Science corner: the natural carbon cycle vs. fossil fuels

We get a steady stream of questions that hinge on a basic confusion over the difference between the natural carbon cycle — the continual uptake and release of carbon dioxide by biological organisms — and the burning of fossil fuels. This post is an attempt to set the record straight. We hope that it achieves the worldwide acclaim of our post on how to turn 6 pounds of carbon into 20 pounds of CO2.

I now present to you the most famous graph in climate science, the Keeling Curve:


This trend line shows in precise detail how atmospheric concentrations of CO2 have changed over past 50 years. Notice two distinct patterns in the graph:

  1. The sawtooth pattern represents the natural carbon cycle. Every summer in the northern hemisphere, grass grows, leaves sprout, and plants flower. These natural processes draw CO2 out of the air. During the northern winters, plants wither and rot, releasing their CO2 back into the air. This sawtooth pattern shows the planet breathing.
  2. The overall upward trend represents CO2 emissions from the burning of fossil fuels. Every year, the baseline of the curve grows. This growth tracks exactly to the consumption of oil and coal.*

Before Charles Keeling started gathering his data in the 1950s, no one expected to see much meaningful systematic variability in atmospheric carbon concentrations, particularly over such short time scales. Keeling’s data was a revelation.

Of the two patterns, it’s that baseline growth that presents a problem. The whole climate change issue hinges on the fact that we are pulling lots of carbon out of the ground where it is safely sequestered in the form of fossil fuel, and then pumping it into the atmosphere.

The natural carbon cycle, on the other hand, is not a problem. If you grow a carrot in your garden and then eat it, you are not contributing to the net amount of carbon in the air. You’re simply riding the ups and down of the Keeling Curve’s sawtooth edge.

Let’s apply the lessons of the Keeling Curve to some of the questions we see most often from readers:

  • Does my wood-burning stove contribute to global warming?

    Not very much. Wood is part of the natural carbon cycle. However, as with most products, some energy is expended harvesting and transporting wood, so it isn’t 100% carbon neutral unless you literally are taking an axe to a tree in your yard. Harvesting a cord of wood typically results in about 500 lbs of carbon emissions. Burning wood does create nasty particulates, but that’s a different problem entirely.

  • If wood is part of the natural carbon cycle, why is deforestation a problem?

    About 20% of global warming is caused by deforestation. The problem with deforestation is that, by definition, the forests aren’t growing back, so the natural cycle is broken. Sustainable harvesting is the key. Don’t fuel your stove with tropical hardwood.

  • Does my breathing contribute to global warming?

    Breathing is the final step in the process of breaking food down into carbon dioxide. And food is part of the natural carbon cycle. So breathing is carbon neutral.

    Again, though, production is an issue. Food has to be grown, harvested, and transported to your house, processes which require fossil fuels. But the issue here isn’t your breathing per se. Like any item you buy, food has a carbon content that reflects the energy required to produce it. It makes sense to shop for local food. It doesn’t make sense to hold your breath.

  • What about my cat’s farts?

    Well, methane is quite a bit worse than CO2, so flatulence from animals is not really part of the natural carbon cycle. This turns out to be a serious issue for the cattle industry. On the other hand, your gassy cat, while certainly unpleasant, is probably not a serious contributor to global warming.

  • TerraPass funds landfill methane flaring projects. But flaring methane just turns it into carbon dioxide. Isn’t carbon dioxide bad?

    Carbon dioxide produced from decomposing garbage is — wait for it — part of the natural carbon cycle. It might seem odd to regard a landfill as part of a natural cycle — certainly garbage dumps don’t grow in the summer and shrink in the winter — but the important distinction is organic carbon vs. carbon pumped or mined from underground.

I’ll update this list as more questions come in. And here’s a bonus tidbit for those jokers who like to point out that fossil fuels originally came from plants. Fossil fuels are indeed organic, so by burning them aren’t we just taking part in a much longer carbon cycle?

Perhaps, but two things to keep in mind: first, fossil fuels accumulated over many millions of years, and we’re burning them over the course of a few decades. The rapidity of the change to our atmosphere will cause us grief.

Second, before all of that carbon accumulated underground, our planet used to be, you guessed it, a whole lot warmer.

* Lest there be any confusion, this statement is uncontroversial. There is (overblown) controversy regarding the link between CO2 and climate change, but no one questions that rising atmospheric concentrations of CO2 are a direct result of industrialization.

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  1. Connor - April 9, 2007

    I was wondering about your point describing breathing as part of the normal carbon cycle. I am curious whether there is a noticible amount of difference between 150 lb man breathing and producing Carbon Dioxide and a 250 lb man breathing and producing Carbon Dioxide. Can being fit (and therefore only eating what you need) be considered a reasonable pro-environment step to take (although its probably not politically correct to tell overweight people they should lose weight, not just for themselves, but for the rest of us.)

  2. Adam Stein - April 9, 2007

    I think the number of calories matters less than where those calories are coming from. Eating lower on the food chain (that is, plants rather than animals) and eating food produced locally (which doesn’t need to be transported as far) are the keys to lowering the carbon content of your diet.
    I can pretty much guarantee that some vegans are going to chime in shortly on the environmental benefits of their diet. They’re right. For many, though, this is a bit of a touchy subject. People have many reasons for making the food choices they do, and they don’t very much like to be told what to eat.

