Many climate deniers suggest that humanity is too insignificant to impact the global climate. As a visualization to counter this claim, I thought of the following exercise.

How much coal, oil, and gas humanity has burned in our history?

This part should be straightforward -- I found this chart of global fossil fuel consumption as a starting point:

enter image description here

How much plant life does that represent?

Basically, how many trees/bushes/grasses does it take to form a ton of coal or a barrel of oil? At some point in history, the coal, oil, and gas we're digging up were plants in a forest, prairie, or swamp.

This Quora answer suggests that 10 tons of plant matter is required to form 1 kg of fossil fuel, but the link to the source is broken, and I'm not able to find any others.

How much land would all those plants cover?

Once we know how many tons of plant matter went in to all the fossil fuels we've burned, we could figure out how many acres of forest, prairie, or swamp that would equate to. This part probably requires a lot of assumptions (density of various ecosystems, etc.), but that's OK.

The result

I'm imagining that this is going to be a vast quantity of land. At least the size of Africa. Perhaps several continents. Once we've got that number, we can visualize it -- imagine burning down a forest the size of [Africa/Asia/some other recognizable land mass] -- now we can understand how humanity's consumption of fossil fuels might impact the climate.

  • This is an interesting question, but I'm not sure about the end goal of contextualizing the burning of fossil fuels. As one of the answers points out, coal has accumulated over time due to plants growing and dying repeatedly in the same area, so you require many earths worth of land (over time) to get one earth-worth of coal. But I don't think that implies that burning some fraction of that coal has a similar environmental impact to burning down all forests on the planet, does it? Commented Sep 27, 2019 at 15:54
  • @NuclearWang agreed, this is all very hypothetical. I'm assuming it's easier for most people to visualize a forest fire than a tree being converted to coal and burned in a furnace. The end result is similar -- carbon (and other chemicals) are released into the atmosphere.
    – LShaver
    Commented Sep 27, 2019 at 16:13
  • Technical/Scientific Point: coal as you state formed from plants. Petroleum is derived from animals (zooplankton) & possibly some algae (phytoplankton). Natural gas (methane), if associated with petroleum deposits (the main source of natural gas, also know as a conventional deposit of gas) is effectively slow pressured cooked petroleum. Lesser amounts of natural gas are formed in some coal deposits or shale deposits. So how many critters died to form 1 L of petroleum? ;-)
    – Fred
    Commented Dec 15, 2019 at 12:31

5 Answers 5


How coal was formed

Regarding coal, from trees, it's not simply a question of the area whether it was a continent-size landmass, but more of the temporal side of things -- how long a time period the land was covered.

Coal was laid down from plant matter during the Carboniferous period, which lasted 60 million (60,000,000) years. For most of that time the micro-organisms that existed were unable to break down the lignin which made up 70% of the plant matter. Effectively the trees didn't rot, they fell down, and more trees grew on top, then died and fell down and more trees grew on top, for millions of years. The carboniferous period ended when the microorganisms evolved the ability to consume the lignin and the coal laying stopped.

Three million generations of trees

So supposing it takes twenty years for 'a tree' to reach maturity, the carboniferous period is three million generations of trees. 'Tree' is a little inaccurate compared to what we currently think of as a tree. The plants were effectively a really tall moss, and it was 80% 'bark' compared to modern trees that are about 10% bark.

All land on earth

During the carboniferous period most of the land on planet was covered with trees, Africa, Asia, the Americas. But to somehow recreate all the coal, you need at least twenty years for a tree to grow, and because there were three million generations, you need three million times the amount of land available on earth's land masses.

Suppose we drained the oceans to create more land for more trees, then you'd only need one million earth's worth of land.

How many trees, how many earths?

Currently there are around three trillion tress on earth -- it's not unreasonable to think that during the carboniferous period when there was nothing else happening, there would be maybe double that, six trillion trees at any moment. Over a 60,000,000 year period, that would be a total of eighteen quintillion trees (eighteen with eighteen zeros after it).

My point is, to recreate the coal that we've consumed alone, ignoring the oil and gas, it's not a matter of planting a few trees, its more a case of planting quintillions of trees on hundreds of thousands of planets. 18 quintillion trees at a rate of 3 trillion per planet requires 6 million planets.

Even if my sums are a few zeros out, its a mind-boggling number of trees and amount of land.

What about the coal we've already used?

In terms of coal that we have already burnt, the World Coal Association figures estimate that the proven coal reserves left in the ground are enough to last another 150 years at current rates -- this is the coal that is easy to dig up with current mining technology. There might be vast quantities of coal located in inaccessible locations, like the bottom of the sea, or spread out too thinly to be cost-effective to mine. This isn't included in the proven reserves figure.

Looking at the Global Fossil Fuel consumption graph in the question, we could estimate that historic coal usage is about 75 year's worth at current consumption rate. At current usage rates, the sixty million years of laying down coal gave us around 225 years of coal that is economically accessible. We're roughly a third of the way through that.

From the earlier calculations, that's three quintillion trees spread out over only two million earth-sized landmasses.

