"The reconstruction of past climate provides an opportunity to learn how the Earth system responded to high concentrations of atmospheric carbon dioxide (CO2). To obtain information about the state of the atmosphere before instrumental records began, combinations of proxies are used in which physical characteristics of past environmental conditions are preserved. Tiny bubbles of ancient air captured in ice cores when new snow accumulating at the top solidified into ice, can be directly measured and give some insight into the composition of the atmosphere in the past"

World Meteorological Organization