| CO2: Past and Future |
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| q | CO2 levels were last at the |
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| 2xCO2 value near 7 Myr ago. |
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| q | CO2 levels were last at the |
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| 4xCO2 value at least before 50 |
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| Myr ago and possible since the |
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| Cretaceous greenhouse world of |
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| 100 Myr ago. |
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