As of 2012, the concentration of carbon dioxide in Earth' s atmosphere is 400 parts per million. For every 1,000,000 gas molecules in our atmosphere, about 400 are carbon dioxide molecules (see the special note about water vapor below). How much is 400 parts per million? The following two diagrams may help you visualize how much 400 parts per million is. Each rectangle is 600 by 400 pixels in size, and therefore contains just a bit fewer than a quarter of a million pixels (240,000 = 600 x 400). At this scale, carbon dioxide represents slightly less than 100 pixels (actually 96 pixels = 400 parts per million times 0.24 million). The first diagram shows 96 black dots (representing carbon dioxide molecules) scattered throughout the rectangle (which represents the whole atmosphere).
In the second diagram, the black dots representing carbon dioxide have been concentrated into a single small rectangle. The black rectangle has dimensions of 12 by 8 pixels and thus contains 96 pixels representing carbon dioxide (the same as the previous diagram).
When referring to gas concentrations in the atmosphere, values in parts per million are typically understood to mean parts per million volume (abbreviated ppmv) as opposed to parts per million mass. This volume concentration essentially tells us what fraction of all atmospheric gas molecules are carbon dioxide molecules.
The amount of water vapor in Earth's atmosphere varies substantially from place to place, from season to season, and as the weather conditions change. Typically, the troposphere (the lowest layer of the atmosphere where we live and most weather happens) contains 1 to 4% water vapor. When atmospheric scientists discuss gas concentrations, they usually subtract out the changing value for water vapor and compare the amounts of various gases in a perfectly dry sample of air. Carbon dioxide, therefore, makes up 400 parts per million by volume of dry air.
How does 400 ppmv for carbon dioxide compare to other gases in Earth's atmosphere? The following diagram shows the amounts of the other major gases in our atmosphere.