According to the data, when were CO2 concentrations the highest in the pre-modern record of the last 160,000 years?

• A. About 120,000 years ago
• B. About 10,000 years ago
• C. About 50,000 years ago
• D. About 200,000 years ago

Answer: (A)

The question hinges on how CO2 varied with the climate over glacial–interglacial cycles captured in ice cores. In those records, CO2 rises during warmer interglacial periods and declines during colder glacial periods, typically ranging from about 180–200 ppm in glacials to about 280–300 ppm in interglacials. In the last 160,000 years, the warmest interval—the last interglacial—a occurred roughly 120,000–130,000 years ago, when CO2 reached its natural peak in that period, near 280–290 ppm. That makes the time around 120,000 years ago the point with the highest pre-modern CO2 within the studied window. The other timings correspond to cooler phases or fall outside the studied interval (for example, 10,000 years ago is within the Holocene at CO2 levels near the baseline interglacial value, and 50,000 years ago was during a cooler phase; 200,000 years ago lies outside the last 160,000-year window).