A Blog by Robert Krulwich

The Fantastically Strange Origin of Most Coal on Earth

This is a story about trees—very, very strange looking trees—and some microbes that failed to show up on time. Their non-appearance happened more than 300 million years ago, and what they didn’t do, or rather what happened because they weren’t there, shapes your life and mine.

All you have to do is walk the streets of Beijing or New Delhi or Mexico City: If there’s a smog-laden sky (and there usually is), all that dust blotting out the sun is there because of this story I’m going to tell.

It begins, appropriately enough, in an ancient forest …

Artist's reconstruction of a forest during the Carboniferous period. From 'Science for All' by Robert Brown (London, c1880). Illustration by World History Archive, Alamy
Artist’s reconstruction of a forest during the Carboniferous period. From ‘Science for All’ by Robert Brown (London, c1880). Illustration by World History Archive, Alamy

… whose trees “would appear fantastic to us in their strangeness,” write Peter Ward and Joseph Kirschvink in their book A New History of Life.

Some of them were giants: 160 feet tall, with delicate fernlike leaves that sat on top of pencil-thin trunks. This was the age when plants were evolving, climbing higher and higher, using cellulose and a tough fiber called lignin to stay upright. Had you been there, you would have felt mouse-sized.

Drawing by Robert Krulwich
Drawing by Robert Krulwich

These trees weren’t just odd looking. “One of their strangest traits was their very shallow root system,” write Ward and Kirschvink. “They grew tall and fell over quite easily.”

Drawing by Robert Krulwich
Drawing by Robert Krulwich

So imagine, then, these stands of towering, fernlike plants mostly growing in swamps. The air is warm and moist, and the land (Europe, the Americas, and Africa were at the time one continuous mass) is covered by millions—no, billions—of trees that are sucking carbon from the air, growing, aging, dying, falling, and releasing oxygen. This is a world littered with dead trees piling on top of each other.

Carboniferous Forest Diorama. Photograph by John Weinstein, Field Museum Library, Getty
Carboniferous Forest Diorama. Photograph by John Weinstein, Field Museum Library, Getty

But when those trees died, the bacteria, fungi, and other microbes that today would have chewed the dead wood into smaller and smaller bits were missing, or as Ward and Kirschvink put it, they “were not yet present.”

Where Are They?

Bacteria existed, of course, but microbes that could ingest lignin and cellulose—the key wood-eaters—had yet to evolve. It’s a curious mismatch. Food to eat but no eaters to eat it. And so enormous loads of wood stayed whole. “Trees would fall and not decompose back,” write Ward and Kirschvink.

Instead, trunks and branches would fall on top of each other, and the weight of all that heavy wood would eventually compress those trees into peat and then, over time, into coal. Had those bacteria been around devouring wood, they’d have broken carbon bonds, releasing carbon and oxygen into the air, but instead the carbon stayed in the wood.

Artist's engraving of a carboniferous forest circa 1754. From The Universe by FA Pouchet (London, 1874). Photograph by UniversalImagesGroup, Getty
Artist’s engraving of a carboniferous forest circa 1754. From The Universe by FA Pouchet (London, 1874). Photograph by UniversalImagesGroup, Getty

We’re talking about a spectacular amount of carbon. Biochemist Nick Lane guesses that the rate of coal formation back then was 600 times the normal rate. Ward and Kirschvink say that 90 percent—yup, 90 percent!—of the coal we burn today (and the coal dust we see flying about Beijing and New Delhi) comes from that single geological period, the Carboniferous period.

That’s why it’s called “carboniferous”—because it produced so much carbon. “The Carboniferous period was the time of forest burial on a spectacular scale,” the writers say.

Take Off Your Helmets and Say Thank You

And therefore, in a just (and biologically aware) world coal miners everywhere would be doffing their helmets to salute the tardy arrival of those teeny earth creatures, the wood-eating bacteria. By not being there 350 million years ago, and by not arriving for another 60 million years, giant seams of black coal now warm us, light us, and muck up our atmosphere. Equal numbers of environmentalists might spend the day throwing darts at these little guys for showing up so late.

