What were the levels of carbon dioxide (CO2) in the pre-industrial era?

The pre-industrial concentration of carbon dioxide was about 280 parts per million.

A value of 200 parts per million is closer to ice-age lows, not the pre-industrial baseline.

A value of 350 parts per million was passed in the late twentieth century as emissions grew.

A value of 420 parts per million reflects recent, present-day levels.

Since industrialisation, burning of fossil fuels has driven carbon dioxide well above the 280 baseline.

This rise is the central driver of the enhanced greenhouse effect and modern warming.