  • Nice. I wonder if there are any estimates of what proportion of the trees growing during that time actually made it into coal? Presumably some fell down and rotted, releasing some portion of their carbon immediately. Are we looking at 90% of the carbon in these trees turning into coal? 2%? I don't think this substantially changes the reasoning of your answer - unless the percent turns out to be vanishingly small, 10^-10% or some such. It would just be nice to have an estimate for completeness. Commented Sep 27, 2019 at 13:02
  • It seems like if you could work out how many trees turned into coal, one further step would be to convert to the number of times you'd have to cover the earth's entire surface area with trees, then wait 20 years for them to grow
    – Chris H
    Commented Sep 27, 2019 at 14:33
  • This is helpful, thanks. However, I'm more interested in how much coal we've burned -- not how much is in the ground. Of the 3 million generations of trees covering the land, how many became coal which we've already burned?
    – LShaver
    Commented Sep 27, 2019 at 15:09
  • @LShaver the last last two paragraphs of the answer, imply that we have already burned about one hundred thousand earth-sized landmasses. Your thought that "Africa/Asia/some other recognizable land mass" is off be several orders of magnitude. Commented Sep 27, 2019 at 15:45

I don't think this question can have a very definite answer because of the enormous time involved. It takes time for mountains to break up into chemical rich dust, and it is really only on land that has risen out of the sea that things could start to grow at first. There may have been enormous tracts of desert for many eons, with plants gradually spreading and making more soil. Another thing to consider is the animal remains that go to make up the peat, especially where the soil was alluvial. It is really a question of how much life went into the making of coal and oil. Did the weight of life on Earth ever exceed that of today with our teeming human billions? I think it is impossible that it could have, because we are using fossilized life in order to swell our own, and this never happened in the past. So when fossilized fuel runs out there will be a huge drop in the weight of life on Earth. Unless of course we use alternative forms of energy. But this will not really work either because of the imbalance of ecology caused by this huge increase in the weight of human life, with its significant increase of heat.

  • 2
    Hello. Welcome to Sustainable Living. Can you add any citations to your answer that support doubting the coal-creation explanation and calculations in Christopher's answer? The explanation of the carboniferous period seems compelling and supported. Commented Oct 15, 2019 at 12:48

Where I live, forests reach a density of 100-200 cubic meters of growing stock per hectare. One cubic meter equals one tonne of carbon dioxide.

From 1870 to 2014, cumulative carbon emissions totaled about 545 GtC. This is about 2000 GtCO2.

So, you need 10-20 gigahectares of land. It is 100-200 million square kilometers.

Unfortunately, world forest area is only 39 million square kilometers.

Perhaps in more temperate regions the forests can reach a higher density. But even then, I'd say we have already used so much fossil fuels that the trees in the forests of the entire world can't contain an equivalent amount of carbon.

  • I found it interesting to compare your answer to that of user3540774. Due to different assumptions the final answer is different by a factor 100. I wonder which answer is more accurate.
    – THelper
    Commented Dec 13, 2019 at 8:53
  • @THelper I have reason to believe my answer is somewhat more accurate, as the other answer assumes 20 gigatonnes of coal is enough. If we assume coal is 100% carbon, the 20 gigatonnes of coal would be emitted in little over 2 years at the current rate of fossil fuel burning.
    – juhist
    Commented Dec 13, 2019 at 17:04

As a guide, mature sitka spruce forest in the UK has 500m3 of wood per hectare report;

This wood has a density of 450 kg/m3 density tables, to that makes 225 tonnes of wood per hectare. The wood is about 50% carbon, so that would convert to 110 tonnes of coal per hectare (the carbon content for wood is discussed by Lamlon in Biomass and Energy).

This doesn't necessarily convert to coal -- this answer addresses the question of how much forest would be needed to match the carbon content. The rate of conversion to coal depends on many factors .. and most (perhaps all?) of the coal we are burning was produced in conditions very different from today.

40,000 TWh of energy (the annual production from coal -- see global fossil fuel consumption) requires 20,000 Tg of coal (http://www.coaleducation.org/lessons/twe/ctele.htm), i.e. 20x10^15 g = 20x10^9 tonnes, which would be equivalent to 0.2 X 10^9 hectare = 2 x 10^6 km2.

That would be about one third of the area of the Amazon jungle (these numbers come with no guarantee, and the Amazon obviously has a different density to Sitka spruce plantations ... but it is interesting to see how you can arrive at an answer).

  • The carbon content for wood is discussed by Lamlon in Biomass and Energy. This doesn't necessarily convert to coal (sorry for missing that) ... my answer addresses a narrow interpretation of the question: how much land would forest cover to hold the carbon content of the coal we burn. Looking at the answer from Christopher Gilmour above, 30% of the plant matter was broken down (in the carboniferous era) and 70% mineralised (converted to coal or gas under pressure), so there should be another factor in the calculation.
    – M Juckes
    Commented Nov 5, 2019 at 8:03
  • 20,000 teragrams of coal is only 20 gigatonnes (20*10^9 tonnes) of coal. My understanding is that from 1870 to 2014, cumulative carbon emissions totaled about 545 GtC. Alone in 2014, carbon emissions were 9.795 gigatonnes of carbon or 35.9 gigatonnes of carbon dioxide.
    – juhist
    Commented Dec 13, 2019 at 17:01
  • Could you please explain where you drew up the figure 40,000 TWh of energy? What does it represent?
    – juhist
    Commented Dec 13, 2019 at 17:08

I was answering the reason given for the question which was, is man's impact significant? Lets look at it another way. plant and animal remains turn into peat, peat turns into coal and oil. They are called fossil fuels because they represent all the life on Earth almost from the beginning of life until more or less the present day. Some, perhaps as much as half, has been lost by volcanism, the movement of the Earth; so it's current volume is less than 100%. Palaeontologists have become very good at measuring the amount of time each layer of earth, including the layers of coal in mines took to compact. Their measurements tend to be in millions of years. How long has man been using fossil fuels? A little over one hundred years. So we are talking about man releasing all the carbon from all the forests in the world at about the rate of a million years of life on Earth, in one year. The area of forests needed to be sustainable at this rate would be at least ten times larger than the sun. I'm sorry if my figures are slightly inaccurate but it is difficult to be accurate when dealing with such astronomical measurements.


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