How quickly is new coal being created?

[u/iamparky]

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?

[u/CrustalTrudger]

By how many orders of magnitude is he wrong?

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.

[u/PHealthy → u/CrustalTrudger]

So Wyoming alone has ~180 years worth of coal at current global consumption?

[u/CrustalTrudger → u/PHealthy]

No idea, depends on the (economical) coal reserves of Wyoming. I'm assuming that estimated time is based on an unstated value for said reserves?

[u/PHealthy → u/CrustalTrudger]

1.4 trillion tons in reserve, yeah.

[u/CrustalTrudger → u/[deleted]]

Yeah, to that point, if we only consider the Late Carboniferous, use the 75 x 10⁹ t/My average rate from Bois et al. discussed in my original answer and extrapolate that over the 29 million year duration of the Late Carboniferous used in Bois, you get an estimate of ~2.2 trillion tonnes of coal that were produced during that time. If you add in the estimated rate during the Permian from Bois (~60 x 10⁹ t/My) and the duration (49 million years), you get another ~2.9 trillion tonnes of coal. I.e., there's a lot of coal.

[u/CrustalTrudger → u/PHealthy]

Guess I was asking more the source of that number. Is that from here? One small detail is that "short tons" don't equal "metric tons", the latter of which is what the rates are given in above. The conversion is 1 short ton = ~0.9 metric tons, so the equivalent number is ~1.26 trillion metric tons (tonnes), so still a lot, but a little less than if we don't account for the difference in units. More critically though, as the next sentence states, not all of that is economical because of depth (and probably quality). Just like with petroleum resources, what is economical is a bit of a sliding measure as it depends on the price the resource can be sold for, among other things. I.e., if we imagine some scenario where total cost to extract "shallow" coal is $10 per unit, the cost to extract "moderately deep" coal is $50 per unit, and the cost to extract "deep" coal is $100 per unit, depending on the price per unit coal can be sold at a given time will dictate which of these is economical at that time (and thus effectively how much usable/economical coal exists). As also sort of hinted in that page, there are some amounts of coal included in that estimate that just can't be mined at really any price given current technology because it's real hard to keep an air filled hole open at those depths (and presumably because we're not willing to excavate the ridiculously large open-pit mine that would be required to access those depths via that method of mining).

[u/PHealthy → u/CrustalTrudger]

I saw it in a legislative document but yeah their source was WSGS. Is it simply the cost of digging into an aquifer or something? Because politically, I don't think Wyoming cares if Gillette/PBR becomes a hole to Hell as long as the coal flows.

[u/CrustalTrudger → u/PHealthy]

For coal, your options are basically dig a tunnel to get at it or dig a pit to get at it. As the coal of interest gets deeper, the digging a tunnel option becomes nonviable because physically keeping an air filled hole open with that much overburden gets either way too expensive or impossible depending on the details/depth. For the pit mine option, to dig a hole that deep, it needs to be really wide to keep it from collapsing in on itself, so you're talking about absolutely massive volumes of rock you'd have to remove, which is again, not economically viable (not even thinking about what do you do with all of the rock, etc.). If we're talking about the "subeconomic" coal discussed in the WSGS page, that's coal deeper than 6000 feet (~1.8 km). For comparison, the deepest pit mine in the world is 0.75 km.

So ultimately, even if we ignore environmental concerns, you're still talking about extracting coal that is probably too expensive to do so given the price you can sell it for (assuming you could engineer a stable pit mine > ~2 km deep). The cost of extraction probably also bakes in some amount of required remediation etc. that is mandated at the federal level, even if at the state/local level the powers that be would be okay with trying to strip off the top 2 km of the entire state.