Final answer:
Climatologists use ice cores, stomatal index, tree rings, and other proxies to estimate past atmospheric CO₂ levels, revealing a historical pattern and a significant increase since industrialization.
Step-by-step explanation:
Climatologists estimate past atmospheric CO₂ concentrations using several indirect methods because direct measurement is not possible for historical data. One primary method is the analysis of ice cores, which contain bubbles of ancient air trapped within. These bubbles provide a sample of the atmosphere from the time when the ice was formed. By examining ice cores from different depths, scientists can construct a timeline of atmospheric gas concentrations, including CO₂, going back hundreds of thousands of years.
Another method is the study of stomatal index in plant leaves, where the number of stomata (the pores in leaf surfaces) can indicate the atmospheric CO₂ concentration during the plant's life. Additional proxies include tree rings, glacier lengths, pollen remains, and ocean sediments. Together, these records paint a historical pattern of CO₂ cycling between lower and higher levels over millennia, but have shown a stark increase due to human activity since the Industrial Revolution.
The well-known "Keeling Curve", which began with measurements in 1958 by Dave Keeling, documents a steady year-by-year increase in atmospheric CO₂ levels, affirming the data obtained from indirect sources.