📚 The Carbon Cycle | 碳循环
Carbon is the fundamental building block of life on Earth. It moves between the atmosphere, living organisms, oceans, and the Earth’s crust in a continuous loop known as the carbon cycle. Understanding this cycle is essential for explaining how ecosystems function and for grasping the impact of human activities on climate.
碳是地球上生命的基本构成元素。它在大气、生物体、海洋和地壳之间不断循环，这个循环被称为碳循环。理解这一循环对于解释生态系统如何运作以及掌握人类活动对气候的影响至关重要。
1. Introduction to the Carbon Cycle | 碳循环简介
The carbon cycle describes how carbon atoms are recycled in nature. Carbon is present in all organic molecules and also in inorganic forms such as carbon dioxide (CO₂) in the air, carbonate ions in seawater, and fossil fuels buried underground. The cycle connects photosynthesis, respiration, decomposition, and combustion.
碳循环描述了碳原子在自然界中如何被循环利用。碳存在于所有有机分子中，也以无机形式存在，例如空气中的二氧化碳（CO₂）、海水中的碳酸根离子以及埋藏在地下的化石燃料。该循环将光合作用、呼吸作用、分解和燃烧连接起来。
Carbon moves between reservoirs – the atmosphere, biomass, oceans, sediments, and rocks – over timescales ranging from days to millions of years. On an IGCSE Edexcel Science level, we focus on the rapid biological exchanges and the slower geological processes that influence climate.
碳在碳库之间移动——大气、生物量、海洋、沉积物和岩石——时间尺度从数天到数百万年不等。在IGCSE Edexcel科学的层次上，我们重点介绍快速的生物交换和较慢的地质过程，这些过程影响着气候。
2. Carbon Reservoirs | 碳库
The major reservoirs that store carbon are: the atmosphere (as CO₂ and methane), the biosphere (in living and dead organic matter), the oceans (dissolved CO₂ and carbonates), and the lithosphere (fossil fuels, limestone, and sedimentary rocks). Each reservoir exchanges carbon with others through natural processes.
储存碳的主要碳库有：大气（以CO₂和甲烷形式存在）、生物圈（活的或死的有机质中）、海洋（溶解的CO₂和碳酸盐）以及岩石圈（化石燃料、石灰岩和沉积岩）。每个碳库通过自然过程与其他碳库交换碳。
- Atmosphere – contains around 0.04% CO₂ by volume but plays a vital role in regulating Earth’s temperature.
- Biosphere – carbon stored in plants, animals, and soil organic matter.
- Oceans – the largest active carbon sink, holding dissolved CO₂ and carbonate minerals.
- Lithosphere – long-term storage in fossil fuels and carbonate rocks like limestone (CaCO₃).
- 大气——体积占比约0.04%的CO₂，对调节地球温度起至关重要作用。
- 生物圈——储存在植物、动物和土壤有机质中的碳。
- 海洋——最大的活跃碳汇，含有溶解的CO₂和碳酸盐矿物。
- 岩石圈——以化石燃料和碳酸盐岩石（如石灰岩CaCO₃）形式长期储存碳。
3. Photosynthesis – Fixing Carbon | 光合作用——固定碳
Photosynthesis removes CO₂ from the atmosphere and converts it into glucose. Green plants and algae use light energy to drive this reaction. The simplified equation is:
光合作用从大气中吸收CO₂并将其转化为葡萄糖。绿色植物和藻类利用光能驱动该反应。简化的方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This process is the primary route by which inorganic carbon enters the living world. Carbon becomes part of carbohydrates, proteins, and fats in plant tissues. On a global scale, photosynthesis fixes approximately 120 billion tonnes of carbon each year.
该过程是无机碳进入生命世界的主要途径。碳成为植物组织中碳水化合物、蛋白质和脂肪的一部分。在全球范围内，光合作用每年固定约1200亿吨碳。
In aquatic environments, phytoplankton perform photosynthesis, fixing dissolved CO₂. This marine photosynthesis accounts for nearly half of the total carbon fixed on Earth, highlighting the ocean’s crucial role in the cycle.
在水生环境中，浮游植物进行光合作用，固定溶解的CO₂。海洋光合作用几乎占地球总固碳量的一半，这凸显了海洋在循环中的关键作用。
4. Respiration – Returning CO₂ | 呼吸作用——释放二氧化碳
All living organisms, including plants, respire to release energy from glucose. Aerobic respiration returns CO₂ to the atmosphere, effectively reversing the capture by photosynthesis:
包括植物在内的所有生物体都通过呼吸作用从葡萄糖中释放能量。有氧呼吸将CO₂释放回大气，基本上逆转了光合作用的捕获过程：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Respiration occurs continuously in cells and is a major natural source of atmospheric CO₂. Decomposers, such as bacteria and fungi, also respire as they break down dead organic matter, contributing significantly to carbon recycling.
