IELTS Reading 105: Clouds and Climate Sensitivity

Adapted from Cesana et al., Nature Climate Change / NASA. IELTS Reading Section 3 difficulty, approximately 1,050 words. Source: https://ntrs.nasa.gov/citations/20210000511

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Reading Passage

A. Of all the uncertainties clouding the forecast of global warming, few matter as much as the clouds themselves. Global climate models broadly agree that rising greenhouse gases will warm the planet, but they disagree fiercely on how much. Doubling the concentration of carbon dioxide, a standard benchmark, raises the world somewhere between 1.5 and 4.5 degrees — a twofold difference that changes every projection of sea level, drought and crop yield. Researchers have long suspected that the principal reason for this spread lies not in the oceans or the ice sheets but in low-lying tropical clouds, and in how they will react as the air above them heats up. A thick deck of low cloud reflects sunlight straight back to space and cools the surface; if such decks thin out or retreat in a warmer world, the warming accelerates. The fear cuts the other way too: if low clouds thicken, they would act as an additional brake on heating. Pinning down which way the balance tips, and by how much, has therefore become one of the central tasks of modern climatology. It is no exaggeration to say that the difference between a two-degree and a four-degree century for human civilisation runs, in large part, through a layer of cloud a few hundred metres above the tropical ocean — a layer so thin that a satellite sensor, rather than a weather balloon, is needed to see it clearly.

B. A new generation of satellite observations has begun to do exactly that. Rather than treating all low clouds as one undifferentiated blob, researchers used a dataset that distinguishes, by shape and altitude, between two tropical varieties. Shallow "fair-weather" cumulus are the scattered puffy little clouds of a trade-wind morning; stratocumulus are the broad, flat, grey sheets that cloak much of the eastern ocean basins. Over a decade of measurements, the team paired these cloud observations with the two environmental factors thought to govern them: the temperature of the sea beneath, and the strength of the temperature inversion that caps the cloudy layer from above. Separating the two cloud types turned out to be decisive, because they behave in remarkably different ways. Cumulus proved almost indifferent to surface warming, while stratocumulus proved highly sensitive to both warmer seas and a weakening inversion. The distinction matters because the two clouds cover different regions and are governed by different physics; lumping them together had hidden exactly the behaviour that now needed to be measured. The ten-year satellite record was long enough to observe how each type responded as the seas warmed a little from one year to the next, but short enough to sidestep the slow, century-scale warming trend that would have confounded the analysis.

C. This matters because the world's climate models appear to have the two clouds backwards. Most of them assume that shallow cumulus, in particular, will retreat as the planet warms, adding a strong positive feedback that amplifies heating. The satellite record shows no such sensitivity at all. Worse, the models also tend to paint stratocumulus across regions where the real world grows only cumulus, and to make those clouds melt away too readily when the sea warms. The result is that, in much of the tropics, the models inflate the very feedback — the thinning of low cloud — that drives their most alarming projections. The observationally inferred low-cloud feedback, the team found, was roughly half the size previously estimated, with a far narrower range.

D. Why do the models get the clouds wrong? The answer leads back to the tropical Pacific. The behaviour of stratocumulus depends partly on the strength of the temperature inversion above it, and that inversion is in turn governed by the east-west contrast in Pacific surface temperatures. Over the past six decades, the eastern Pacific has warmed only weakly relative to the west — a pattern that holds the inversion strong and the low-cloud feedback in check. Yet most models fail to reproduce this observed trend. Instead they assume the eastern Pacific will warm as quickly as the west, weakening the inversion, thinning the clouds, and pushing the system toward a higher sensitivity. If the historical pattern of slow eastern-Pacific warming continues, the team calculates, the likely warming from a doubling of carbon dioxide lands near 3.5 degrees — a moderate figure, squarely in the middle of the traditional range. The result is not a prediction but a constraint: it narrows the range of plausible futures from above, leaving the lower end largely untouched. Policymakers reading such work should not conclude that the clouds will save them; they should conclude, rather, that the worst case looks a little less likely than it did.

E. The finding does not close the debate. It implies that the highest-sensitivity models overstate low-cloud feedback by a factor of two, while the lowest-sensitivity models, which predict almost no feedback at all, err in the opposite direction — compensating areas of positive and negative effect that cancel out implausibly neatly. It also rests on an assumption about the future — that the Pacific will continue to behave as it has for the past half-century — which could itself be overturned if the eastern basin suddenly begins to warm faster. Even so, it illustrates how observational constraints can narrow the vast gulf between climate models. Rather than arguing from simulations alone, climatologists are now measuring the present-day behaviour of real clouds, and using those measurements to test whether the virtual clouds in their computers look anything like the sky above. The lesson is not that the threat of warming has evaporated, but that the upper end of the possible range looks less likely than it once did — provided, of course, that the Pacific keeps its old habits. Should it break them, the models may yet prove right after all.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. How satellites tell the two low clouds apart ii. Why models' low-cloud feedback is inflated iii. The tropical Pacific — and why it matters iv. What the constraint does (and does not) settle v. Why clouds are the biggest source of uncertainty vi. The history of weather satellites vii. How cumulus clouds form from moisture

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

Choose the correct letter, A, B, C or D.

5. Why do low tropical clouds matter so much to climate projections? A. They reflect sunlight and their possible thinning amplifies warming. B. They produce most of the world's rainfall. C. They are the main source of carbon dioxide. D. They absorb all incoming heat.

6. What did the satellite observations reveal about shallow cumulus? A. They are highly sensitive to warming. B. They are almost insensitive to surface warming. C. They disappear completely in a warmer world. D. They grow thicker as the sea cools.

7. Why do the researchers say models have the clouds "backwards"? A. Models predict cumulus will retreat, but observations show they do not. B. Models make clouds too high in the atmosphere. C. Models ignore stratocumulus entirely. D. Models use data from the Arctic only.

8. What climate sensitivity does the study estimate if Pacific trends continue? A. Around 1.5 degrees. B. Around 3.5 degrees. C. Around 6 degrees. D. Exactly 4.5 degrees.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. Doubling carbon dioxide raises global temperature by between 1.5 and 4.5 degrees in models.
10. Stratocumulus clouds are insensitive to changes in the temperature inversion above them.
11. Most climate models reproduce the historical Pacific warming trend accurately.
12. The study concludes that global warming is no longer a serious threat.
13. The CASCCAD satellite dataset covers the years 2007 to 2016.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

The behaviour of stratocumulus depends partly on the strength of the temperature (14) __________ above it, which is governed by the east-west contrast in Pacific surface (15) __________.

Answers & Explanations

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