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"Each part per million by volume of $CO_2$ in the atmosphere represents approximately 2.13 gigatonnes of carbon". Per a_donda's comment,"During the carboniferous, atmospheric $CO_2$ sank from ~1500 to ~250ppm", so, about 2.7 trillion tonnes. "There are an estimated 1.1 trillion tonnes of proven coal reserves worldwide." The latter number does not include ...

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