Final answer:
The calibration of radiocarbon dates is achieved by comparing the ratio of carbon-14 to carbon-12 in the object being dated to the ratio in the present-day atmosphere.
Step-by-step explanation:
The calibration of radiocarbon dates is necessary because atmospheric radioactivity has not remained constant in the past. This calibration is achieved by comparing the ratio of carbon-14 (¹⁴C) to carbon-12 (¹²C) in the object being dated to the ratio of ¹⁴C to ¹²C in the present-day atmosphere. By doing so, researchers can adjust the measured radiocarbon dates to account for variations in atmospheric radioactivity over time.