Scientists have been monitoring CO2 in the atmosphere from Mauna Loa in Hawaii since 1958. Which of the following units is used by scientists to measure CO2 in the atmosphere?

• A. Percent
• B. Parts per billion (ppb)
• C. Parts per million (ppm)
• D. Grams per cubic meter

Answer: (C)

The key idea is expressing the amount of CO2 as a mixing ratio—the fraction of air that is CO2. In the atmosphere, CO2 is present at a few hundred molecules per million air molecules, so scientists use parts per million to describe its concentration. On the Mauna Loa measurements, reporting CO2 in parts per million lets researchers track long-term trends precisely; for example, about 400 CO2 molecules per 1,000,000 air molecules corresponds to roughly 0.04% of the air being CO2. Using percent would compress that small value into a single hundredth of a percent, which is less precise for monitoring changes over decades, and parts per billion would place CO2 far too small to be practical since its concentration is much higher than many other trace gases. Grams per cubic meter would introduce dependence on temperature and pressure, making comparisons across times and places harder. Therefore, parts per million is the standard and most informative unit for atmospheric CO2.