Tuesday, September 11, 2012


Note the use of "C" rather than "CO2"

This has been bugging me for a bit, because I am not a scientist -- well, mostly not -- and I get confused:

Pg = petragram = 10^15 grams = 10^12 kilograms = one billion tonnes = one gigatonne

They're all the same . . . news stories tend to say "billion tons," which is understandable, but why scientists cannot agree on either petragrams (Pg) or gigatonnes (Gt) I have no idea . . .

Tg = tetragram = 10^12 grams = 10^9 kilograms = one million tonnes = one megatonne

While we're on the subject:

There is one ton of carbon per 3.67 tons of carbon dioxide. So when we talk carbon emissions or carbon taxes, it's important to note whether we're talking the mass of the carbon alone, or the mass of the carbon dioxide.

The mass of the atmosphere is about 5.1480×1018 kg, so 1ppm (part per million) is 5.1480×1012 kg, or (using our new en-smartening conversion above), about 5 gigtonnes (Gt) of CO2. While CO2 emissions are conventionally reported by mass, CO2 in the atmosphere is reported by volume, in parts per million of the total volume of the atmosphere. One part per million (ppm) of CO2 is 7.81 billion tons. The carbon itself weighs 2.13 billion tons.

A little more than half of that goes into natural sinks (for the moment, knock on wood) so it takes about 15 Gt of human CO2 to bump atmospheric CO2 by 1ppm.

UPDATE: Thanks to anon for pointing out the error above.

Further info at Skeptical Science, the Carbon Dioxide Information Analysis Center, and CO2Now.


  1. I think you missed 1 thing. CO2 in the atmosphere is expressed in ppm by volume. But CO2 is roughly 50% denser than "air" (the rest of the atmosphere), so 1 ppmv actually has a mass of about 7.8Gt and 390 ppmv of CO2 is about 3,000Gt, which is the total CO2 in the atmosphere.
    Each year, we put out about 30 Gt of CO2, (1% of the total) and as you say, half goes into sinks so CO2 rises by 0.5% or 2 ppm.

  2. http://micpohling.wordpress.com/2007/03/30/math-how-much-co2-by-weight-in-the-atmosphere/ for more info