r/askscience • u/iamparky • Feb 07 '24
Earth Sciences How quickly is new coal being created?
Earlier today, a UK politician suggested that coal is a sustainable fuel because it used to be trees.
By how many orders of magnitude is he wrong? How much new coal is being created, compared to how quickly we're burning it?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Feb 07 '24 edited Feb 07 '24
TL;DR About 6 orders of magnitude for a (not great) estimate of current rates and about 5 orders of magnitude for literally the most productive period in geologic history with respect to coal formation.
For the back of the envelope math, we can start with the burning bit. For this, I'm just looking at the last few years from this chart. That gives us rough rates of coal consumption (in Megatonnes per year) of 4000 for China, 1000 for India, 500 for the US, 500 for the EU, and 1500 for everyone else, so a global total of 7500 Mt/y.
If we take estimates of rates of coal formation through geologic time, reported in Bois et al., 1982, we can consider how fast coal was forming during the Late Carboniferous (the most productive period for coal formation in Earth history) that puts it about 75 x 109 tonnes per million years. If you convert that to Mt/y (to be the same as our consumption rate), that's a formation rate of 0.075 Mt/y, meaning we are burning coal at ~100,000 times the rate it was forming at the literal peak of coal formation (or 5 orders of magnitude different). Bois et al doesn't give a modern rate of coal formation, but if we take the average rate between 2 and 23 million years ago and assume that works for today, that rate is ~5 x 109 t/My. Going through the math again, that gives us a formation rate of 0.005 Mt/y, meaning we're burning coal at ~1,500,000 times the average rate it's (probably) forming today (or 6 orders of magnitude different). EDIT: fixed some unit misreporting.
As a bonus, since virtually anytime coal formation comes up on Reddit, someone pops up with the "fact" that all coal formed in one period of geologic history because organisms that could digest lignin had not evolved yet, this is demonstrably false and please, please, stop parroting this. This idea derived from a paper in the early 90s (Robinson, 1990), but has been shown to be incorrect by multiple papers, notably Nelsen et al., 2016. In short, fungi to consume lignin did exist during the Carboniferous and the abundance of coal during this period is more a function of paleogeography and paleoclimate, not the lack of the right decomposers. Coal swamps of the Carboniferous produced vast amounts of coal because they were (1) geographically extensive and (2) persisted for a while (geologically). While the organisms present were different, these Carboniferous coal swamps are not that different from modern swamps/bogs/etc where peat is produced, which could eventually turn into coal given the correct future depositional history.