One is parts per million. Imagine a million M&Ms. Three of them are blue and the rest are yellow; this means that three parts per million are blue. If you had 10 million M&Ms, and 30 of them were blue and the rest were yellow, you would still have three parts per million blue M&Ms. For every million M&Ms, three of them are blue. Parts per million is a relative measurement, just like a percentage is a relative measurement.
When measuring carbon in the atmosphere, we are referring to parts per million by volume. This means that if we took the total volume of the atmosphere and divided it into a million parts, a given number of those parts would be carbon. If we took a much smaller volume of the atmosphere—for instance, the air in a box—and divided it into a million parts, the same number of those parts would be carbon. Parts per million doesn’t tell you exactly how much total carbon is in the atmosphere, but rather how much carbon would be in any given volume of the atmosphere.
Parts per million is abbreviated Ppm. You may sometimes see Ppmv, which refers to parts per million by volume. Carbon is denoted as “C”, so when talking about total amounts of carbon you may see the units Ppm, Ppmv, or PpmvC.
The other unit scientists use to measure carbon is a more typical measurement—the weight of carbon. For this, scientists, scientists use petagram or gigaton, which are the same units of measure. “Peta-“ is 1015, for a total of 1015 grams; “giga” is 1 billion, for a total of 1 billion metric tons; one metric ton is 1,000 kilograms and one kilogram is 1,000 grams.
1gigaton = 1,000,000,000,000,000grams
Petagrams are abbreviated as “Pg”, and gigatons are abbreviated as “Gt”. When talking about total amounts of carbon you may see the units PgC of GtC.