呼吸作用在细胞中持续发生，是大气CO₂的主要自然来源。分解者，如细菌和真菌，在分解死亡有机质时也会进行呼吸作用，对碳的循环再利用贡献巨大。
It is important to remember that while green plants take up CO₂ during the day by photosynthesis, they also release CO₂ at night through respiration. The net effect of a growing plant is carbon removal from the air, but the balance depends on the stage of life and environmental conditions.
需要牢记的是，虽然绿色植物在白天通过光合作用吸收CO₂，但夜晚也会通过呼吸作用释放CO₂。一株生长中的植物对空气的净效果是去除碳，但平衡取决于生命阶段和环境条件。
5. Feeding and Decomposition | 摄食与分解
Carbon moves through food chains when animals consume plants and other animals. The organic carbon in the food is used for growth, energy, and building tissues. Through digestion and assimilation, carbon becomes part of the animal’s body or is released as CO₂ during respiration.
当动物食用植物和其他动物时，碳便沿着食物链移动。食物中的有机碳被用于生长、供能和构建组织。通过消化和同化作用，碳成为动物身体的一部分，或在呼吸作用中作为CO₂释放。
When organisms die, decomposers break down complex organic molecules into simpler substances. This decomposition releases CO₂ through respiration and returns mineral nutrients, including carbon compounds, to the soil and atmosphere. Without decomposition, nutrients would remain locked in dead bodies, and life could not be sustained.
当生物死亡时，分解者将复杂的有机分子分解为较简单的物质。分解过程通过呼吸作用释放CO₂，并将包括含碳化合物在内的矿物质养分归还至土壤和大气。没有分解，养分将被锁定在遗骸中，生命将无法持续。
In waterlogged or oxygen-poor conditions, such as peat bogs, decomposition is incomplete. Over geological time, partially decomposed plant material can form peat, which can eventually turn into coal. This sequesters carbon away from the atmosphere for millions of years.
在积水或贫氧的环境中，如泥炭沼泽，分解作用不完全。经过地质时间，部分分解的植物物质可形成泥炭，最终变为煤。这使碳远离大气，被固定数百万年。
6. Combustion of Fossil Fuels | 化石燃料的燃烧
Fossil fuels – coal, oil, and natural gas – formed from the remains of ancient organisms buried under heat and pressure. When burned, they combine with oxygen in a rapid oxidation reaction, releasing large amounts of CO₂ and energy. The general equation for hydrocarbon combustion is:
化石燃料——煤、石油和天然气——是由古代生物遗骸在高温高压下形成的。燃烧时，它们与氧气发生快速氧化反应，释放出大量CO₂和能量。碳氢化合物燃烧的通式为：
hydrocarbon + O₂ → CO₂ + H₂O + energy
Human industrial activity, transport, and power generation have dramatically increased the rate of fossil fuel combustion since the Industrial Revolution. This releases carbon that had been locked away for millions of years, upsetting the natural carbon balance and increasing atmospheric CO₂ levels.
自工业革命以来，人类的工业活动、运输和发电大大加快了化石燃料的燃烧速度。这将原本被封存数百万年的碳释放出来，打破了自然的碳平衡，导致大气CO₂浓度升高。
In IGCSE Edexcel Science, you should be able to contrast natural sources of CO₂ (respiration, volcanic eruptions) with anthropogenic sources (burning fossil fuels, deforestation, cement production). The latter are primarily responsible for the rapid rise in greenhouse gas concentrations.
在IGCSE Edexcel科学中，你应当能区分CO₂的自然来源（呼吸作用、火山喷发）与人为来源（燃烧化石燃料、砍伐森林、水泥生产）。后者是温室气体浓度快速上升的主要原因。
7. The Role of Oceans | 海洋的作用
Oceans play a dual role in the carbon cycle. CO₂ dissolves directly into surface water, forming a weak carbonic acid (H₂CO₃). This dissolved CO₂ can be used by marine plants for photosynthesis. The reaction is:
海洋在碳循环中扮演双重角色。CO₂直接溶解到表层水中，形成弱碳酸（H₂CO₃）。溶解的CO₂可被海洋植物用于光合作用。反应为：
CO₂ + H₂O ⇌ H₂CO₃
Additionally, marine organisms extract dissolved carbonates to build shells and skeletons made of calcium carbonate (CaCO₃). When these organisms die, their shells sink, eventually forming limestone and chalk sediments. This process locks carbon into the lithosphere over millions of years.