A coal miner plants explosives in a coal mine. Photograph by H. Mark Weidman Photography, Alamy
A coal miner plants explosives in a coal mine. Photograph by H. Mark Weidman Photography, Alamy

And Now … in Spectacular Magnification, Let Me Introduce …

But enough of me talking about them. It’s time for you to take a close—and I mean close—look at these amazing wood-eaters. They come in many forms, but I’m choosing microbes called Trichonympha because they’re so tiny, so squirmy, and so, well, crazily busy. They’re single-celled and can be found, yes, inside a termite gut. They look, says photographer Richard Howey (who studies them), like teardrops, or pears “wearing wigs.”

Here they are in this Nikon Small World award-winning video by Danielle Parsons and Wonder Science TV:

When I first saw this video, I was shocked by the commotion. I had thought wood-eaters would be mellow, sluggish, and, well, a little less clumped together. So I had questions. A web search brought me to Richard Howey in Wyoming, who has written about and photographed Trichonympha, and I asked him to take a look at the video so I could pepper him with questions. Which is what I did …

Me: Wow! This is crazy. So much motion!
Richard Howey: Yes, it looks almost like a game of bumper cars.
Me: So why are they so squished together?
RH: I’m not sure. I was really stunned [when you showed this to me]. It seems like Macy’s on Christmas Eve. [Pause.] I know they reproduce at an incredible rate.
Me: What do you mean? Are we watching them having sex?
RH: They might be [laughs]. Their reproductive process is incredibly complicated … [goes on to discuss mating types]
Me: But mostly they’re eating, right?
RH: Oh, definitely. You see those little white crystals jiggling around?
Me: Yeah, those shiny, stonelike things? What are those?
RH: Those are little cellulose bits; the termite has chewed and shredded the wood, and now these bits have reached its intestines. The microbes scoop them up …
Me: And once they get them inside?
RH: They produce a dissolving agent that’s going to reduce those bits to starches and sugars that the termite can eat.
Me: I like their little wiggly nose-like tops.
RH: Those aren’t noses.
Me: Well, heads then …
RH: Actually … They’re kind of like legs. They have little locomotive hairs, flagella, attached there, and that’s how they propel.
Me: It’s weird. It looks like they know where they’re going …
RH: That’s an illusion. I think they just … go.
Me: Why don’t they stop? Do they ever rest?
RH: No, those flagella are very motile—they keep moving and moving and eating and eating …
Me: That’s it?
RH: That’s what they do. Always.

And we should be oh-so-thankful they do it. Because of them, dead trees get recycled. Soil gets replenished. Smaller organisms get fed. And miners can mine—which is only to say: Sometimes very little creatures make a very big difference.

Editor’s Note: The image of coal featured in this post was updated for accuracy.

21 thoughts on “The Fantastically Strange Origin of Most Coal on Earth

  1. Wonderful and very entertaining article. I always enjoy Robert Krulwich.
    Unfortunately, the picture selected of the Belle Ayr Mine in Wyoming does not mine coal formed during the Carboniferous period 300+ million years ago. Rather, the coal in Wyoming that it mines is Tertiary age, or less than 66 million years old.
    More importantly, Nick Lane is not correct if he says that 90% of the coal burned today is Carboniferous. In fact, only about half the coal burned in the US is Carboniferous, and the other half is Tertiary Age. And much of coal mined in Indonesia, the largest producer of steam coal in the world, is Tertiary.
    So that begs the follow up question: if the lack of these microorganisms is why coal formed during the Carboniferous, when why did so much form during the Tertiary?

    1. Thanks so much for pointing out the discrepancy in the mining photo with the type of coal discussed in the post. The image has been removed from the post. As for the rest of your questions, I’ll have to let Robert speak to those. Thanks again, Bill!

