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Why does methane have less total atmospheric concentration than carbon dioxide?

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World map shaded to show annual carbon dioxide emissions by country in 2022, with China, the United States, and India highlighted as top emitters in dark red. A bar chart below lists the top ten emitting countries by million metric tons. — Greenhouse gas Map of annual carbon dioxide emissions by country in 2014.

Methane (CH $_{4}$ ) exists in significantly lower atmospheric concentrations than carbon dioxide (CO $_{2}$ ) primarily due to its much shorter residence time in the atmosphere.12

While carbon dioxide remains in the atmosphere for hundreds of years, methane has a residence time of only about 10 years. This is because methane is highly reactive and is continuously removed from the atmosphere through natural sinks. The primary natural sink is the reaction of methane with the hydroxyl radical within the troposphere, which converts it into carbon dioxide and water vapor. Additionally, methane is oxidized by bacteria in soil and is destroyed upon reaching the stratosphere.123

Because of these rapid removal processes, methane concentrations are generally measured in parts per billion (ppb), whereas carbon dioxide concentrations are measured in parts per million (ppm). Despite its lower concentration, methane is a more potent greenhouse gas than carbon dioxide because it produces greater radiative forcing per molecule.12