Final answer:
To estimate the atmospheric conditions (CO2 concentrations) of the past, scientists can analyze ice cores or examine plant stomatal index.
Step-by-step explanation:
One way to estimate the atmospheric conditions (CO2 concentrations) of the past is by examining ice cores. Scientists can measure the amount of CO2 trapped in ice to determine atmospheric concentrations from hundreds of thousands of years ago. Another method is to study stomatal index, which is the percentage of epidermal cells on plant leaves that contain stomata. By analyzing the stomatal index of plant samples collected in the past, scientists can infer the atmospheric CO2 concentrations during those times.