Scientists analyzing ice cores from Antarctica discovered that current atmospheric CO2 levels are higher than at any point in the past 800,000 years. During previous warm periods, CO2 levels peaked at around 280 parts per million. Today's levels exceed 420 parts per million and continue to rise annually.
The passage suggests that
CO2 levels have always remained constant throughout history
ice cores cannot provide accurate historical data
previous warm periods were warmer than today
current atmospheric conditions are unprecedented in recent geological history
Correct Answer: D
Choice D is the best answer. Current CO2 levels exceed anything in 800,000 years.
- Context clues: Current levels are "higher than at any point in the past 800,000 years."
- Meaning: Exceeding all historical precedents makes current conditions unprecedented.
- Verify: The comparison of 420 ppm to previous maximum of 280 ppm shows a major departure.
💡 Strategy: When data shows something happening for the "first time" in a long period, infer unprecedented conditions.
Choice A is incorrect because CO2 fluctuated between periods. Choice B is incorrect because the scientists used ice cores as valid evidence. Choice C is incorrect because higher CO2 suggests potentially warmer conditions now.