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The prehistoric CO2 low during the past ice Age was 180 ppm. This is only 30 ppm above the CO2 level (150 ppm) where C3 plants, which make up 95% of all plant species and 100% of all marine plants, are unable to grow. So during the past ice Age we can very close to a total collapse of all ecosystems that are dependent upon C3 plants. The overall long term trend with CO2 levels has been downward, starting at 7000 ppm during the Cambrian era as a portion of the carbon captured by plants doesn’t get recycled back into the atmosphere, but instead is permanently stored in the ground in the form of limestone, coal, and oil. In the absence of human activity, it is likely that during the next ice Age, CO2 levels would drop below the 150 ppm that would result in the death of all C3 plants and the animals that depend on them. So, looking in the long term, human releases of CO2 into the atmosphere is a good thing for the long term health of the biosphere, putting off the day when the earth’s atmosphere no longer holds enough CO2 to support photosynthesis.
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