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Concentration of CO2 in the Atmosphere

Carbon Dioxide (CO₂), the Greenhouse Gas

Let’s get down to the basics:
what is CO₂ and why should we even care?

Ball-and-stick model of the carbon dioxide molecule, one of the most important chemical compounds in the world. It is vital for life as we know it but catastrophic at excess levels. Color code: carbon, C is black; oxygen, O is red. Image from Creative Commons.

Worth Gretter

We all know, unless we are in climate-change denial, that the Earth is warming, and it won’t be good for us. We also know that burning fossil fuels produces carbon dioxide (CO₂), which is a greenhouse gas that traps heat. But how does that all work, anyway?

When you read that your car produces several tons of CO₂ each year, that sounds quite unreasonable. After all, CO₂ is a gas, and gases are light in weight, so “tons of CO₂” does not seem to make sense. Let’s look at the chemistry.

Gasoline is a mixture of hydrocarbons. If we look at a very simple hydrocarbon, ethane, it has a formula of C2H6, i.e. two carbon atoms and six hydrogen atoms. This would seem like mostly hydrogen, but it’s not. Hydrogen only has an atomic weight of one and carbon has an atomic weight of 12. So, ethane weighs in at 30, but only 6 of that is hydrogen and the other 24 is carbon.

Things get worse when we burn the ethane. One atom of carbon, with a weight of 12, combines with two oxygen atoms, each weighing 16. This yields CO₂, weighing 44. And ethane has two carbon atoms, so we get two times 44, or 88. This example can be scaled up from atomic weight to any units you like, such as pounds. So, burning 30 pounds of ethane produces 88 pounds of carbon dioxide – almost three times as much. (If you are wondering what happens to the hydrogen, it combines with oxygen to form H2O – water.)

The gasoline you put in your car weighs about 6 pounds per gallon. If you fill up with 10 gallons, or 60 pounds, that will turn into about 150 pounds of CO₂ when burned. If you use 10 gallons a week, then every week you are putting 150 pounds of CO₂ into the atmosphere. (You may notice that 150 pounds of CO₂ is only 2-1/2 times the 60 pounds of gasoline, not 3 times, because gasoline is not ethane but rather a mix of hydrocarbons.)

But we know that the atmosphere is enormous. Even if every human adds 150 pounds of CO₂ each week, how can that affect the climate? This gets even more perplexing when you consider that the atmosphere is (approximately) 78% nitrogen, 21% oxygen, and 1% argon. That adds up to 100%, so where is the CO₂?

It turns out there are trace gases, and CO₂ is one. It is present in the atmosphere at 0.04%, which is 400 parts per million. (This is where the environmental group 350.org gets its name, from the desire to keep CO₂ below that level.) But how can 0.04% of the atmosphere even matter?

When sunlight strikes the earth it warms all surfaces, and some of that warmth is re-radiated outward as infrared light, i.e. heat energy. Some of the heat is lost to space, but some is captured by molecules in the atmosphere and re-radiated back to Earth again.

It turns out that most molecules in the atmosphere can’t re-radiate energy. Nitrogen and oxygen are diatomic gases (N2 and O2) made of two identical atoms. Argon is monatomic, a single atom. These forms can’t store infrared energy and re-radiate it. CO₂, however, is made of two different kinds of atoms and can store energy temporarily within its molecular structure. That energy is re-radiated, some to space and some back to Earth. Methane and water vapor are also involved in re-radiating energy, making them greenhouse gases as well.

Worth Gretter

Greenhouse gases help keep the earth warm, which is a good thing. But Earth is getting too warm (as we are learning) simply because greenhouse gas concentrations are much higher now than in recent history. Humans are causing it by burning fossil fuels and creating carbon dioxide.

So, we see that there are two factors at work here: 1) we are all putting more CO₂ in the atmosphere than we realize; and 2) most of the atmosphere is not greenhouse gases, so a small change in CO₂ makes a big difference.

And why does this even matter? Because there are now almost eight billion people releasing CO₂. That is a problem! Earth’s climate is very complicated, and no one can predict exactly what changes will occur. But we have already had a taste of the future with intensifying tropical storms, floods, and wildfires. I think even some of the climate- change deniers are starting to agree that we need to make some changes if we want to survive.

Worth Gretter is a retired electrical engineer for Knolls Atomic Power Laboratory. He lives in Menands, New York and volunteers at Ten Broeck Mansion, a historic house museum.

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