  3. Connor - April 9, 2007

    I had heard the eat locally and lower on the food chain arguments before, and it makes sense. I do my best to eat locally even though I don’t agree with eating as a vegetarian (to each their own.) I’ve seen other tips like “remove everything you can from your car” and the fact that airlines charge me for putting extra in my luggage. It seems like the dirty secret in all this is that one’s weight really can make a significant difference. Why should I pay the same for my airline seat as someone much heavier (if I haven’t made up the difference with luggage)? If removing bags of salt and extraneous other things make a difference with car fuel efficiency, I have to be doing better driving my car than someone much heavier. Finally, it seems pretty clear that being overweight correlates with eating choices that are NOT eating locally or lower on the food chain.

  4. Tom Arnold - April 9, 2007


    If you have a real issue, the solution is just around the corner. I am sure it could be remade to fit:

  5. Aaron A. - April 9, 2007

    Can being fit (and therefore only eating what you need) be considered a reasonable pro-environment step to take

    As Adam said, it’s not always about quantity, but also quality. That goes for both nutrition and CO2. America is rather unique in that our obesity problem hits hardest among the lower class. Why? Processed foods, o’course. To compensate for a lack of flavor and nutritive value, the processors add more fats and refined sugars. You don’t need to be a dietician to know what that means.

    My answer? Community gardens. We need more of ’em. They create a shared space for residents to interact, build a sense of community, and grow fruits and vegetables for a fraction of what Big*Mart would charge.

  6. Woody - April 11, 2007

    Yes Aaron, but don’t clear thick, natural vegetation to make space for the garden. The more substantial natural plants will do more to convert CO2 to O2 than the seasonal garden plants, which need fertilizers & human energy input.

    Switching gears, the real kicker is our population. We wouldn’t be in this mess if there were far fewer people doing things outside the natural carbon cycle. And how about the billions of people currently living in poverty who contribute little to the greenhouse effect? They deserve better lives, but at the expense of heating the planet further? The answer is to work to reduce world population to a level that the Earth can sustain AND learn how to live as close to carbon neutral as possible.

  7. Chad - April 11, 2007

    Can someone actually point me to some evidence that “buying local” decreases one’s carbon footprint? I am rather skeptical of this claim. Having worked for both large and small companies, I would hazard to guess that the efficiencies of scale are more than enough to offset the additional transportation costs. I know that the journal Environmental Science and Technology (a peer-reviewed journal affiliated with the American Chemical Society) recently had an article showing that organic foods were pretty much a wash environmentally for these very kinds of reasons.
    Also, you would be surprised at what fraction of the “transportation costs” of food consist of your trip to and from the store. If you go a few miles out of your way to the local farmer’s market a couple times a month, you probably have negated your carbon savings via the extra driving alone.

  8. Geoff - April 11, 2007

    Nice FAQ, though you might mention that the biggest benefit of turning landfill methane into CO2 is that you’re reducing its effect on the climate by a factor of 21:1. So even though the remaining CO2 contributes to warming, it causes much less harm than the original methane would have.

  9. Adam Stein - April 11, 2007

    Chad — there have been some good academic studies on this. The bibliography in Omnivore’s Dilemma is useful. Unfortunately, I don’t have my copy handy. When I do, I’ll try to remember to post some references. The short of it, though, is that production doesn’t matter nearly as much as transportation, so I don’t think efficiencies of scale are the major factor. Also, it is well known that organic produce doesn’t have a considerably lower carbon content than conventionally produced food, again because transportation is the overriding factor. Incidentally, organic food is often produced at industrial scale these days.
    You raise a good point about the “last mile” effect of shopping habits, and I don’t know of any studies on that (although they probably exist). I’d be curious to find out more about this.
    Of course, some would argue that the longer term solution is to create more demand for locally grown food, which will improve availability. Also, there are some pretty clearcut cases where the “buy local” rule of thumb comes in handy. Namely, skip the strawberries flown in from Chile when you’re shopping at Safeway.
    Geoff — good point. I will update when I get a chance.

  10. Bob - April 11, 2007

    Would not food delivered by efficient bulk haulers be safer for the Earth than the same quantity delivered by many small lot delivery vehicles?
    Or, to rephrase the question. Would high volume production and delivery be less destructive to the Earth than the equal volume produced and delivered from and by low volume producers?

  11. Kevin - April 12, 2007

    I’ve wondered about the claim of landfills being carbon neutral. In fact, much of the mass in landfills is plastics that were synthesized from oil. I understand oxidizing the methane to CO2 to reduce the greenhouse effect of that carbon, but that carbon isn’t initially part of the usual carbon cycle.
    If anyone has numbers, I’d be interested in seeing what fraction of landfill-generated CO2 arises from the fossil fuels used to produce the mass of plastic that is discarded into these sites.
    On a slightly different topic, has anyone seen reports on the energy available from all the methane generated by all the landfills domestically. Since we’re oxidizing the methane to CO2 anyway, does it make sense (engineering-wise and commercially) to make use of the energy released?

