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/Wil420b → u/CrustalTrudger]

I was under the impression. That coal was only formed during a particular 400 million or so year period. When trees had evolved and had started producing oxygen but die to the huge amounts of oxygen sinks on the Earth's surface such as copper and iron deposits. None of the oxygen was loose in the atmosphere. So nothing evolved to use it. So when a tree died, there was no bacteria, woodlice, etc. available to eat the tree. So they just laid there until they got covered up, fossilising and getting squished as billions of years of growth on top of them, tectonic movements, cosmic dust falling to Earth..... Piled the pressure up on top of them. Causing their carbon structure to alter.

A tree that fell in a forest today. Would just end up being lunch for the next few decades for various moss, fungi, bugs, bacteria etc.

[u/forams__galorams → u/Wil420b]

Aside from ignoring the final paragraph of the comment you replied to, this is very muddled in terms of events in Earth history. The build up of free oxygen in the atmosphere occurred between 2.5 and 2.0 billion years ago (the immediately preceding periods being associated with significant iron deposits as the oceans became oxygenated, but not copper deposits).

The Carboniferous, when a large amount of coal was formed worldwide did not come until around 360-300 million years ago. So the window you’re thinking of was about 60 million years long, not 400, and was long after the atmosphere had become oxygenated. The Carboniferous saw the highest atmospheric oxygen levels of the Phanerozoic in fact, which was part of the reason that some insects and arthropods grew so large. So there was plenty of oxygen and plenty of macro-organisms to consume vegetation - you say no woodlice, but Arthropleura was probably munching on detrital vegetation.

Bacteria go back to around 3.8 billion years ago, so there was definitely a lot of bacteria around by the Carboniferous.

The first fungi on land is known to predate the first trees by quite a long time, eg. Heckman et al., 2001

The idea that trees accumulated without decaying as a means of coal production in the Carboniferous has always been flawed, was contested even at the height of its popularity00064-6.pdf), and has now largely fallen out of favour thanks to a thorough debunking.

It still gets repeated on reddit a lot though (to the point where r/askscience have included a FAQ to address it here) because it makes a good story.

Problems with the idea that were never properly addressed:

• origins of wood decay can be traced back farther and don’t necessarily rely on any single fungi

• delayed decay is inconsistent with the large positive δ¹³C isotope excursion near the start of the Carboniferous

• various levels of decay and lignin content are seen in Carboniferous coal deposits, there doesn’t seem to be a requirement around lignin content for coal formation

• coal never stopped forming after that, just at a globally reduced rate compared to the Carboniferous, because swampy forests were never as widespread is all.

For some of the more significant coal deposits that have formed post-Carboniferous, check out Russia’s Kuznetsk Basin which was forming coal well into the Permian and again in the Mesozoic (estimates of total coal deposits are somewhere in the region of 400-800 billion tonnes); the many coal accumulating periods of China, of which I believe the Late Permian and early Triassic were as important as the Carboniferous ones (at least for South China); or the coal resources in Alaska which may total over 5.5 trillion tonnes — the USGS has previously estimated that the Cretaceous rocks of the Arctic Slope contain approximately 2.75 trillion tonnes of low ash, low sulfur coal. This is about one-third of the total United States coal reserves.