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Scientists have long assumed that Antarctic ice sheets are essentially stable. In theory, snowfall replenishes ice sheets, replacing the mass lost when ice from the sheets flows outward in solid streams and sheds into the ocean as icebergs. However, satellite measurements show that the West Antarctic ice sheet has been losing mass and that its ice streams are accelerating.
To explain these changes, geophysicist Robin Bell and her colleagues have been studying the role played by liquid water under the ice sheet. They have found that although the base of the ice sheet is insulated from changing atmospheric temperatures, it can melt because of friction as it moves over underlying rock. The meltwater lubricates and accelerates the ice streams. Furthermore, some lakes below the ice sheets are draining, adding more lubrication that further destabilizes the streams.
Ordinarily, ice streams cause ice shelves to accumulate at the streams' ocean outlets. These shelves block and slow the streams. But since ice shelves are sensitive to ocean temperatures, global warming can melt them. In 2002, ice streams flowing into the Larsen B ice shelf in West Antarctica accelerated dramatically after much of the shelf collapsed into the ocean. This suggests that global warming could allow far more land-based ice to flow into the ocean, raising sea levels rapidly.
The passage most strongly suggests that some ice streams have accelerated because
| Text from Passage | Analysis |
|---|---|
| Scientists have long assumed that Antarctic ice sheets are essentially stable. | What it says: For a long time, scientists believed Antarctic ice doesn't change much What it does: Sets up the traditional view that will be challenged Source/Type: Scientists' assumptions (what researchers believed) Connection to Previous Sentences: This is our starting point - no previous information to connect to Visualization: Imagine Antarctic ice sheets like a steady lake - scientists thought the ice level stayed roughly the same year after year Reading Strategy Insight: Watch for challenges to this assumption coming next |
| In theory, snowfall replenishes ice sheets, replacing the mass lost when ice from the sheets flows outward in solid streams and sheds into the ocean as icebergs. | What it says: The theory says: new snow adds ice while ice streams and icebergs take ice away, keeping things balanced What it does: Explains WHY scientists thought ice sheets were stable Source/Type: Scientific theory/explanation Connection to Previous Sentences: This builds on sentence 1 by explaining the reasoning behind the "stable" assumption. This is elaboration, not new complexity Visualization: Think of a bathtub: snowfall = water flowing IN, ice streams/icebergs = water flowing OUT. Theory says IN = OUT, so water level stays constant What We Know So Far: Scientists thought Antarctic ice was stable because snowfall IN should equal ice flow OUT Reading Strategy Insight: Feel confident here - this is just explaining the logic of sentence 1, not adding complexity |
| However, satellite measurements show that the West Antarctic ice sheet has been losing mass and that its ice streams are accelerating. | What it says: But actual satellite data shows West Antarctic ice is shrinking and ice streams are moving faster What it does: Contrasts reality with the theory - introduces the problem Source/Type: Factual evidence from satellites Connection to Previous Sentences: This directly contradicts sentences 1-2. The "However" signals: Theory said stable, but reality shows instability Visualization: Back to our bathtub - the water level is actually DROPPING, meaning more water flows OUT than flows IN. Ice streams are like drains opening wider What We Know So Far: Theory vs. Reality problem established. Theory predicted stability, satellites show loss What We Don't Know Yet: WHY this is happening Reading Strategy Insight: Classic RC pattern: set up expectation, then show it's wrong. Expect explanation of WHY next |
| To explain these changes, geophysicist Robin Bell and her colleagues have been studying the role played by liquid water under the ice sheet. | What it says: Robin Bell's team is researching underground water to understand why ice sheets are losing mass What it does: Introduces the researcher and her focus area - sets up the explanation Source/Type: Research approach by specific scientists Connection to Previous Sentences: This builds on sentence 3 by saying "To explain these changes" - directly answering the WHY question that sentence 3 raised Visualization: Think of detectives investigating a mystery. The mystery = "Why is ice disappearing?" The clue they're following = "water under the ice" Reading Strategy Insight: Transition sentence - author is organizing by saying "here's who studied the problem and what they focused on" |
