385 parts per million
This picture represents carbon dioxide in the atmosphere. Air is a mixture of gases. In this picture, air molecules are represented as white dots - 25, 488 of them. Their separation corresponds to the average separation of air molecules at sea level (about 11 molecule diameters).
The blue circled dots are 'natural' carbon dioxide.
In 1750, 280 out of every million air molecules was carbon dioxide. This proportion is represented by the blue circled dots. It is a small number compared to the other molecules that make up the air (mostly nitrogen and oxygen) which shows what potent stuff carbon dioxide is. Without these blue dots the world would freeze and all plants would die.
The red circled dots are carbon dioxide molecules that we have added to the air. In 2008, 385 of every 1million air molecules was. The extra carbon dioxide molecules that we humans have added are shown as red dots. Because carbon dioxide is so potent, it doesn't take much to have a significant impact.
A detail from the image can be found here: www.flickr.com/photos/carbonquilt/4319248098/