此外，海洋生物提取溶解的碳酸盐以构建由碳酸钙（CaCO₃）构成的贝壳和骨骼。当这些生物死亡后，贝壳沉入海底，最终形成石灰岩和白垩沉积物。这一过程将碳锁入岩石圈，时间长达数百万年。
However, increasing atmospheric CO₂ leads to more dissolution in oceans, causing ocean acidification. The lower pH adversely affects organisms that rely on carbonate for shells, such as corals and molluscs, disrupting marine ecosystems.
然而，大气中CO₂的增加导致更多碳溶解到海洋中，引起海洋酸化。较低的pH值会对依赖碳酸盐形成外壳的生物（如珊瑚和软体动物）产生不利影响，从而破坏海洋生态系统。
8. Human Impact on the Carbon Cycle | 人类对碳循环的影响
Two major human activities upset the carbon cycle: burning fossil fuels and deforestation. Combustion releases ancient carbon at a rate far exceeding natural removal processes. Deforestation reduces the number of trees that absorb CO₂ through photosynthesis; it can also release carbon stored in forest biomass and soils when trees are burned or decay.
两个主要的人类活动打乱了碳循环：燃烧化石燃料和砍伐森林。燃烧释放古老碳的速度远远超过自然消除过程。砍伐森林减少了通过光合作用吸收CO₂的树木数量；当树木被烧毁或腐烂时，还会释放储存在森林生物量和土壤中的碳。
Agricultural practices also contribute: livestock release methane (CH₄), a potent greenhouse gas, through enteric fermentation, and rice paddies emit methane from anaerobic decomposition. Industrial processes, notably cement manufacturing, release CO₂ when limestone is heated.
农业实践也是因素之一：牲畜通过肠道发酵释放甲烷（CH₄）这一强效温室气体，稻田在缺氧分解中也会排放甲烷。工业过程，特别是水泥制造，在石灰岩加热时释放CO₂。
Combined, these activities have raised atmospheric CO₂ from about 280 parts per million (ppm) before the Industrial Revolution to over 420 ppm today. In IGCSE exams, you may be asked to interpret graphs showing this rapid increase and relate it to the enhanced greenhouse effect.
这些活动合在一起已将大气CO₂从工业革命前的约280 ppm（百万分之）提高至如今的420 ppm以上。在IGCSE考试中，你可能会被要求解读展示这种快速上升的图表，并将其与增强的温室效应联系起来。
9. The Greenhouse Effect and Carbon | 温室效应与碳
CO₂ and CH₄ are greenhouse gases that trap infrared radiation reflected from the Earth’s surface, warming the lower atmosphere. This natural greenhouse effect is essential – without it, Earth would be too cold for most life. However, additional emissions intensify the effect, leading to global warming.
CO₂和CH₄是温室气体，能够捕获从地球表面反射的红外辐射，使低层大气变暖。这种天然的温室效应是不可或缺的——没有它，地球将会太冷而不适合多数生命生存。然而，额外的排放增强了这一效应，导致全球变暖。
Through the carbon cycle, we can see why carbon is so central to climate regulation. Disturbing the balance between carbon sources and sinks directly alters the planet’s energy budget. Feedback mechanisms, such as melting permafrost releasing methane, can further accelerate warming.
通过碳循环可以看出碳为何对气候调节如此关键。扰乱碳源和碳汇之间的平衡会直接改变地球的能量收支。反馈机制，例如永冻层融化释放甲烷，会进一步加速变暖。
In Edexcel IGCSE Science, candidates are expected to describe the consequences of global warming: rising sea levels, changing weather patterns, loss of biodiversity, and melting ice caps. All these are linked, fundamentally, to increased carbon emissions disrupting the cycle.
在Edexcel IGCSE科学中，考生需要描述全球变暖的后果：海平面上升、天气模式变化、生物多样性丧失以及冰盖融化。从根本上说，所有这些都与碳排放增加打乱碳循环密切相关。
10. Balancing the Carbon Cycle | 平衡碳循环
Restoring balance requires both reducing emissions and enhancing natural sinks. Strategies include switching to renewable energy sources (solar, wind, hydropower), improving energy efficiency, planting trees (afforestation), protecting existing forests, and developing carbon capture and storage technology.
恢复平衡需要减少排放并增强自然碳汇。策略包括转向可再生能源（太阳能、风能、水力能）、提高能源效率、植树造林、保护现有森林以及开发碳捕获与储存技术。
On an individual level, reducing energy consumption, recycling, and choosing a plant-based diet can lower one’s carbon footprint. While individual actions alone are insufficient, widespread behavioural change, combined with government policies, can make a difference.
在个人层面上，减少能源消耗、回收利用并选择以植物为主的饮食方式可以降低碳足迹。虽然仅靠个人行动还不够，但普遍的行为改变加上政府政策可以产生影响。
From a scientific perspective, understanding the carbon cycle helps us predict how different interventions might affect CO₂ levels. For example, reforestation initially removes carbon, but mature forests eventually reach an equilibrium where respiration balances photosynthesis, limiting further uptake.