| They have found that although the base of the ice sheet is insulated from changing atmospheric temperatures, it can melt because of friction as it moves over underlying rock. | What it says: Bell's team discovered: even though surface temperature changes don't reach the bottom of ice sheets, the bottom can still melt from friction when ice rubs against rock underneath What it does: Provides the first part of Bell's explanation - explains how water forms under ice Source/Type: Research findings from Bell's team Connection to Previous Sentences: This answers the question raised in sentence 4 - it explains HOW liquid water gets under the ice sheet (through friction, not atmospheric warming) Visualization: Like rubbing your hands together fast - they get warm from friction even if the air around you is cold. Ice sheet rubbing on rock = heat = melting at the bottom What We Know So Far: Underground water exists because of friction melting, not surface warming What We Don't Know Yet: How this connects to the accelerating ice streams from sentence 3 Reading Strategy Insight: Notice how this explains ONE piece of the puzzle logically |
| The meltwater lubricates and accelerates the ice streams. | What it says: The underground water acts like oil, making ice streams slide faster What it does: Connects the underground water to the accelerating ice streams Source/Type: Research conclusion from Bell's team Connection to Previous Sentences: KEY CONNECTION! This links sentence 5 (how water forms) to sentence 3 (accelerating ice streams). This is the direct cause-and-effect explanation Visualization: Like putting oil under a heavy box - suddenly it slides much faster. Underground water = oil, ice streams = heavy box sliding toward ocean What We Know So Far: Complete cause-effect chain: friction creates meltwater → meltwater lubricates → ice streams accelerate Reading Strategy Insight: Feel relieved - this SHORT sentence connects everything we've learned. The author is helping by making the connection crystal clear |
| Furthermore, some lakes below the ice sheets are draining, adding more lubrication that further destabilizes the streams. | What it says: Underground lakes are also emptying out, creating even more water that makes ice streams even more unstable What it does: Adds another source of lubrication - intensifies the same problem Source/Type: Additional research finding Connection to Previous Sentences: This builds on sentence 6 by providing MORE of the same thing. "Furthermore" signals: here's additional evidence for the same process, not a new concept Visualization: Not just the "oil from friction" but also "stored oil tanks draining" - even MORE lubrication making ice streams slide faster Reading Strategy Insight: This is reinforcement, not new complexity. Same concept (water lubricates ice) with additional source (draining lakes) |
| Ordinarily, ice streams cause ice shelves to accumulate at the streams' ocean outlets. | What it says: Normally, ice streams create ice shelf build-ups where they meet the ocean What it does: Introduces a new element (ice shelves) and describes normal conditions Source/Type: General scientific fact Connection to Previous Sentences: This shifts focus from what's happening inland (sentences 5-7) to what happens at the ocean. New geographic area but still part of the same ice system Visualization: Think of rivers flowing into a delta - ice streams are like rivers, ice shelves are like the delta buildup where river meets ocean What We Don't Know Yet: How ice shelves relate to the acceleration problem discussed earlier Reading Strategy Insight: "Ordinarily" suggests this normal pattern is about to be disrupted - expect contrast |
| These shelves block and slow the streams. | What it says: Ice shelves act like dams, blocking and slowing down the ice streams What it does: Explains the function of ice shelves in the normal system Source/Type: Scientific explanation Connection to Previous Sentences: This simplifies and clarifies sentence 8. Instead of complex "accumulate at outlets," we get simple "block and slow" Visualization: Ice shelves = natural speed bumps or traffic barriers slowing down the ice stream "traffic" heading to the ocean What We Know So Far: Normal system has ice shelves that slow ice streams down Reading Strategy Insight: Author is helping by restating sentence 8 in simpler terms - feel more confident, not less |
