One of the biggest source of confusion and errors in climate discussions concerns “carbon” versus “carbon dioxide.”
I was reminded of this last week because of my post Economics Stunner: “Oil and Coal-Fired Power Plants Have Air Pollution Damages Larger Than Their Value Added.” I noticed that Skeptical Science innocently confused C and CO2.
The economists had written, “We assume that the central estimate of the social cost of carbon is $27 per ton of carbon” (which is to say a measly $7.36 per ton of CO2). SkS assumed the authors meant CO2 couldn’t believe that leading economists could possibly have lowballed climate impacts so much. As SkS writes in their correction, this puts their social cost of carbon “near the lower limit we have seen in recent economic studies!”
SkS needn’t feel bad. I’ve seen this mistake made a dozen times — in fact, I got an email from the Director for Modeling and Analysis at one of the largest fossil fuel companies in the world about my post, and he got it backwards!
The paragraph I usually include in my writing:
Some people use carbon rather than carbon dioxide as a metric. The fraction of carbon in carbon dioxide is the ratio of their weights. The atomic weight of carbon is 12 atomic mass units, while the weight of carbon dioxide is 44, because it includes two oxygen atoms that each weigh 16. So, to switch from one to the other, use the formula: One ton of carbon equals 44/12 = 11/3 = 3.67 tons of carbon dioxide. Thus 11 tons of carbon dioxide equals 3 tons of carbon, and a price of $30 per ton of carbon dioxide equals a price of $110 per ton of carbon.
The reason this confusion arises so much is that scientists usually use carbon, because they are studying the carbon cycle, and governments also usually use carbon, because the scientists do. But “carbon” is not intuitive, whereas carbon dioxide is what we all emit — that is why businesses and the public typically report numbers in terms of carbon dioxide.
“Point Carbon” for instance, reports prices in the European market for CO2 allowances (in euros, of course). Still, they use the terms interchangeably, as I and almost everyone else who writes in this space does, which only adds to the confusion. You’ll read things like “The price of carbon is $10 a ton of CO2.”
But the central climate number in this whole arena is the atmospheric concentration of carbon dioxide. The media is typically caught in between, sometimes using one and sometimes using the other, and sometimes making a mistake or not being clear.
In my books, I have tried to consistently use CO2, for clarity’s sake, but haven’t been quite as consistent in the blog. So I am going to try to more consistently use CO2. Where relevant I will also include one conversion to carbon, too, without bombarding you with too many numbers.
And whenever I write a post where tons or cost per ton matters, I’ll link to this post. If nothing else, it’ll remind me to check my numbers.
So hopefully, from now on, if I fail to be clear, you should make the default assumption I am talking carbon dioxide.
I would recommend all blogs and journalists clearly state their “carbon dioxide policy” — and be sure to check when reporting on studies or articles or business action that they know whether they are talking carbon or carbon dioxide. Don’t assume the authors of the original piece got their math right.
Oh, and people often switch metric tons (tonnes) and short tons. Here’s that conversion:
