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C02 550 million years ago was significantly higher than today. But it was around this period of time that life flourished, and the Cambrian explosion occured. So, C02 could actually be an extremely good thing for Terra's biosphere. Even the not so optimistic Rothmann model does not show a significant increase in C02 during the Holocene era.
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Of course CO2 levels have been higher. But that was 550 million years ago. Life was adapted to such conditions, something that life is not adapted to now. Therein lies the problem. The rate that anthropogenic global warming is and will increase is much higher than it ever has in the past. Ever noticed that every diversity explosion was preceeded by a mass extinction. In the past increases in atmospheric CO2 were relatively slow.
The carbon we put back in the atmosphere now was locked away since the Carboniferous, when trees in massive forests fell into swamps and became fossilised as coal. Oil comes from sea creatures, in which the carbon moved up the food chain before the dead were buried and compressed.
This is millions of years worth of CO2 and it is suddenly being returned to the atmosphere. It is true that years ago the Earth was much warmer, the Cenozoic is far cooler. We are actually in an Ice Age now, no other period had such large ice caps, and the temperatures cycle between full blown ice age and milder periods such as now. That is what life on earth is adapted to. Sudden increases in temperature will lead to mass extinction, and we are part of the ecosystem, whether we like it or not. We have evolved to prey on many of the planets creatures, and if they go, we go.
Even if warming triggers crash cooling, our society may not survive. We survived before, but as "cavemen". We live very differently now, and we each specialise in individual skills that require everyone else to be of use. Few of us could survive in the wild.
Cooling would be caused by the cessation of the Gulf stream. The northern hemisphere owes its unusually warm climate to warm water flowing from the Carrribean and Gulf of Mexico north to America and western europe. Decreasing salinity due to the melting ice cap can stop the stream completely, resulting in rapid cooling. Studies show that is is already weakening.