Attempts to determine past levels of atmospehric CO 2 concentrations have been made by using a variety of techniques, including direct measurements of trapped air in polar ice cores, indirect determination from carbon isotopes in tree rings, spectroscopic data, and carbon and oxygen isotopic changes in carbonate sediments in deep-ocean cores.
(2 concentrations; by using; a variety of techniques, including direct measurements of trapped air in polar ice cores)
2 concentrations
by using
a variety of techniques, including direct measurements of trapped air in polar ice cores