How do scientists learn about the past conditions of Earth’s atmosphere?

Scientists are able to learn about the past conditions of Earth's atmosphere through direct chemical records and proxies of chemical records. The study of Earth's past climate is called paleoclimatology.

Polar ice cores are the only direct chemical records that exist on Earth. Scientists have removed large cores of ice from polar ice sheets such as Greenland and Antarctica - these polar cores act as a timeline for Earth's climate that reaches back almost one million years. In fact, the search for a true 1 mya ice core is currently underway. Scientists can analyze the air trapped within bubbles in the ice to infer temperature, ocean levels, and atmospheric carbon dioxide.

Other paleoclimate records exist indirectly in marine sediment cores, lake sediment cores, speleothems, tree rings cores, and fossils. Unlike ice cores, these records do not have anything to measure directly. (Ice cores contain ancient air, which is the same air that existed in the past!) Instead, these proxies record the effects of climate, which can be used to tell the entire paleoclimate story.