| But since ice shelves are sensitive to ocean temperatures, global warming can melt them. | What it says: Because ice shelves can be melted by warmer ocean water, global warming threatens them What it does: Introduces the threat to ice shelves (connects to global warming) Source/Type: Scientific reasoning Connection to Previous Sentences: This contrasts with sentences 8-9. If shelves normally slow streams, but warming melts shelves, then streams won't be slowed Visualization: If ice shelves are traffic barriers, global warming is like removing the barriers - nothing to slow the ice stream traffic anymore Reading Strategy Insight: Pattern recognition: setting up another problem that makes the original problem (accelerating streams) worse |
| In 2002, ice streams flowing into the Larsen B ice shelf in West Antarctica accelerated dramatically after much of the shelf collapsed into the ocean. | What it says: Real example: when the Larsen B ice shelf collapsed in 2002, the ice streams feeding into it sped up dramatically What it does: Provides concrete evidence/example of the ice shelf theory Source/Type: Historical fact/evidence Connection to Previous Sentences: This proves sentence 10's prediction with real data. Perfect example of theory → evidence pattern Visualization: 2002: Larsen B ice shelf (the "speed bump") disappears → ice streams (the "traffic") suddenly speeds up dramatically What We Know So Far: Both underground lubrication AND ice shelf removal cause stream acceleration - and we have proof it happened Reading Strategy Insight: This is confirmation, not new complexity. The abstract idea gets concrete proof |
| This suggests that global warming could allow far more land-based ice to flow into the ocean, raising sea levels rapidly. | What it says: The Larsen B example suggests global warming could cause much more ice to flow to the ocean, making sea levels rise quickly What it does: Concludes with the broader implication and consequence Source/Type: Author's conclusion based on evidence Connection to Previous Sentences: This takes the specific Larsen B case (sentence 11) and extrapolates to the global consequence. This is the "so what?" conclusion that ties everything together Visualization: If one ice shelf collapse caused dramatic acceleration, imagine multiple collapses worldwide = massive amounts of ice → ocean → rapidly rising sea levels What We Now Understand: Complete story: friction + draining lakes = lubrication → faster streams + global warming = shelf collapse → even faster streams → rapid sea level rise Reading Strategy Insight: Classic ending: specific evidence supports broad conclusion. Feel confident - the passage has built logically to this final point |
To explain why Antarctic ice sheets are unstable despite scientists' long-held belief that they were stable, by describing new research that reveals the causes of accelerating ice loss.
In this passage, the author walks us through a scientific mystery and its solution:
Antarctic ice sheets are not stable as scientists once believed, and the combination of underground water lubrication and global warming removing natural barriers could cause ice to flow into the ocean much faster, leading to rapid sea level rise.
The question asks us to identify what the passage most strongly suggests as the cause of ice stream acceleration. This is asking for the primary mechanism that explains why some ice streams have sped up, based on the evidence and explanations provided in the passage.
From our passage analysis, we identified two main causes of ice stream acceleration:
The passage provides concrete evidence for the second mechanism with the 2002 Larsen B example: "ice streams flowing into the Larsen B ice shelf in West Antarctica accelerated dramatically after much of the shelf collapsed into the ocean." Our analysis noted this as "perfect example of theory → evidence pattern."
While the passage discusses multiple factors contributing to ice stream acceleration, the question asks what the passage "most strongly suggests." The Larsen B example provides the clearest cause-and-effect evidence: ice shelf collapse (caused by warming ocean temperatures) directly led to dramatic acceleration. The passage explicitly states that ice shelves "are sensitive to ocean temperatures" and that "global warming can melt them." This creates a direct causal chain: increased ocean temperatures → ice shelf melting → removal of barriers that slow ice streams → acceleration.
Why It's Right:
Key Evidence: "In 2002, ice streams flowing into the Larsen B ice shelf in West Antarctica accelerated dramatically after much of the shelf collapsed into the ocean."
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