  2. Another great article, Robert. A quick note from an animal nutritionist. As I understand it, lignin is not a fiber – at least not like cellulose, hemicellulose, and others are in the family of sugar-based polymers. Lignins are organic polymers generally constructed from benzene rings. They function rather like cement in plant tissues, holding cellulose sheets in plants together and giving them rigidity (think plant stems). Lignins are not digestible to animals, or to most microbes – and even to those that can disassemble it, its breakdown is a rather slow process. In modern herbivores, much effort is put into selecting plant parts that are the least lignified.

    I also wonder about the assertion that cellulolytic bacteria didn’t exist in the Carboniferous. That they were far less common and diverse then seems possible, but I don’t see how the evolution of herbivorous animals (including arthropods) could have proceeded far without the availability of those organisms. Given that herbivores did exist at that time (insects, followed by the expansion of terrestrial vertebrates) I wonder if there were other self-protective mechanisms at work in the plants of the time which contributed to their low rate of decay. To say the ancient trees did not decay because cellulolytic microbes were not present or sufficiently numerous is possible, but I suspect it may oversimplify the real case.

  3. I agree with the commenters that the explanation does not add up. I go back to the sentence in the closing paragraph, “Soil gets replenished”. These giant fern like trees must weigh several tons each. A single tree has to convert at the very least the same weight in soil material over its lifespan. What is soil composed of but broken down biological matter coming from dead plants, trees and animals. So these cellulose decomposing organism has to already exist to break down dead plants and trees. Thus allowing the creation of these gigantic fern forest.

    1. It’s not the case that a tree’s mass is equal to the amount of soil taken up and converted to tree matter. Much of the carbon in the tree (which eventually ends up as coal in the trees described here) comes from the air and water (which falls from the sky as rain). This is actually covered in another Krulwich piece on this notion as described by Richard Feynman. http://www.npr.org/sections/krulwich/2012/09/25/161753383/trees-come-from-out-of-the-air-says-nobel-laureate-richard-feynman-really

      I wouldn’t bother to correct you, except that this idea is critical to understanding why burning coal contributes to climate change. The trees described in the article pulled massive quantities of carbon from the air over a long period of time and sequestered it away underground back in the Carboniferous era.

      Now in the last 150 years, we’ve dug up millions of years worth of this sequestered carbon and, by burning coal (and other fossil fuels), released it back into the air. We’ve done this, in comparative geological time, all at once. The consequences of all that carbon dioxide thrust back into the air is an increased greenhouse effect and resultant climate change.

      So that’s why it’s important to understand that these trees pulled much the carbon from the air, rather than the soil.

      1. Mike is right. Trees don’t get their trunks, their branches, their bulk from the soil. Mostly, as Richard Feynman says, they “eat air” and, with sunshine (and photosynthesis), turn it into carbon, into wood, into roots and heavy stuff. Trees do need some minerals. But a tree is not displaced soil. It’s recycled sunshine.

        There’s a

  4. This will be shorter than the comments I lost by not putting in the damn robot stuff when I posted the comment. Peat is accumulating today in swamps like the okefenokee in Georgia and in arctic areas like northern Canada. This is ascribed to low oxygen and high acidity (tannic and other organic acid) conditions limiting biologic activity. Termites are oxygen breathers, so if their bacteria require the termite host, then they would not be source of plant material breakdown. Methanogenic bacteria would function, though they operate more slowly than aerobic bacteria. The accumulation of peat may simply be a mass balance problem; organic material may accumulate on the floors of relatively stagnant water ponds and lagoons faster than the bacteria able to function under the conditions there can digest it. The Carboniferous coal measures are swamp deposits formed in deltas and coastal swamps. As a previous commenter has noted, the large deposits in Wyoming and Colorado are Cretaceous in age and there are commercial lignite deposits in Mississippi that are Cenozoic in age, so the absence of organisms able to break down the plant matter because they haven’t evolved yet is not an explanation for the more recent coal.

