Introduction to Physical Geology Practice Exam

Carbon dioxide is a trace gas in the atmosphere. Its concentration is measured in parts per million, and today it sits at about 400 ppm, which is roughly 0.04% of the air by volume. That is well below one tenth of a percent, so the correct approximate portion is less than 0.1%. If CO2 were at 0.5%, 1%, or 5%, it would be a much larger share of the atmosphere and would have produced much more noticeable effects on climate and air properties than we currently observe.