uncleO writes "The current issue of Scientific American has an interesting article, Impact from the Deep, about the possible causes for the five major global extinctions. It contends that only the most recent one was caused by a 'dinosaur killer' asteroid impact. Evidence suggests that the others were caused by 'great bubbles of toxic H2S gas erupting into the atmosphere' from the oceans due to anoxia." From the article: "The so-called thermal extinction at the end of the Paleocene began when atmospheric CO2 was just under 1,000 parts per million (ppm). At the end of the Triassic, CO2 was just above 1,000 ppm. Today with CO2 around 385 ppm...climbing at an annual rate of 2 ppm...to 3 ppm, levels could approach 900 ppm by the end of the next century."
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