What was the approximate concentration of CO2 in the Earth's atmosphere before the industrial revolution?

The concentration of carbon dioxide (CO2) in the Earth's atmosphere before the industrial revolution is widely accepted to have been around 280 parts per million (ppm). This figure is significant because it serves as a baseline for understanding the changes in atmospheric CO2 levels resulting from human activities such as fossil fuel combustion and deforestation, which have led to sharp increases in CO2 concentrations since the late 18th century.

Prior to industrialization, natural processes such as respiration, decomposition, and ocean-atmosphere exchanges dictated CO2 levels, and 280 ppm reflects a relatively stable carbon cycle. This concentration is often highlighted in climate studies as it represents a point in time that predates significant human impact, thereby providing context for the dramatic rise in CO2 levels observed in more recent decades. Recognizing this historical baseline is crucial in discussions about climate change and its implications for global warming and climate policy.