The truly safe level would be the pre-industrial level before we started fossil-fuel emissions, when it was within the natural variation range. Carbon dioxide comprises about 387 parts per million by volume (ppmv). The level in 1832 was 284 ppmv, which is roughly the limit of normal levels (200-280 ppmv from ice age to interglacial maximum), which is 73.4% of the current concentration..
There is a total of 5,000,000 gigatonnes (5×10^15 t) of air, and carbon dioxide makes up about 0.046% of it by weight, or 2300 Gt. Now, assuming the same air density, 73.4% of that is 1700 Gt. That’s a difference of 600 Gt, or 600,000,000,000 tons. Impressive amount, I’d say.
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