Final answer:
Pre-industrial CO2 levels were stable at approximately 280 parts per million (ppm) for at least a thousand years before the rise of industrial activity began to increase greenhouse gas concentrations.
Step-by-step explanation:
The pre-industrial CO2 level, before the impact of the Industrial Revolution, was approximately 280 parts per million (ppm). Over the past 700,000 years, CO2 levels have typically ranged between 200-300 ppm, according to ice core samples. However, with the onset of industrialization, these levels have risen sharply due to human activities. Ice cores from both Greenland and Antarctica confirm that the pre-industrial concentration of CO2 was stable at around 280 ppm for at least a thousand years prior to the fossil fuel era, indicating a baseline before human influence.