Across the past 400,000 years, between which values did atmospheric carbon dioxide cycle?

1. Antarctic ice cores trap bubbles of ancient air at known depths. 2. Deeper ice is older, so the cores read like a record running backwards in time. 3. Carbon dioxide measured in those bubbles goes up and down in repeating cycles. 4. The low point of those cycles is about 180 parts per million by volume. 5. The high point is about 300 parts per million by volume. 6. Moving from one to the other took roughly 50,000 years. 7. The 280 to 392 range describes the modern industrial rise, not the long ice core cycle. 8. A ceiling of 500 parts per million never appears in the ice core record at all. _Source: OpenStax Biology 1e (CC BY 4.0), Chs 44 and 46, section 44.5 Climate and the Effects of Global Climate Change_