What is the typical CO2 concentration range during the coldest period in the last 160,000 years?

• A. Between 280 and 300 ppm
• B. Between 350 and 400 ppm
• C. Between 190 and 200 ppm
• D. Between 150 and 170 ppm

Answer: (C)

During the coldest phases of the last 160,000 years, atmospheric CO2 concentrations were around 190–200 ppm. This lower level reflects how the carbon cycle behaves in cooler climates: the ocean tends to absorb more CO2 and the land biosphere stores less, shifting carbon into the ocean and away from the atmosphere. Warmer interglacial periods show CO2 near 280–300 ppm, which is noticeably higher. The other ranges don’t fit the ice-core evidence for these glacial cycles: 280–300 ppm is too high for the coldest times, 350–400 ppm corresponds to much warmer conditions than those ice-core records indicate, and 150–170 ppm is not what the data show for the recent 160,000 years. So, 190–200 ppm best matches the typical CO2 during the coldest periods.