The following passage was written in the mid-1990s.
Topic
The evidence for global warming, the role of greenhouse gases, and evaluating alternative explanations for the recent unprecedented rise in Earth's atmospheric temperature.
Framework
Answer / Explain (with a touch of Present Debate: alternative explanations vs. greenhouse effect)
Main Point
Recent scientific evidence shows that the Earth’s average temperature has risen to levels not seen in the past 1,000 years, and while some alternative explanations exist, the best explanation for this unprecedented warming is the effect of greenhouse gases. The Most Valuable Sentence: Appears at the end of the last paragraph —
P1: Evidence for Unprecedented Warming and the Greenhouse Effect
The passage opens by telling us that compelling new evidence shows Earth’s atmosphere has recently warmed in a way that hasn’t happened in the last 1,000 years. This is based on analyzing things like ice samples, tree rings, and coral. The idea that greenhouse gases are causing this rise is getting stronger, even though it’s still debated.
P2: Addressing Problems with Early Models & the Role of Sulfates
Early computer models predicted even more warming than we’re seeing, which critics used to question the greenhouse theory. But new research shows that airborne sulfates (from volcanoes and pollution) actually reflect sunlight and offset some of the warming, explaining the mismatch and closing the gap between past predictions and what has actually happened.
P3: Considering Solar Variations as an Alternative Explanation
Another possible reason for the warming is changes in solar energy, but even though these solar cycles seem to track with temperature ups and downs over the decades, solar models can’t account for the full extent of the recent warming. The current rise exceeds what solar variation alone could produce, leading the author to conclude that greenhouse gases are still the main culprit.