    You are in error when you state that it is the weight of overlying plant matter that compresses the peat into coal. All weight matters (though buoyancy needs to be subtracted from the overlying material to get effective loading). Most of the loading is from stream sediments deposited on top of the former peat, compressing it and where deeply buried also resulting in increasing its temperature by the insulating effect of the overburden. The earth has a heat flux, called the thermal gradient, that matches the temperature at the ground surface and increases the temperature with depth. The deeper the burial, the more the hydrogen component of the peat is driven off, leaving the carbon behind. Peat goes to lignite with a few hundred feet of cover, as in the coastal plain of Mississippi, to bituminous coal with a few thousand feet of cover, to anthracite when in mountain building areas like the folded Appalachians of Pennsylvania, and to graphitic schist in the metamorphic areas of mountain building.

    You are incorrect about the digestion of plant matter releasing oxygen. I am not a biologist, so I don’t know the relative amounts of oxygen to the hydrogen and carbon components of living tissue, but the reason the bottom of swamps is anaerobic is because all the available oxygen is consumed by the decay of plant material. There is none left over to be released. It may go to water or to carbon dioxide. You will have to ask a biologist or look it up. I don’t have time to look this up myself. I have answered this just from memory, so I may be wrong in some details. You should verify my comments in appropriate sources.

    1. He is right about the digestion of plant matter releasing oxygen if he is referring to oxygen (and carbon and hydrogen) atoms, rather than molecular oxygen.

      ie the atoms of matter (in the form of carbon dioxide and water) the plant pulled out of the atmosphere/environment and locked into its wood, returned to the environment once released from the wood by the processes of decay (also in the form of carbon dioxide and water, mostly)

  5. The article could be interesting which isn’t as it messed up with reasoning of coal formation.
    And it content coal formation of particular geological time frame.

    National Geography may initiate a large article, different types of coals and the formation mechanism. These are available in web but separately, and most easy rrachong and reading are having serious flaws which could not be ignored. Like here in this article flaws is in reasoning how coal was compressed. It was not because trees of carboniferous era fall on one another….reason is sedimentation and weight of earth mass. Another reason why the reasoning is erroneous here, carboniferous trees were not densed wood like today, so impact of new fall to the older layers is not significant, and if the layers are formed in swamp or swallow region then almost no impact….
    Hope new management of National Geography would come up with large article contributed by multi-disciplinary scholars.

  6. Very enjoyable article, and thank you for the critical questions from Bill and Marc. I look forward to reading the replies.

    1. Bill Meister raises an interesting question: how much of the world’s coal is Carboniferous? Ward and Kirschvink (not Nick Ward) say it’s about 90 percent . Bill says, What about Tertiary coal? I’ve written W& K to ask where they got their numbers, but in the meantime, I think its safe to say that an overwhelming amount of black coal on this planet, (in the Eastern US, Europe, Canada, Russia, India (where its called Gondwana), Australia, Antarctica and South America and Africa, is, as W&K say, Carboniferous in origin. I can’t vouch for the 90%. That’s their percentage. And when they write me, I’ll report what they say. But from what I can tell, Bill, black coal is lopsidedly Carboniferous. You’re right that in the western USA and in some parts of Asia there are predominantly Teritary coal fields. That coal, I notice, is often called “Brown coal” and seems to have more moisture and less carbon content. But if it’s the good coal you want, the coal-iest coal, its overlwhelmingly Carboniferous.

      1. Please allow me to clarify my comments with regard to how much coal is Carboniferous. Although I don’t believe it is as high as 90%, nonetheless it is true that the majority of coal in the ground was formed during the Carboniferous; the amount of bituminous coal formed during this age is truly phenomenal and unprecedented before or after and represents at least 90% of the hard or black coal. But because it is generally cheaper to mine, younger brown coal of Tertiary or Cretaceous age, represents a very significant amount of the coal that is currently burned to produce electricity (50% or so in the US). The younger brown, or sub-bituminous coal is oftentimes low in sulfur which helps reduce sulfur dioxide emissions but since it is lower in heating content than bituminous coal more of it needs to be burned in order to generate the same amount of electricity.

