One in a million

Earth atmosphereThe amount of carbon dioxide in the atmosphere is rising. But by how much? Of the Earth’s atmosphere, almost 80% is nitrogen. Much of what is left is oxygen. Carbon dioxide makes up less than 1% of the atmosphere. The amount of carbon dioxide is so small that percentages are not used. A unit called parts per million, or ppm, is used instead.

Parts per million is a useful unit when the amount being measured is relatively tiny. A concentration of one part per million means that for every million units there is only one of the unit of interest. Take an Australian classroom of 22 students. The population of Australia is 22 million. This means one part per million of the Australian population is a member of that class.

Parts per million is useful for small amounts with a big impact. Small concentrations can be hard to imagine, but parts per million allows us to express them in a way that’s easier to understand.

Parts per million is not just used for individual objects, it is mainly used to measure concentrations by mass or volume. For example, a scientist might want to measure small amounts of gases in the atmosphere. This brings us back to the initial question about rising carbon dioxide levels. About 250 years ago, the concentration of carbon dioxide was about 260 parts per million.

There are places on Earth where the concentration of carbon dioxide has recently risen above 400 parts per million for the first time in recorded history. While this is still not much in absolute terms, it represents around a 50% increase in carbon dioxide levels over the past 250 years. As carbon dioxide is one of the gases responsible for trapping just enough heat to make Earth habitable, rising levels could have a significant impact on the global climate.

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