A group activity designed for introductory biology labs (originally created for Life103), this activity leads students through creating a stomatal peel of a chosen leaf and using it to calculate stomatal index. Once students have calculated a stomatal index, they compare their data to data taken from leaf fossils used to calculate prehistoric CO2 concentrations. By comparing their stomatal index to prehistoric data, students make predictions about the current concentration of CO2 in our atmosphere and how changes to the CO2 concentration in the future might impact plants.