|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.
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.