Burning Fossil Fuels Makes Atmospheric CO2 Levels Rise
We’ll get to the data in a second, but first—why does burning fossil fuels emit CO2?
The answer is simple:
combustion is reverse photosynthesis.
When a plant grows, it makes its own food through photosynthesis. At its most oversimplified, during photosynthesis, the plant takes CO2 from the air and absorbs light energy from the sun to split the CO2 into carbon (C) and oxygen (O2). The plant keeps the carbon and emits the oxygen as a waste product. The sun’s light energy stays in the plant as chemical energy the plant can use.
So wood is essentially a block of carbon and stored chemical energy.
When you burn a log, all you’re doing is reversing the photosynthesis. Normally, oxygen in the air just bounces off carbon molecules in wood—that’s why trees aren’t constantly on fire. But when an oxygen molecule gets moving fast enough and smashes into a log’s carbon molecule, they snap together and the oxygen and carbon are reunited again as CO2. This snapping releases chemical energy, which knocks into other nearby oxygen molecules, causing them to get going fast—and if they get going fast enough, they’ll snap together with another of the log’s carbon molecules, which releases more chemical energy. This causes a chain reaction, and the log is now on fire. So a log burning is the process of the carbon in the log combining with oxygen in the air and floating off as CO2.
Of course, that’s all irrelevant to the person burning the log—what they care about is the energy released during all of this CO2 formation. The release of all of the log’s stored chemical energy creates a glorious blaze of heat and light. The tree spent years quietly absorbing carbon molecules and sunshine joules, and all at once, during combustion, that carbon and sunshine explode back out into the world.
To put it another way, photosynthesis just kidnaps carbon and sun energy out of the atmosphere, and after years of holding them hostage, combustion sets them both free—the carbon as a billowing eruption of newly reunited CO2, and the sun energy as fire—meaning that fire is essentially just tightly packed sunshine.
But burning a log and releasing all that CO2 does not tamper with the atmosphere’s carbon levels. Why? Because the carbon that’s being released was recently in the atmosphere, and if you hadn’t set the log on fire, it would have likely decomposed, which would release the carbon back into the world anyway. The log’s carbon was only being held temporarily hostage, and releasing it through combustion has little effect.
Carbon flows from the atmosphere into plants and animals, into the ground and water, and then back out of all those things into the atmosphere—that’s called the carbon cycle. At any given point in time, the Earth’s active carbon cycle contains a specific amount of carbon. Burning a log doesn’t change that level because the carbon cycle “expects” that carbon to be hanging around the ground, water, or air.
But sometimes, a small portion of carbon in the cycle drops out of the cycle for the long term—it happens when a plant or animal dies but for some reason doesn’t decay normally. Instead, before it can decay and release its carbon back into the cycle, it’s buried underground. Over time, that lost carbon adds up. And today, the Earth’s fossil fuels make up a huge mass of lost carbon—carbon that long ago was taken hostage permanently, and carbon that the carbon cycle does not expect to be involved in its routine.
When humans discovered all of this underground kidnapped carbon, you have to remember that for them, the carbon wasn’t the point. They were staring at an endless sea of 300 million-year-old, densely packed sunshine—trillions of ancient plants with their joules intact—and since there are no laws protecting the estates of Carboniferous plants, we could seize it all for ourselves. The grandest joule theft in history.
And as we helped ourselves, we didn’t worry about the fact that extracting those joules also meant extracting carbon that had been buried as far back as the Precambrian period—there were locomotives to fuel and cars to power and buildings to heat, and the joules were irresistible.
And those joules have gone a long way—you can thank them for the comforts and quality of your life today. But those carbon molecules have gone a long way too.
Starting in 1958, scientist Charles Keeling started measuring atmospheric CO2 levels from an observatory on Mauna Loa in Hawaii. Those measurements are still going on today. Here’s what they show:
The zig-zaggy motion of the line is due to the level falling each year in the summer when plants are sucking up CO2 and rising up again during the winter when the leaves are dead. But the overarching trend is unmistakable. To put that into context, ice drilling technology allows scientists to collect accurate data on what CO2 levels have been throughout the last 400,000 years. Here’s what they’ve found:
So atmospheric CO2 levels have oscillated between about 180 and 300 parts per million over the last 400,000 years, never eclipsing 300, and suddenly in the last century the level has vaulted up to 400 (it’s currently at 403ppm).
So instead of the atmosphere being .02% or .03% carbon, it’s now .04% carbon and maybe moving towards .05% and higher.