Which of the following is a method upon which historical CO2 levels are measured? examination of tree rings. Ocoring of stony corals. coring of ancient ice sheets. examination of stomata on fossils of plants. all are ways in which historical CO2 levels can be estimated. Question 25 (2 points) One example of an aspect that scientists would use to classify an organism is how the organism obtains energy. whether it is unicellular or multicellular. morphology (physical characteristics). all of the above.