  12. Adam Stein - April 12, 2007

    I don’t have those numbers, but landfill methane is referred to as biogenic, which I take as an indication that the primary source is organic matter. As far as I know, plastic doesn’t decompose into methane (although anyone with more specific knowledge should chime in).
    Often methane flared from landfills is used to generate electricity. I don’t know enough about the specific factors that determine when it makes sense to do so, although I imagine it has a lot to do with the amount and concentration of the methane being flared.

  13. Anonymous - April 13, 2007

    Bob — yep. Transporting goods by train is far and away more (carbon) efficient than transporting via fleets of gas-guzzling semis. But hey, gas is cheap! (Or was, anyway.) And trucks can make it to places trains can’t, without building more railroads. Hence the train has died out, and railroads that were the settlers’ proud accomplishment, and the main transportation method at one point, now are being converted to (mostly unused) bike trails, or simply left to rust. But I have a feeling one day we will regret this. After all, cheap gas can’t last forever….

  14. Mike - April 18, 2007

    I have heard and read the arguement that the rise in CO2 is not driving up temperatures, it is the rise in atmospheric temperature that is driving up CO2 levels. Can you address the reasoning behind this arguement?

  15. Adam Stein - April 18, 2007

    Ummm…there is no reasoning behind that argument. CO2 levels began rising at the same time that we began burning fossil fuels. CO2 levels rise in direct proportion to the amount of fossil fuels we burn. Put these facts together with the fact that burning fossil fuels creates CO2, which then goes into the air and stays there, and you’ve got a pretty good idea where the CO2 is coming from.

  16. James Ray - April 19, 2007

    So I’m wondering is the live-cut Christmas tree industry part of the solution or part of the problem? It would seem that plastic Christmas trees from China would be part of the problem, on several accounts. Petro based products manufactured into plastic trees, probably requiring lots of electricity produced by coal fired plants and then of course the high transportation costs to ship all these plastic trees to the US. But live cut trees have to be cut down with gas power chain saws and distributed by diesel powered trucks carrying trees from the evergreen tree growing regions to everywhere else in the US. And cutting down trees stops the natural cycle, unless of course one believes without a market for Chrsitams trees, the farmer would plant something else or plant nothing at all. So my question is: is buying a live cut Christmas tree this season instead of a plastic one in a box going to help or hurt the environment?

  17. Adam Stein - April 19, 2007

    Cutting down Christmas trees does not stop the natural cycle. So far as I know, Christmas trees are farmed and not a source of deforestation. (I admit, though, that I’m not an expert on Yule happenings.)
    Given that plastic trees are shipped from China and real trees are produced domestically, my guess is that real trees are better. They also smell nice.

  18. Enviro-mentalist - April 23, 2007

    Can you point me to a published study that supports your point #2 (eg. a carbon mass balance):
    “The overall upward trend represents CO2 emissions from the burning of fossil fuels. Every year, the baseline of the curve grows. This growth tracks exactly to the consumption of oil and coal.*”

  19. joseph hegedus - June 23, 2007

    Would the idea of letting forests burn as part of the “natural way” release more carbon into the atmosphere than cutting the forests and keeping the carbon in the form of 2×4’s. Joseph

  20. Adam Stein - June 24, 2007

    Neither of the above, really. Deforestation is the issue. Creating 2×4’s is not such a problem if the wood is harvested sustainably. Forest fires have been happening for as long as forests have existed, and are part of the natural life cycle of forests. The issue, to oversimplify, is removing forests permanently.

  21. ian joseph - November 28, 2007

    well, i kind of agree with that but wouldn’t the burning of forests just release more carbon into the atmosphere, i thought that was the main problem?

  22. Adam Stein - November 28, 2007

    Yes, but: when you look at the relative scale of various sources of deforestation, the big problem is clear-cutting in tropical rainforests.

  23. river - February 24, 2008

    can i ask ur a question?
    how to determine the fossil carbon fraction (out of total carbon content) of some materials, like plastic, texitle,paper?
    is there any equipment of methods which can measure fossil carbon fraction of certain material?

  24. Adam Stein - February 25, 2008

    Hi river —
    There’s no instrument that can measure carbon content, because fossil fuels aren’t literally an ingredient of (most of) these materials. Rather, fossil fuels are used to power the manufacturing process. Aluminum production, for example, requires a tremendous amount of energy.
    Organizations such as the World Resource Institute provide some of this data. Otherwise, Google is your friend.

  25. Jay - December 1, 2008

    Since plastic takes eons to decompose, it’s safe to conclude that it can’t be much of a source of methane.

  26. Jay - December 1, 2008

    My guess would be that the real problem is that high volume production is currently being done with lots of fossil fuel inputs and with techniques that are destructive to the soil. But all things being equal, it still depends on average carbon intensity of the transport times the average distance transported, so lots of short hauls with biodiesel trucks might be better than massive, long-distance hauls with petroleum diesel trucks. I suspect that buying from local organic producers has enough benefits to outweigh the efficiency of bulk hauling over long distances.