rules of ecology and evolution, seems less likely Of course, for the initial thousands of years after the crisis, such effects could be imagined, but 10 million years is a long time for any group of organisms to remain in stasis. After the crisis, many of the surviving species would have been present in only small population sizes. Under normal conditions, each species has an effective population structure, in which individuals breed widely, and with movement between populations. This keeps up a good genetic ferment, and maintains a breadth of genetic possibilities for evolution, should that be required.
Low population sizes can lead to problems. The scope of genetic variation is severely limited. Indeed, whole swathes of the genome have been lost, and the survival of up to 10% of species into the earliest Triassic implies an even more severe curtailment of the overall number and variation of genes. So the rules of evolution were almost certainly different for a while, until population sizes of species built up again. It is hard to believe, however, that unnaturally low population sizes could have been maintained for 10 million years. The delay in recovery must have had more to do with the abysmal Early Triassic world.
As if that were not enough, there was a follow-up extinction event, towards the end of the Olenekian stage, some 2 or so million years after the end-Permian crisis. Ammonoids were again hit hard, almost disappearing
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about the size of a terrier dog, so it would have been within the dietary range of some of the predatory birds.
This sort of short-lived experiment is typical of recoveries. The first species to become established may have a good evolutionary run for their money. New species can proliferate at a faster than normal rate as vacated niches fill up, and as ecosystems reconstruct themselves. But some of the first-comers may not be able to cling on to their positions, if, for instance, they are not as well adapted to their roles as other species that come in and take over. In the end, the giant terror birds gave way in most places to mammalian predators, the ancestors of cats and dogs. So, during a post-extinction rebound there can be a phase of rapid niche-filling, and then comes the inevitable sorting and stabilization phase when many extinctions happen, and ecosystems readjust to a pattern that may then hold sway for many million years or more of relative stability.
Panic or complacency?
Lessons from the past can be read in two ways. The ecological activist would emphasize how human activities are destroying biodiversity and how this could turn into a cascade of death as species after species becomes extinct. As tropical forests are cleared and reefs are poisoned, we are losing not only species, but whole habitats. The palaeontological record of mass extinctions then makes grim comparisons. We know that after a mass extinction, life takes a long time to recover. The geologist may say that 10 million years is a short time, but measured in human lifetimes, it is effectively infinity.
Low levels of extinction can turn into high levels. Destroying species and habitats piecemeal might lead to a runaway crisis, as seems to have happened in the past. Once the world becomes locked into a spiral of downward decline, it is impossible to see how any intervention by humans could turn it back. It could be, for example, that removing one or two species from an ecosystem does little damage. The remaining species can adapt and plug the gaps. But if another few species are picked off, then another few, and then a few more, a point may be reached when that ecosystem will collapse. Better to stop destroying the environment before we become locked into such a catastrophic sequence of events. The natural world is complex, and consequences are often unpredictable.
Destruction of forests can kill ocean fish, for example. Plants take up carbon dioxide during photosynthesis and pump out oxygen. Animals require oxygen, but produce carbon dioxide as a waste product. There is a balance here, and that balance could be perturbed by destroying too much of the world’s forests. Cycling of carbon is important too, as dead plants and animals are incorporated into the soil, as organic carbon builds up in the bottoms of lakes or is washed into the sea. Nutrients from these sources then circulate in the oceans, providing sustenance for fishes.
A political conservative could also claim justification from the fossil record. Such a person would note that life has always bounced back, even from a mass extinction as profound as the end-Permian event. Evidently, each species locks up a huge evolutionary potential in its genes and given the chance to explore the extent of its full capabilities, most species seem to be able to proliferate and expand into new niches. Indeed, the conservative, warming to the theme, might suggest that species that have been killed by human intervention were obviously rather feeble, and a bit of extinction is good for the moral fiber. Who needs dodos and great auks anyway?
This conservative viewpoint has gained ground in some political circles. Bjorn Lomborg,9 a Danish statistician and one-time green campaigner, argues that world resources are not running out, that forest cover across the world has increased and that the world’s species are not disappearing at an alarming rate. His views have inevitably been greeted with outrage by many, who claim that he has selected narrow definitions of natural phenomena in order to make his case: farmed, temperate-climate tree nurseries differ from ancient, complex tropical forests. It is startling none the less that it is still difficult to make definitive and universally convincing statements about the state of the natural world today.
Coming back to reality
Much as one might wish to accept such reassuring claims, they are too complacent. Of course some life will survive human depredations. It may be cockroaches or rats, but to claim that humans cannot drive all life to extinction is hardly cause for congratulation.
There are lessons to be learnt from the past. Human activities have done more than eliminate just one or two species here and there. The first Maoris in New Zealand killed all the moas, some 13 or more species of impressive, large, flightless birds, thus eliminating an entire family of birds, the Dinornithidae, sometime before 1770. The Maoris also killed off other, smaller families of native birds. Similarly, after Europeans arrived on the Hawaiian islands in 1788, 18 bird species disappeared, and another 12 species may be extinct. Of 980 species of native Hawaiian plants, 84 have already been eliminated, and a further 133 have wild populations numbering fewer than 100 individuals.
