Digital ACT Reading Practice Question #1583

Based on the passage, an atmospheric CO2 concentration of 180 ppm historically correlated with: Passage Excerpt (Natural Science): Drilling four kilometers into the Antarctic ice sheet at Dome Concordia, paleoclimatologists retrieved cylindrical ice cores spanning 800,000 years of continuous atmospheric history. Each annual layer of compacted snow traps microscopic bubbles of ancient air—literal fossilized samples of Earth's past atmosphere. Analysis of stable oxygen isotopes (delta-18O) reveals ancient local temperatures, while gas chromatography of the trapped bubbles measures greenhouse gas concentrations. Across eight glacial cycles, the data reveals an airtight coupling: whenever atmospheric carbon dioxide fell below 180 parts per million (ppm), continental ice sheets expanded over North America and Eurasia; whenever CO2 climbed toward 280 ppm, Earth entered a balmy interglacial epoch.