Quite right! That’s the total in the biocarbon on *Earth*. I read it off my spreadsheet late at night. Glad someone checked the numbers! I’ll correct it accordingly. ]]>

40 exajoules per gigaton times 294,000 gigatons required, gives me 12 million exajoules, not 111,000.

Did I miss something here?

You stopped before you finished dividing the numbers you calculated. 12 million exajoules required divided by 675,000 exajoules per year would 17 years. But photosynthesis is about 2% efficient (under good conditions), so this is 900 years. Add in your assumption of only planting half the planet (and ignoring the fact that the half you picked has more than average sunlight), and I get a little under two thousand years by my figuring (twenty years with your original numbers).

(I’m only keeping the first one or two significant figures)