>>Fact is that none of the Earth global warming events known to us today were preceded by an increase of atmosphere CO2 content...
Stuff to think about.<<
That may not be true.
250 million years ago massive volcanic action may have raised the co2 levels and temerperature dramatically, that in turn started the melting of methane hydrates which are 23 times the greenhouse gas of CO2 and raised the temperature even more.
95% of all life on earth was destroyed!
As Dr Steven Chu, nobel prize winner in physics said: "we are boiling the planet".
Read how it may have happened below.
"What caused the long dying seems to a complex operation as to what happened. The root cause was the Siberian Traps. They belched lava and carbon dioxide over nearly a million years. The amount of lava they belched has no parallel. Not even the Deccan Traps come close. The important part was the CO2 though. It apparently was enough to warm the planet 5 degrees C above what it was already: which was already warmer than now by a considerable bit (a few degrees C).
From there, the oceans warmed. This had two effects. The oceans of the Permian seem to have been very stratified. That is to say that the oceans did not intermix from the bottom depths to the ocean surface. There were very distinct layers. Additionally, as the world warmed, the ice caps melted. This decreased the salinity of the top waters and made the stratification worse. This caused the bottom of the ocean to be anoxic (ie very little or no oxygen dissolved in the various layers). The warming made the anoxic conditions worse. Warm water holds less oxygen than does cold. Life in the water would be...less than comfortable. With the melting of the very extensive ice caps, the oceans would rise and the anoxic layers would be brought up to depths that would originally have been habitat for the Permian marine organisms (This is Hallam's regression-transgression-anoxia theory).
Then the warmed ocean unleashes a nasty surprise for the surface and atmosphere: we get methane hydrate burps. There is a large amount of methane sequestered in the ocean in the form of ices. When the water warms too much, the methane is released from the ice. This makes it way into the atmospehre. Methane is a much better greenhouse gas than carbon dioxide. This ended up causing a further 5 degree C atmospheric rise. This happened very quickly and helped to pummel the End-Permian lifeforms even more. If the simulations (PDF) of Jeff Kiehl and Christine Shields of NCAR hold up to the fossil record, this means that the methane in the atmosphere end up whacking the ozone and unleashes a lot of normally blocked ultraviolet light [here too] above and beyond the norm which helps kill off the terrrestrial ecosystems as well.
As if to add insult to injury, the hypoxic-anoxic oceans encouraged growth of anaerobic bacteria that produce lots of hydrogen sulfide. Hydrogen sulfide is nasty stuff and highly toxic. It would help kill even more in the oceans than even oxygen deprivation would. It would, futhermore, bubble out of the oceans and kill right and left anything that took a whif on land. However, to make things even worse, Kiehl et al state that it increases that length of time that the methane remains in the atmosphere. This increases the amount and heating and away we go.
In the end, what killed life at the end of the Permian? It was a negative feedback that in effect, even in such a hellaciously hot climate (72 C at the equator!), snowballed. If ever there was a time that we, the planet Earth, was dangerously close to tripping over the edge into a run away greenhouse, that would have been it. Not the Eocene. The mechanisms that regulate the CO2 content in the atmosphere broke down, got depressed, and nearly shot itself in the head; in effect nearly ending all life.
To me, based on all the evidence that's coming in about the PT Extinction, it seems that what happened is pretty settled. Details and refinements need to be done. We're a long ways from being able to close the book, say it's been written, and that we completely understand why life nearly died then. We are far closer and the alternative scenarios are, with the pace that the supporting science for the above theory is growing, falling further and further behind.
The new question, that probably ought to be asked sometime in the next decade is not why life died at the PT Extinction, but rather:
Why did it survive at all?
After all, if the carbon cycle choked and died, how did it get restarted? It looks like in the case of Venus, it didn't and that world died. Why did it not here? Perhaps a hint is in Erwin's book, Extinction, where he states that a colleague of his noticed that the fossils that people have been attributing to the 'fungal spike' looked a lot more like algae fossils to him. Perhaps the humblest of photosynethesizers saved us all. That's a bit of research for people in the future to determine though.
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