Here’s a cool recent discovery – the reason the earth has so much coal is that it took fungi 60 million years to figure out how to digest lignin, the compound that gives wood its strength.  Before that the trees were free to coat the earth.  They just lay where they fell until they burned or were buried in sediments.  This was the Carboniferous Era, the time from about 360 to 300 million years ago when almost all coal was laid down.  There was just an enormous amount of plant life on land then.  The oxygen level hit 35% (vs 21% today), as the trees consumed CO₂ to build themselves and released O₂.   Invertebrates, which have to breathe through their shells, reached enormous size, meters long.   Don’t click on this if you’re arachnophobic.   Plants with roots and leaves first appeared about 390 My ago, so that marks the start of the era.

So what caused the end?   People have thought that it was climate change or a reconfiguring of the continents, or the evolution of new kinds of trees.   This fungal explanation appeared in 2012, as published in Science,  “The Paleozoic Origin of Enzymatic Lignin Decomposition Reconstructed from 31 Fungal Genomes” by Dmitrios Floudas of Clark University in Worcester, and 70 other authors at 25 other institutions.   This is not easy work!   A lay explanation from Scientific American can be found here.

White rot fungus eating oak wood

Lignin is the brown stuff in wood.  Most of the rest is relatively clear cellulose.  The lignin is what gets removed to make paper.  It’s a tough molecule that consists of long chains of carbon clusters with lots of cross links.   It repels water, and apparently originally evolved to form the surface of tubes to transport water throughout a plant. Then it became the main structural compound of woody plants.  Once plants could hold themselves off the ground, they raced upwards to capture as much sunlight as possible, since tall plants shade out short ones.

Nothing could eat this stuff until white rot fungi came along.   People still don’t know quite how the enzymes in it work, but they can track the genes that make them across a lot of different fungal species.  By looking at the number of random mutations in those genes, and knowing the rate at which those mutations accumulate, they can work backward to when the common ancestor to all those species began to diverge.   This happened about 290 million years ago.  Given the inaccuracies here, that’s pretty close to when the Carboniferous closed.

The trees of the Carboniferous weren’t like those today – they were mainly an extinct species called Lepidodendrales, or scale trees.    Here’s a fossil of one that I found near the coal town of Joggins, Nova Scotia:

That’s how common these trees were – you can find them lying by the side of the road 300 million years later.  They were huge, up to 30 m tall, with meter-thick trunks of mostly bark.  The bark was green and photo-synthesized, unlike modern trees.  Those little indents were actually pores on the trunk for taking in air and water.   They weren’t as strong as modern trees, but they didn’t need to be because white rot wasn’t around.  They went for maximum growth instead of defense from attackers.   That worked great until it didn’t.

A long time after this a vertebrate came along with an unusually developed nervous system.   It found that the remains of these trees could be digested by steam engines, and used for rapid locomotion and for the construction of artificial skins out of wool and cotton.  Thus began the Anti-Carboniferous, when all these ancient deposits began to be consumed.

There appears to be about 1100 billion tons of mine-able coal today, and a couple of trillion more tonnes that cannot be mined.   The number keeps going up as new deposits are discovered and mining techniques improve.  There’s no telling how much was laid down originally, since it can be destroyed by plate tectonics.    Compare that 4000 billion tonnes total to the 750 billion tons of carbon in the atmosphere, and the 600 billion that are in modern-day trees.  Those scale trees sure took out a lot of CO2.  Maybe the Earth would have been Venus were it not for these incompetent fungi.

Humanity has burned somewhere between 300 and 400 billion tons of coal since the early 19th century.   It’s hard to tell because there are only good worldwide statistics for recent decades.   The oil company BP has stats from 1981 to 2018 in this spreadsheet.  This probably covers the majority of the coal burned, since the world population is so much higher now and so much more industrialized than it was earlier.

According to BP, humanity has burned 216 billion tons of coal in the last four decades.  The breakdown by region looks like this:

China now consumes far more than anyone else, but India is coming on strongly. Both countries are ruining their air as a result. Both are still building coal plants, while Europe and the US have been shutting them down as uneconomic.

All this consumption is one piece of evidence that there have been no other industrial species on Earth before us. They would have mined this stuff too. We’ve consumed 20-30% of it in just two hundred years, and there won’t be much left after ten thousand. Anyone before us would have taken it all, along with all the readily available iron, copper, and gold.

The Anti-Carboniferous is going to be a lot shorter than the Carboniferous! For our sake it had better end in the next couple of decades. Let’s hope the trees can then put all this carbon back into the ground where it belongs.