Final answer:
The composition of air from hundreds of thousands of years ago is primarily obtained from the analysis of ice cores, which contain trapped gases like carbon dioxide, allowing scientists to determine past atmospheric conditions and climate changes.
Step-by-step explanation:
Data on the carbon dioxide and methane composition of air dating back hundreds of thousands of years comes primarily from analyses of ice cores. Ice cores are samples taken from ice sheets or glaciers, which contain air bubbles and other biological evidence that can reveal past atmospheric gases and climate conditions. By drilling into the ice and extracting these cores, scientists can measure the carbon dioxide levels trapped within the ice to determine past atmospheric carbon dioxide concentrations.
For example, ice cores from the Russian Vostok station in East Antarctica, which were laid down over the course of 420,000 years and reached a depth of over 3,000 meters, have been crucial in understanding the historical levels of carbon dioxide in our atmosphere. These analyses give us a record of temperatures and carbon dioxide concentrations that allow us to observe natural cycles and the impact of human activities on the Earth's climate.
Analyses of tree rings, while useful for reconstructions of past climates and understanding changes in Earth's atmosphere, serve a different role compared to ice cores and are not the primary source of data for carbon dioxide and methane compositions dating back such long periods.