Earth's atmospheric composition, the gases that make up our atmosphere, has not been constant over time. Earth's early atmosphere was most likely primarily helium, methane, ammonia, and hydrogen. Today, Earth's atmospheric composition still varies. For example, the amount of carbon dioxide, water vapor, and methane in our atmosphere is rising. Nitrogen is constantly being circulated between the atmosphere, biosphere, hydrosphere, and geosphere.

The amount of water vapor in Earth's atmosphere varies over time and place. It changes depending upon the season, with altitude, and latitude. For example, air over the oceans contains more water vapor than over deserts. Air on the east coast of the United States is more humid than the Intermountain West. Tropical air near the equator also contains more water vapor than other latitude zones of our planet.

Carbon dioxide has been increasing in concentrations in our atmosphere, due primarily to the burning of fossil fuels (coal, oil, and natural gas). It has also naturally fluctuated throughout Earth's history, but has never previously reached concentrations that match modern day levels.