What was the level of carbon dioxide (CO2) during the pre-industrial era?

Concept: Atmospheric carbon dioxide is reported in parts per million (ppm) — the number of CO2 molecules present in every one million molecules of dry air. The "pre-industrial era" is the reference period before large-scale fossil-fuel burning began, conventionally fixed at about 1750 CE; its air composition is not measured directly but reconstructed from air bubbles sealed inside polar ice cores. Every statement about modern greenhouse warming is expressed as a departure from this baseline, so the baseline value itself is a standard figure to remember.

Application: Ice-core series such as Law Dome and EPICA give an atmospheric CO2 concentration of roughly 275–285 ppm for the interval AD 1000–1750, and the IPCC adopts 280 ppm (about 278 ppm in the year 1750) as its standard pre-industrial reference, a figure NOAA and WMO also quote as "about 280 ppm". The pre-industrial level asked for is therefore 280 ppm.

Cross-check: Place the other listed concentrations on the same record and the reference value stands out:

• 200 ppm — the depth to which CO2 fell during glacial maxima, for example the coldest phase of the last ice age about 20,000 years ago; an ice-age value, not the state of the atmosphere in AD 1750.

• 350 ppm — crossed by the continuous Mauna Loa measurements in the late 1980s, already about two centuries into industrialisation.

• 420 ppm — reached only in the 2020s, after more than 250 years of accumulated emissions.

• Arithmetic check: 420 − 280 = 140 ppm, i.e. atmospheric CO2 now stands roughly 50% above its pre-industrial baseline — exactly the framing the IPCC and WMO use, which only works if the baseline is 280 ppm.