The famous ‘red books’ of the International Union for the Conservation of Nature classify different levels of threat to present species. Thousands of species are listed as in danger of extinction, and the lists become longer and longer each time they are revised. Species under threat include much-loved forms such as pandas, tigers and blue whales. Millions of dollars are spent by governments and charities in order to try to con-serve such species. Special breeding programs in zoos help the effort, and sometimes – rarely – these huge efforts allow a species to come off the endangered list. But at what cost? We can eliminate a species in a moment, but conservation is expensive. And of course, while people will pay for the rescue of the panda, the California condor, even the Kerry slug, who will pay for the protection of the countless uninteresting beetles, bugs, scorpions, frogs, snakes and tropical plants that are just as close to extinction? And what of the threatened species we don’t even know about?
The extinction estimates quoted by Al Gore have a firm basis in fact. The only substantial reason to question them is that there may be levels of extinction resistance among species. What we have mentioned so far is the extinction, to a large extent, of extinction-prone forms, endemics restricted to single islands for example. If there is such a sliding scale, and we are busily eliminating the more precarious species, perhaps rates of extinction will decline as humans tackle the more recalcitrant species, the ones that just refuse to give in and die.
On the other hand, human populations are increasing exponentially. The time it takes for the human population to double keeps diminishing. Global populations rose from 100 million to 200 million between the time of Christ and ijoo. The 400 million mark was achieved by 1700,800 million by 1800,1600 million by 1900, 3200 million by 1980, and with current levels at nearly 6000 million (6 billion), the doubling time is down to 25- years. At today’s levels of human population, some 40% of global productivity has been sequestered for our benefit – including all humans and their domesticated plants and animals. This means that all other species have somehow to get by on only 60% of the oxygen and carbon (well it’s more than 60% since human and domestic waste goes into the ‘wild’ systems) that they had available to them in the days of Julius Caesar.
And even though the exponential rise in global human population is damped, or slowed down, by famines and wars, the rate continues to go up. This brings a pressure that might cancel out any tendency to reduction of current extinction rates. So, argue many, the debate about extinction-prone and extinction-resistant species is irrelevant. They’re all going to go anyway, as wealthy nations pump pollutants into the atmosphere, and poorer peoples replace natural habitats with poor-quality farm land. There is no room for complacency.
Ironically, extinctions in the distant past are better understood than the current crisis. Reversing that well-worn maxim, it may be that ‘the past is the key to the present’. Normally, geologists and paleontologists bow humbly before scientists who work on modern phenomena. To understand how rivers worked in the Permo-Triassic of the Karoo Basin, the geologist seeks advice from geographers and geomorphologists who study modern river systems. To understand what Archaeopteryx looked like, the paleontologist consults an ornithologist. In extinction studies, paleontology has (some of) the answers.
As we saw earlier, biologists have failed to estimate current biodiversity, with estimates ranging from j to 100 million species. Biologists have also failed to estimate current extinction rates, and there are robust debates around the figures quoted at the beginning of the chapter by Al Gore. Paleontologists, on the other hand, can give good estimates of extinction rates, certainly at family and generic levels, and they have relatively reliable ways of turning those into estimates of species extinction rates. So we know how profound the end-Permian crisis was and the scale of the KT event. We know also how long the recovery took, since time-scales were long.
Paleontologists, of course, become more hesitant when pressed about how long any particular extinction crisis lasted. Their weakness is short
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I have tried to show in this book, by weaving history and science, how arguments are often re-run generation after generation. Scientists are evidently human too. They can be prejudiced, they can be scared or constrained. Catastrophic extinctions in the geological past is a beautiful example of an idea that was presented in the 1820s, that was firmly crushed by Lyell, and could only raise its dangerous head again in the 1980s. It took years for geologists to dare to accept the obvious, that there truly have been mass extinctions in the past and that structures on the Earth’s surface that look like impact craters actually are impact craters.
To have lived through the tail-end of this switch-over has been fascinating. I was taught by anti-catastrophists, but I now preach asteroids and mass extinctions to my students. Even more rapid has been the accumulation of knowledge about the end-Permian event. Everything changed between 1992 and 2001. In that time, the event focused down from 10 million years to a few thousand years. The Siberian Traps flowed into view as the main culprit. Gas hydrates, undreamt of before the1970s, suddenly became the answer to abrupt climate changes in the past, and perhaps crucial as part of a runaway greenhouse model for the end-Permian environmental crash. Long drawn-out post-apocalyptic anoxia is every-where in the Early Triassic.
Such rapid accumulation of knowledge and ideas has its risks. This book may thereby have a short shelf-life as scientists disprove all the wild-eyed theories that were published in the last years of the twentieth century. Fin desiede excess, they will claim. Perhaps not, though.
As one grows older, one realizes how little one knows: ‘the more you learn, the more ignorant you become’. The joy of being a scientist is to discover this. When I was beginning my career, I felt that scientific research was a line of work that led to ever greater complexity. As one accumulated information about how the Earth works, all the simple questions would be answered. Then the questions would have to become more intricate and harder to solve.
But the unanswered questions are as big and as simple as you could wish for (although the answers may be so intricate as to be unattainable). How diverse is life? How does the world react to human intervention?