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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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