> oil and coal are far more energy dense than wood
In addition, a block of wood is a lot more dense than an acre of forest, which is a lot more dense than the difference in carbon content between an acre of forest and an acre of charred burned forest.
While reading the article, I mentally substituted "a forest the area of Africa" with "an oil tank the size of Africa and as deep as a forest is tall", which is clearly unreasonable. And they mention "4.81 tons" in passing - a ton is a lot, and surely 4.81 tons is a huge volume of gas, but how much?
We need to run the numbers. Crude petroleum has a density of about 0.8 g/cc = 0.8 ton/m^3 and a carbon composition of 85%.
4.81 tons of carbon x (1 kg crude / 0.85 kg carbon) x (1 m^3 / 0.8 ton crude)
= 7 m^3 of crude. Looking it up, a typical tractor-trailer tanker truck might carry 40 m^3 of crude...so this can be stated as mankind burning a yearly volume of one truckload of fuel for every 6 acres in Africa, which is a lot more believable.
Going back to the oil-per-acre analogy, an acre is a little more than 4000 square meters, so (assuming it's all oil - ignoring coal, natural gas, and other CO2 producers like, I don't know, human-initiated forest fires) this is a film of oil over the continent of Africa 1.7mm deep. That's a lot more believable than imagining a towering petroleum forest.
I think the math is somewhat wrong. The original source link for that "4.81 metric tons per acre" number is broken, but it appears to be talking about tons of carbon, not CO2 (which is roughly 27% carbon by mass). The article underestimates the amount of CO2 released by forest fires by roughly a factor of 4, and therefore overestimates the required land area by the same factor.
It's still a pretty staggeringly huge amount of land, though.
This would have been my explanation too, but to my surprise, wood is only about 1/3 of the energy density of diesel fuel. I'm not sure why i expected it to be an order of magnitude difference.
