The biggest source of mistakes: C vs. CO2

Probably the biggest source of confusion and errors in climate discussions concerns “carbon” versus “carbon dioxide.” I was reminded of this last week when I saw an analysis done for a major environmental group that confused the two and hence was wrong by a large factor (3.67). 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.

I confess that in my books I have tried to consistently use CO2, for clarity’s sake, but have failed to embrace that strategy in the blog. I now realize that was a mistake after receiving an e-mail from a long time a reader who was confused as to whether the price I quoted in a recent post was dollars per ton of carbon or carbon dioxide (even though I had said in the post it was “the price 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).


And, indeed, the central climate number in this whole area 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.

So I am going to try to be consistent and use CO2. Where relevant I will also include one conversion to carbon, too, without bombarding you with too many 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.