从科学的角度看，理解碳循环有助于我们预测不同的干预措施可能如何影响CO₂水平。例如，植树造林最初会移除碳，但成熟森林最终会达到一个平衡，此时呼吸作用与光合作用持平，限制了进一步的吸收。
11. Summary of Key Processes | 关键过程总结
The carbon cycle can be summarised by tracking a carbon atom. It might start in the atmosphere as CO₂, be fixed into glucose by a leaf, become part of a caterpillar when the leaf is eaten, return to the air through the caterpillar’s respiration, dissolve into the ocean, precipitate as calcium carbonate, and eventually be subducted into the Earth’s mantle over eons.
碳循环可以通过追踪一个碳原子来总结。它可能从大气中作为CO₂开始，被叶片固定为葡萄糖，当叶片被吃掉时成为毛虫身体的一部分，通过毛虫的呼吸作用返回空气中，溶入海洋，以碳酸钙形式沉淀，最终在亿万年间被俯冲进入地幔。
| Process | Carbon Movement |
|---|---|
| Photosynthesis | Atmosphere → Biosphere |
| Respiration | Biosphere → Atmosphere |
| Decomposition | Dead biomass → Soil/Atmosphere |
| Combustion | Lithosphere → Atmosphere |
| Dissolving in oceans | Atmosphere → Hydrosphere |
| Shell formation | Hydrosphere → Lithosphere |
| Volcanic activity | Lithosphere → Atmosphere |
过程与碳移动：光合作用：大气→生物圈；呼吸作用：生物圈→大气；分解：死亡生物质→土壤/大气；燃烧：岩石圈→大气；海洋溶解：大气→水圈；贝壳形成：水圈→岩石圈；火山活动：岩石圈→大气。
In an IGCSE exam, you may be asked to draw or label a diagram of the carbon cycle. Emphasise the arrows and clearly state the processes that add or remove CO₂ from the atmosphere. Remember that photosynthesis and dissolution in oceans are major removals; respiration, decomposition, combustion, and volcanic eruptions are major additions.
在IGCSE考试中，你可能需要画出或标注碳循环示意图。强调箭头，并清楚地说明增加或减少大气中CO₂的过程。请记住，光合作用和海洋溶解是主要的移除过程；呼吸作用、分解、燃烧和火山喷发是主要的增加过程。
12. Exam Tips for IGCSE Edexcel Science | IGCSE Edexcel科学考试技巧
When answering carbon cycle questions, link each process to specific organisms or reservoirs: for example, 'phytoplankton perform photosynthesis, absorbing CO₂ from ocean water.' Use precise terminology: 'carbon sink', 'sequestration', 'greenhouse gas', and 'anthropogenic emissions'. Avoid vague statements like 'carbon goes into the air'.
回答碳循环问题时，要将每个过程与特定的生物体或碳库联系起来：例如，“浮游植物进行光合作用，从海水中吸收CO₂”。使用准确的术语：“碳汇”、“固存”、“温室气体”和“人为排放”。避免模糊的表述，如“碳进入到空气中”。
Be prepared to interpret line graphs showing seasonal fluctuations in CO₂ levels. The annual rise and fall reflect the balance between photosynthesis in summer (lower CO₂) and respiration in winter (higher CO₂), especially in the Northern Hemisphere where most landmass exists.
准备好解读展示CO₂浓度季节波动的折线图。年度的升降反映了夏季光合作用（CO₂较低）与冬季呼吸作用（CO₂较高）之间的平衡，尤其是在拥有最多陆地的北半球。
Candidates often lose marks by confusing the role of plants – plants fix carbon in photosynthesis but also release CO₂ by respiration. Ensure you specify the net effect over a 24-hour cycle or across a growing season. Also, remember that deforestation has a double impact: it reduces CO₂ uptake and often releases stored carbon through burning.
考生常因混淆植物的作用而失分——植物在光合作用中固定碳，但也通过呼吸作用释放CO₂。要确保指明在24小时周期或整个生长季节中的净效应。此外，要记住砍伐森林有双重影响：减少CO₂的吸收，并常通过燃烧释放储存的碳。
Lastly, carbon cycle questions often integrate with other topics: food webs, energy transfer, evolution, and even acid rain. A solid understanding of the cycle allows you to draw connections across the entire IGCSE Edexcel Science syllabus.
最后，碳循环类题目经常与其他主题综合在一起：食物网、能量传递、进化甚至酸雨。扎实理解碳循环，使你能够跨越整个IGCSE Edexcel科学教学大纲建立联系。
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