雅思阅读 45: The Carbon Awakening in Frozen Ground（冻土中的碳苏醒）

改编由 Science Advances / Frontiers / Alfred Wegener Institute（2025-2026）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2025.1654065/full

Reading Passage

A. For tens of thousands of years, the soils of the circumpolar north have held their breath. Layer upon layer of plant roots, leaves and animal remains, frozen solid by Arctic winters, have accumulated in permafrost — ground that remains at or below zero Celsius for two or more consecutive years. Buried in that frozen material is twice as much carbon as currently floats in the atmosphere. Until recently, the assumption among climate modellers was that this vast reservoir would remain locked away, accessible to microorganisms only slowly as the surface warmed. That assumption is now being dismantled by a wave of field studies showing that the Arctic is warming roughly four times faster than the rest of the planet — a phenomenon called Arctic amplification, driven by the loss of reflective sea ice and the exposure of dark, heat-absorbing ocean — and that the thaw unlocks ancient carbon at a rate that accelerates rather than slows as temperatures rise. The result is a self-reinforcing feedback loop: warming releases carbon, carbon warms the atmosphere, more warming releases more carbon. Climatologists refer to this as a positive feedback, meaning one that amplifies the original change rather than dampening it. Such loops, once they cross a threshold, can run away on their own.

B. The mechanism is microbial. When permafrost thaws, bacteria and fungi that have been dormant in the ice awaken and begin to digest the newly accessible organic matter. In well-drained soils exposed to air, the digestion produces carbon dioxide. In waterlogged bogs and lake beds, where oxygen is scarce, the same digestion produces methane — a greenhouse gas more than eighty times more potent than carbon dioxide over a twenty-year horizon, though it breaks down within decades. Researchers from a range of institutions, publishing in journals such as Frontiers in Microbiology and Science Advances in 2025, have shown that the microbial response is remarkably consistent across permafrost samples from Alaska, Siberia and northern Canada. Common bacterial families rise to dominance within weeks of thaw, and the ratio of methane to carbon dioxide shifts in favour of methane as temperatures climb. The surprise was not that microbes respond to warmth — that is textbook biochemistry — but that the response is so consistent across soils that have been frozen for tens of millennia, as though the same dormant communities were waiting everywhere for the signal to wake.

C. A further surprise came from looking at plant roots. A 2025 study used carbon-isotope labelling — growing Arctic grasses in an atmosphere enriched with a rare, heavy form of carbon dioxide, then tracking where that carbon ended up — to show that live roots in thawing soils actively accelerate the breakdown of ancient organic matter. The phenomenon is called "rhizosphere priming": root exudates, which are sugar-rich compounds pumped into the soil to feed beneficial fungi, also feed the general microbial community, which in turn decomposes older, frozen carbon faster than it would without the plants' stimulation. Across a range of Arctic and sub-Arctic soils, the presence of roots increased carbon dioxide loss derived from soil organic matter by about thirty-one per cent over 370 days. The effect lasted longer in permafrost than in the overlying "active layer" that thaws every summer, suggesting that deep, previously frozen carbon is more vulnerable to plant-driven priming than models had assumed. This matters because Arctic vegetation is itself spreading northward as the climate warms, meaning the very shrubs and trees that might sequester carbon on the surface are simultaneously waking carbon from below.

D. Yet not all of the released carbon reaches the atmosphere. A separate 2026 study led by the Alfred Wegener Institute found that a large fraction of ancient permafrost carbon eroded into the Laptev Sea ends up trapped in seabed sediments, where it is broken down slowly rather than instantaneously. The team measured how much carbon was delivered to the sea during the last deglaciation, roughly eleven thousand years ago, by analysing sediment cores, and compared it with current coastal erosion rates. They concluded that the ocean acts as a buffer — but a finite one. Deep Arctic lake sediments, meanwhile, tell a less reassuring story: a 2025 analysis showed that methane produced anaerobically in those deep sediments has twice the global warming potential of shallow, aerobic carbon dioxide at warmer temperatures, and that these deep sources have been overlooked in most estimates. The discrepancy means that models which assume shallow, oxygen-rich decomposition may be underestimating the climate impact of thaw by a factor that researchers are only beginning to quantify. A single unmodelled lake basin, the authors note, could account for a measurable fraction of the permafrost feedback.

E. The policy implications are awkward. Even if human societies achieved net-zero carbon dioxide emissions tomorrow, a 2025 Science Advances modelling paper found, permafrost would continue to lose carbon for centuries, accumulating about fourteen petagrams of carbon by the end of the simulations — an amount roughly equivalent to all the carbon released by the United States over the past three decades. The methane fraction of that loss would grow as warming continued. The finding does not excuse fossil-fuel emissions, but it does mean that stabilising the climate requires even deeper cuts than models that ignore permafrost have assumed. It also means that the Arctic's frozen archive — once imagined as a permanent vault — is a slow-release tap that no global treaty has yet managed to turn off. Some researchers have proposed geoengineering schemes to re-freeze the thawing ground, but these are untested at scale and carry their own ecological risks. The consensus, for now, is that the most reliable way to slow the permafrost feedback is to cut global emissions faster, before the tap opens wider. Any delay, the authors warn, commits future generations to a feedback that no geoengineering scheme is known to reverse.

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 microbes unlock frozen carbon ii. Why Arctic winters are longer than tropical nights iii. An unexpected role for living plant roots iv. The ocean as an imperfect buffer v. Why the policy problem is especially difficult vi. A history of Arctic exploration vii. How methane is used as cooking fuel

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. How much faster is the Arctic warming than the rest of the planet? A. About the same rate. B. Roughly twice as fast. C. About four times faster. D. Ten times faster.

6. What is "rhizosphere priming"? A. The freezing of plant roots in winter. B. Live roots accelerating the microbial breakdown of ancient soil carbon. C. A type of Arctic plant. D. The chemical treatment of farmland.

7. What did the Alfred Wegener Institute study conclude? A. All permafrost carbon immediately reaches the atmosphere. B. Much permafrost carbon eroded into the sea ends up trapped in sediments. C. The Laptev Sea is warming faster than the land. D. Coastal erosion has stopped entirely.

8. According to the 2025 Science Advances model, how much carbon could permafrost continue to lose after net-zero? A. About 14 petagrams. B. About 14 million tonnes. C. None at all. D. About 1.4 kilograms.

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. Permafrost holds roughly twice as much carbon as is currently in the atmosphere.
10. Methane is less potent than carbon dioxide over a twenty-year horizon.
11. Rhizosphere priming was found to last longer in permafrost than in the active layer.
12. Deep Arctic lake sediments produce only carbon dioxide, never methane.
13. The permafrost carbon feedback was first modelled by researchers at the Massachusetts Institute of Technology.

Questions 14-15

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

When permafrost thaws, dormant microbes awaken and digest ancient organic matter, producing either carbon dioxide in well-drained soils or (14) __________ in waterlogged bogs. Even after net-zero emissions, permafrost would continue to lose (15) __________ for centuries.

答案与解析

← 上一篇 | 返回雅思焦点 | 下一篇 →