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hanerozoic Carbon Dioxide.png - Wikimedia Commons
Faint young Sun paradox
The Last Time the Globe Warmed - YouTube - the
Paleocene–Eocene Thermal Maximum (PETM)
Anoxic event
Permian–Triassic extinction event
End-Permian extinction, which wiped out most of Earth's species, was instantaneous in geological time
The Earth has had more carbon dioxide in its atmosphere in its past, about 10 times as much in the Jurassic and 25 times as much in the Cambrian. However, their greenhouse effects produced temperatures close to present-day ones, at least if the occurrence of tropical forests is a good guide. I use that because it is hard to get lots of plant growth in a very hot and/or arid environment -- water either evaporates too fast or there is not much water to begin with.
The solution is a curious fact about the Sun. It is slowly getting brighter as helium accumulates in its core and makes it more difficult for light to get out of it - mostly X-rays at the core's temperatures. When the Sun started nuclear fusion, it was only 70% as bright as it is today, and it was hard for there to be much liquid water on the Earth's surface without a very strong greenhouse effect. This is the "faint young Sun paradox".
So as the Sun gets brighter and brighter, it slowly pushes the silicate-carbonate mechanism into making less and less CO2 in the atmosphere, and in a billion years or thereabouts, the CO2 level may be too low to support most plant life. This also means that it took more CO2 in the past to produce temperatures much like present-day temperatures. Which is what we find.
But there were events in the past with CO2 well in excess of what the silicate-carbonate thermostat tends to maintain. Like the Paleocene-Eocene Thermal Maximum (PETM) of about 55.5 million years ago. It had a big injection of CO2 into the atmosphere that raised temperatures by about 5 - 8 C. It made nice temperate forests in the Arctic - and 36 C temperatures in equatorial oceans. Not surprisingly, it had a mass extinction.
There were somewhat similar events earlier - oceanic anoxic events. A warmed ocean dissolves less O2, meaning less decomposition, meaning deposits of undecayed organisms in sediments - mostly algae. This results in black shales, and baked black shales make petroleum and natural gas. So global warming is a likely source of our oil and gas deposits. OAE's also produce mass extinctions.
Then the Permo-Triassic mass-extinction event. It was huge; it is the biggest-known mass-extinction event, even bigger than the K-Pg event, the killer of the (non-avian) dinosaurs. It drove something like 96% of marine species and 70% of land-vertebrate species into extinction. Major groups were driven into extinction, like trilobites and eurypterids (sea scorpions), or else got drastically reduced in diversity without ever recovering, like brachiopods and crinoids (sea lilies). Brachiopods look superficially like bivalves - very superficially. They were abundant over much of the Paleozoic, but afterward, it was bivalves that became abundant.
A curious consequence of the mass extinction was that rivers in South Africa's Karoo Basin shifted from meandering to braided, consistent with much of the ground cover being lost as a result of that mass extinction.
The P-Tr event had a big injection of CO2 into the atmosphere, and also an 8 C temperature rise.
So we have big injections of CO2 and big temperature rises in the PETM, the P-Tr event, and likely OAE's. Injecting CO2 into the atmosphere is what we are doing right now with fossil fuels.
hanerozoic Carbon Dioxide.png - Wikimedia Commons