Dear Umbra,

I know CO2 is a gas as other greenhouse gases are, and gases are sometimes lighter than air. So I’m wondering: how can gases be weighed in tons? That’s a hell of a lot of gas to weigh even one ton, let alone the millions of tons that are reported to be causing climate change. (No, I’m not Oklahoma Sen. James Inhofe in disguise.)

James N.
Oklahoma City, Okla.

Dearest James,

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Science. It is confusing. Oh, for the Dark Ages, when all was explained by unquestioned religious authorities, and the common people never needed to have a critical thought.

The unbearable heaviness of being.

I, too, have puzzled over the concept of “tons” of carbon dioxide. Not only how, but why? Why are we talking about the weight of this gas when most of us, including yourself, have trouble picturing how much carbon dioxide is in a ton (and can’t imagine how a gas can get this hefty to begin with)? As it happens, the how and why are connected. Today, I will try to explain these entwined matters and help us all understand a little more about the elephantine gas portions entering our atmosphere. Wish me luck.

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The overall problem regarding the planet is that greenhouse-gas concentration in the atmosphere is increasing. Greenhouse gases like CO2 trap and radiate sunlight and heat, so as more of them enter the atmosphere, the global mean temperature rises. To discover, chart, and study the problem — and then talk to each other about it — scientists everywhere need to measure atmospheric gases in the same way. It turns out that weight and mass are some of the easiest standard measurements to use.

But how can we quantify something that looks like nothing? Carbon dioxide and other gases are made of atoms — and everything made of atoms has mass. Each carbon atom, for example, has an atomic mass of 12 (because each has six protons and six neutrons in its nucleus, if you must know). The two oxygen atoms in a CO2 molecule have mass as well. By using these factoids and other standardized measures such as the “mole” (no relation to the TV series), scientists have a way to calculate the tonnage of CO2 produced by humankind’s bad habits. So what is a mole, you badger? If I tried to describe it, all the actual scientists who are still reading this would fall down laughing, so I leave that to you and your own research, dear readers. Suffice it to say that a mole of carbon dioxide weighs 44 grams. When enough grams of CO2 pile up, you’ve got a ton.

As to how the gas coming from your personal fossil-fuel combustion can be weighed — well, do you remember chemical equations? Scientists have calculated how much CO2 results from all kinds of chemical reactions, from burning fuel in your car to burning natural gas (aka methane) in your furnace, all using chemical equations. Here’s the equation for burning methane in the presence of oxygen: CH4 + 2O2 => CO2 + 2H20. (I know it’s pretty meaningless to all us liberal arts majors, but I just want us to occasionally be near some actual Science.) That’s “one” methane on the left, turning into “one” CO2 and some water on the right. (I think we’ve now gotten close enough to actual Science and can step back to a safe distance.)

And, yes, a ton is a lot of anything, including carbon dioxide. But the atmosphere itself is quite immense — with a total mean mass of 5.1480 x 1018 kilograms, according to the National Center for Atmospheric Research. Today, there are about 385 parts per million of carbon dioxide in the atmosphere,* or some 782 gigatons of carbon — a gigaton being a billion metric tons. Before the Industrial Revolution, the level was about 280 ppm.

In other words, the time to act has long been before us and is now starting to be behind us. Alas.


*[Correction, 06 Aug 2008: This article originally stated that there are about 367 parts per million of carbon dioxide in the atmosphere, but according to NOAA, the current number is closer to 385 ppm.]