The pre-industrial concentration of carbon dioxide in the atmosphere was about 280 parts per million (ppm). This means that, before large-scale industrialization, roughly 280 molecules in every million dry-air molecules were carbon dioxide. Ice-core records place late-pre-industrial values near 275–285 ppm, so 280 ppm is the standard rounded figure used in climate science.
The benchmark usually refers to the period around 1750, before fossil-fuel combustion and industrial land-use changes caused a sustained rise. Carbon dioxide levels had varied naturally over geological time, but they were relatively stable during the centuries immediately before the Industrial Revolution.
Since then, burning coal, oil, and natural gas, along with cement production and land-use change, has increased atmospheric CO₂ substantially. Plants, oceans, and soils absorb part of human emissions, but not all of them. A frequent mistake is confusing 280 ppm with today’s concentration or with the much older range measured in ice cores. The unit ppm describes concentration, not the percentage of emissions caused by humans.