During the coldest period in the past 160,000 years, what was the approximate concentration of CO2?

• 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 period in the last 160,000 years, atmospheric CO2 was about 190–200 parts per million. This comes from ice-core records, where ancient air trapped in bubbles in Antarctic ice gives direct measurements of past CO2 levels. At the last glacial maximum, the climate was cooler and the ocean absorbed more CO2, so the atmospheric concentration dropped well below the pre-industrial level of around 280 ppm and far below today’s levels. The other ranges correspond to warmer periods or modern times, not the glacial maximum.