During ice ages, atmospheric CO2 levels dropped to around 180 parts per million, while peak interglacial periods saw levels around 280 ppm. These fluctuations occurred over tens of thousands of years. Current CO2 levels exceed 420 ppm, with most of the increase occurring in just 150 years since the Industrial Revolution—a rate of change unprecedented in the geological record.
It can be inferred from the text that
CO2 levels have never varied throughout Earth's history
the Industrial Revolution had no effect on atmospheric CO2
the current rate of atmospheric change may exceed anything in the planet's recent geological history
ice ages occurred because of human activity
Correct Answer: C
Choice C is the best answer. The speed of recent change is explicitly unprecedented.
- Context clues: Natural changes took "tens of thousands of years"; current change in "just 150 years"—"unprecedented."
- Meaning: The rate, not just the level, is historically exceptional.
- Verify: Previous 100 ppm changes took millennia; recent 140 ppm change took 150 years.
💡 Strategy: When the speed of change is compared to historical rates, infer the significance of that speed.
Choice A is incorrect because the passage describes past fluctuations. Choice B is incorrect because most increase occurred "since the Industrial Revolution." Choice D is incorrect because ice ages happened before human industrial activity.