Question

Between 400,000 years ago until 1950, the levels of carbon dioxide in the atmosphere never exceeded
parts per million. Today, carbon dioxide levels have exceeded
parts per million.

Answer 1

Between 400,000 years ago until 1950, the levels of carbon dioxide in the atmosphere never exceeded about 300 parts per million. Today, carbon dioxide levels have exceeded 400 parts per million (currently roughly 420–425 ppm in the early 2020s).