Question 23·Medium·Central Ideas and Details
Climate scientists use natural archives to reconstruct Earth’s past temperatures. Tree rings reveal annual growth patterns extending roughly 2,000 years into the past, while the chemical composition of coral skeletons records changes over several centuries. To examine climate fluctuations on glacial time scales, however, researchers study ice cores extracted from Antarctica and Greenland, whose compressed layers of snowfall preserve atmospheric information dating back more than 800,000 years.
What does the text indicate about ice cores?
For questions asking what the text indicates or states about something, first underline the exact sentence or two that mention that thing. Note any specific numbers, comparisons, or contrasts given (like time spans, quantities, or locations). Then go through the choices and cross out any that (1) bring in ideas not mentioned (like precision or difficulty), (2) exaggerate with words like “cannot,” “always,” or “never,” or (3) contradict the details you underlined. Choose the option that most directly restates or summarizes the relevant part of the passage without adding new information.
Hints
Find the description of ice cores
Look for the sentence that talks specifically about ice cores and how researchers use them. What key detail is given in that sentence?
Focus on time periods
The passage gives different time ranges for tree rings, coral skeletons, and ice cores. How does the time range for ice cores compare to the other two?
Beware of extra claims
Eliminate any answer choice that mentions details the passage never states, such as precision, ease of collection, or special kinds of phenomena, if those are not clearly described in the text.
Step-by-step Explanation
Identify the task
The question asks: “What does the text indicate about ice cores?” This means you must base your answer only on what the passage directly states about ice cores, not on outside knowledge or guesses.
Locate the key sentence about ice cores
Find where ice cores are mentioned: “To examine climate fluctuations on glacial time scales, however, researchers study ice cores extracted from Antarctica and Greenland, whose compressed layers of snowfall preserve atmospheric information dating back more than 800,000 years.” This sentence tells you what is special about ice cores, especially in terms of the time period they cover.
Compare time spans for all archives
Now look at the time spans the passage gives:
- Tree rings: “extending roughly 2,000 years into the past”
- Coral skeletons: “records changes over several centuries”
- Ice cores: “dating back more than 800,000 years” The clear contrast is that ice cores go back far longer in time than tree rings and coral.
Match that idea to the best answer choice
Scan the answer choices for the one that correctly captures this contrast in time span, without adding claims the passage does not support. The only choice that directly reflects that ice cores cover a much longer period than tree rings and coral is: They provide climate data over a longer time span than tree rings and coral skeletons do.