雅思阅读 85: When Wetlands Release Their Frozen Carbon（湿地释放冻结碳之时）

改编自 Science Advances / AGU（2025年2月）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://www.science.org/doi/10.1126/sciadv.adn1056

Reading Passage

A. When people picture climate change, they usually imagine factories, exhaust pipes and smokestacks. Yet among the largest single sources of methane on Earth, a greenhouse gas more than eighty times more powerful than carbon dioxide over a twenty-year horizon, are places that look almost innocent: bogs, marshes, peatlands and Arctic tundra. Natural wetlands emit roughly 160 million tonnes of methane every year, close to a third of the total entering the atmosphere. For millennia these wetlands have functioned as gentle regulators, fixing carbon from the air, releasing a portion back as methane, and keeping the rest buried in waterlogged peat where oxygen cannot reach it. That balance is now being disturbed. As the planet warms, and especially as the frozen soils of the high north begin to thaw, scientists are asking whether the world's wetlands are about to tip from a steady, manageable source into an accelerating one — a feedback loop in which warming causes emissions, which cause more warming, and which no human policy can easily reverse once it is under way.

B. The chemistry at work is older than agriculture. In a waterlogged wetland, plant roots and dead leaves fall into an oxygen-free layer. Microbes called methanogens consume that organic matter in the absence of oxygen and excrete methane, which bubbles out through the mud, escapes through plant stems, or is trapped in frozen soil until spring. Opposing them are methanotrophs, bacteria that live in the surface layer and consume much of the rising methane before it reaches the air. The net emission is the small difference between these two processes, and it is extraordinarily sensitive to temperature and water level. A warmer soil speeds both groups of microbes, but the methanogens tend to respond faster; a drier soil, by contrast, allows oxygen in, which suppresses methane production and encourages its consumption. A wetland that floods for a week can flip from a sink to a source and back again within a single season. Wetlands are therefore not simple pipes. They are ecosystems in balance, and the balance can shift either way depending on how warming rearranges the water. This is why model projections disagree so sharply: small changes in assumed drainage, plant community or microbial community can move the estimated global emission by tens of millions of tonnes a year.

C. The Arctic stores something else entirely: permafrost. Across Siberia, Alaska, northern Canada and the Hudson Bay Lowlands, frozen ground has held undecomposed plant material in deep freeze for tens of thousands of years. Estimates suggest the carbon locked in permafrost is roughly twice the amount currently in the atmosphere. As the climate warms, the active layer above this ice deepens, and previously frozen organic matter thaws. If the thaw produces a wet landscape — flooded depressions, thaw lakes, saturated peat — anaerobic microbes immediately go to work, and methane, rather than carbon dioxide, is the product. Recent satellite-based studies of the Hudson Bay Lowlands, one of the largest contiguous peatlands on Earth, have detected summertime methane enhancements closely tracking soil temperature, with the strongest signals appearing after 2021. Warmer and wetter soils emitted more; spatial variation, however, outweighed year-to-year variation, meaning that local drainage matters as much as the thermometer. A single thaw lake can out-gas a square kilometre of dry peat, and a single season of unusually heavy rain can double a site's annual emission.

D. A study published in Science Advances in 2025 added a twist that had been overlooked. Most models had assumed that the future change in wetland methane would be driven mainly by rising temperature and carbon dioxide, which fertilise plants and so supply more organic fuel to methanogens. The new analysis found that a third factor was just as important: the decline of sulphate deposition. Across Europe and North America, decades of clean-air regulation have cut sulphur emissions from power stations, and the sulphate that once fell on wetlands has diminished. Sulphate-loving bacteria outcompete methanogens for the same organic fuel; with less sulphate around, methanogens get a larger share. Combining this effect with rising CO2, the researchers projected that global wetland methane emissions could rise by as much as 85 million tonnes per year by the end of the century under a high-emissions scenario — a substantial addition to a greenhouse gas already rising faster than policy-makers expected. The authors stressed that the numbers carry large uncertainty, but the direction was consistent across models.

E. Why should anyone who does not live in a bog care? Methane's atmospheric lifetime is short — around a decade — which means that rapid cuts to methane produce rapid benefits, and it is partly for this reason that governments have pledged to reduce human-caused methane emissions by thirty per cent by 2030. The pledge, known as the Global Methane Pledge, now includes over a hundred countries. Yet wetlands are not, however, an emissions source that can simply be regulated like a gas pipeline. They are living systems responding to temperature, drainage, plant community and chemistry in ways that vary from swamp to swamp. Drying a peatland can reduce methane in the short term but exposes stored peat to fire and decomposition, releasing carbon dioxide for centuries. Rewetting a drained bog can bring methane back. The most defensible conclusion, researchers say, is not that wetlands should be drained or left alone, but that their feedbacks must be modelled honestly — and that the fastest, safest lever governments still hold is cutting fossil-fuel methane, which gives the climate a chance to stabilise before the wetlands themselves begin to run away. Whether the Paris Agreement's one-and-a-half-degree target leaves enough headroom for the northern wetlands is, at the time of writing, one of the open questions in climate science.

Questions 1-4

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

List of Headings i. The biological balance beneath the water ii. Why natural wetlands are the largest single source of methane iii. The carbon store locked in frozen northern soils iv. An overlooked factor: falling sulphate deposition v. Why wetland management is neither simple nor optional vi. The history of clean-air legislation in Europe vii. How methane is produced by burning fossil fuels

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. According to the passage, why is methane especially concerning? A. It remains in the atmosphere for thousands of years. B. It is far more effective than CO2 at trapping heat over two decades. C. It is released only by human activity. D. It cannot be measured from satellites.

6. What determines whether a wetland emits more or less methane? A. Only the number of fish living in the water. B. The balance between methanogens and methanotrophs, modulated by water and temperature. C. Whether the wetland is located in a national park. D. The amount of sulphate already present in fossil fuels.

7. What did the Hudson Bay Lowlands satellite study find? A. Methane emissions were entirely unrelated to soil temperature. B. Local differences in drainage mattered more than year-to-year changes. C. Peatlands in the region had stopped emitting methane. D. The strongest signals appeared before 2021.

8. Why did declining sulphate deposition increase methane emissions? A. Sulphate directly absorbs methane in wetland water. B. Sulphate-loving bacteria no longer outcompete methanogens for organic fuel. C. Sulphate causes plants to grow more slowly. D. Sulphate is a greenhouse gas stronger than methane.

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. Natural wetlands contribute roughly one third of annual global methane emissions.
10. Permafrost currently stores about the same amount of carbon as the atmosphere.
11. Drying a peatland always reduces its net climate impact.
12. Methane has an atmospheric lifetime of around one decade.
13. Most governments have already met their 2030 methane reduction pledge.

Questions 14-15

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

In waterlogged soils, (14) __________ produce methane in the absence of oxygen, while (15) __________ consume much of it at the surface.

答案与解析

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