        So Robert is correct that the Carboniferous age coal can be considered “good quality” due to higher heating content, in some ways the younger aged coal contributes more to the coal dust blowing around in Beijing or New Delhi simply because so much of it is burned worldwide.

  7. Another very important issue is that the Oxygen content of the atmosphere was very high in this time-period (all of those fast growing plants of course).

    Forest Fires were unstoppable with Oxygen levels exceeding 30% for most of the period. A signle Forest Fire could burn right across a continent and days and days of rain would be required to stop it. Much of the coal is, in fact, the burnt-up remnants of those fast growing forests.

    Another issue is that sea level changes were particularly rapid in the Carboniferous. Gondwana was right over the South Pole and was repeatedly glaciated over and then melted over realtrively quick periods. The ice age lasted 50M years. North America and Europe were equatorial so did well even in a glaciated world.

    But the rapid sea level changes repeatedly buried the forest remnants and burnt forest remnants

    1. The oxygen was high not only because the plants were growing rapidly, but also because the plants were not decomposing quickly. All the oxygen a plant releases in its lifetime is consumed if the plant’s organic (ie carbon) content is fully decomposed. That carbon is reacted back with the oxygen and converted back into carbon dioxide, directly or indirectly, during the processes of decay.

      But because the organic material in those trees was not decomposing quickly, and instead getting buried and sequestered away (and turned into coal), allowing the oxygen to stay in the air.

      If humans somehow managed to dig up and burn ALL of that carboniferous coal, we’d end up reducing the atmospheric carbon content to pre-carboniferous levels, which was probably below 10%. (This won’t ever happen because a lot of that sequestered carboniferous carbon is now in forms that are accessible to our mining)

  8. I am having trouble envisioning the soil the promoted millions of generations of these trees. The trees start falling down, not decomposing, but instead getting deeper and deeper. In fact, fabulously deep. In other words, the soil content gradually becomes radically different than how it started. It’s hard to understand how trees could stay viable with a soil littered miles deep with their undecomposed remains.

    1. I do not think the trees would be ENTIRELY undecomposed. There may not have been bacteria around capable of digesting cellulose and lignin, but trees are made of more than just those two polymers. Everything else would have decomposed normally, and the remaining cellulose and lignin would have been broken up mechanically over time into small chunks, and fires would also reduce some of it into forms that organisms could extract nutrients from.

  9. It would be interesting to imagine the world where the microorganisms had been present during the Carboniferous age. The trees wouldn’t have stacked up like they did but would have decomposed as they do now. This would not have tied up carbon in the tree trunks but the carbon, in the form of carbon dioxide. Would this not have led to higher levels of carbon dioxide later on. This could have led to a runaway greenhouse effect in the Permian and later ages. It could have doomed earth to a Venus-like condition and life would have ceased on Earth. Perhaps, we a owe a lot to those late-arriving microorganisms.

  10. This article mentions lignin-degrading microbes, and then describes the symbiotic bacteria found in the guts of termites. What it fails to mention is the fungi that currently degrade woody material. It is these fungi that were missing during the Carboniferous period. Interestingly, recent research has calculated the origin of fungal lignin degrading enzymes to happen sometime shortly after the carbiniferous, which agrees with the decreased production of coal due to degradation of woody biomass.

    Once again, the vital role of fungi in the ecosystem is overlooked.

    1. Ben Auxier proposes that maybe what was missing all those many years ago…were, well, maybe wood-degrading bacteria…but perhaps more importantly, wood-degrading fungi. Fungi are hugely important in the life (and death) of plants; they were present when plants first tiptoe out of the oceans onto the land; plants need them for minerals, and when plants die, fungi do, indeed, help degrade them back into soil. As it happens, over at Radiolab, we’re deeply, richly into fungi at the moment, so I just want to tell Ben, they may have been ignored here, but they will not be overlooked. Stay tuned.

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