Decomposition

There are two things to note about the previous screenshot:

• CO₂ levels in the air are steadily rising over time.
• There are dips and then bumps in the levels of CO₂, but the result still ends up rising overall. These dips and bumps happen on a regular pattern.

The first point is what is known to statisticians as a trend. You may already be familiar with the notion of a Trend Line from Microsoft Excel. A trend is a kind of pattern that describes gradual change over time. In our case, it is quite clear that the trend is upward.

The second point is called seasonality—for very apt reasons, as it may turn out. Seasonality describes the pattern of variance that happens regularly. If you carefully look at the chart, typically at around August to October of each year, the CO₂ levels drop to the lowest point of the year. After which, they rise steadily again until...