📚 Water and Carbon Cycles | 水循环与碳循环
Water and carbon cycles are fundamental Earth systems that shape climate, ecosystems and human life. For Edexcel A-Level Geography, you need to understand how these cycles operate as open and closed systems, how energy drives their stores and fluxes, and how feedback loops respond to natural and human pressures.
水循环和碳循环是塑造气候、生态系统和人类生活的基本地球系统。在Edexcel A-Level地理中，你需要理解这些循环如何作为开放系统和封闭系统运行、能量如何驱动其储存和通量，以及反馈回路如何响应自然和人为压力。
1. Systems Framework | 系统框架
A system is a set of interacting components that form a linked whole. In physical geography, systems have stores, flows, inputs and outputs. The global water cycle is often treated as a closed system because no water enters or leaves the Earth-atmosphere system, while a drainage basin is an open system with inputs from precipitation and outputs to the sea or atmosphere.
系统是一组相互作用、构成联系整体的组成部分。在自然地理学中，系统具有储存、流动、输入和输出。全球水循环通常被视为封闭系统，因为水不会进入或离开地球-大气系统，而流域是一个开放系统，输入来自降水，输出进入海洋或大气。
Energy and matter behave differently: matter cycles through the system, but energy flows through and is eventually radiated back to space. This distinction helps explain why cycles remain balanced over long timescales even though short-term imbalances occur.
能量与物质的行为不同：物质在系统中循环，但能量流经系统并最终辐射回太空。这一区别有助于解释为什么循环在长时间尺度上保持平衡，即使短期不平衡也会发生。
2. The Global Water Cycle | 全球水循环
The global water cycle describes the continuous movement of water between the atmosphere, oceans, land and biosphere. The main stores are oceans (about 97% of all water), ice caps and glaciers, groundwater, surface water, soil moisture and atmospheric water vapour. Fluxes include evaporation, condensation, precipitation, transpiration, runoff and infiltration.
全球水循环描述了水在大气、海洋、陆地和生物圈之间的持续运动。主要储存包括海洋（约占所有水的97%）、冰盖和冰川、地下水、地表水、土壤水分和大气水汽。通量包括蒸发、凝结、降水、蒸腾、径流和下渗。
- Ocean store: ~1,370,000 × 10³ km³ | 海洋储存：约1,370,000 × 10³ km³
- Ice sheets and glaciers: ~29,000 × 10³ km³ | 冰盖和冰川：约29,000 × 10³ km³
- Groundwater: ~23,000 × 10³ km³ | 地下水：约23,000 × 10³ km³
- Atmosphere: ~13 × 10³ km³ | 大气：约13 × 10³ km³
Residence times vary greatly: water vapour remains in the atmosphere for about 9 days, while deep groundwater can stay for thousands of years. These differences influence how quickly water responds to climate change.
停留时间差异很大：水汽在大气中停留约9天，而深层地下水可停留数千年。这些差异影响水对气候变化的响应速度。
3. Drainage Basin Processes | 流域过程
A drainage basin is an area of land drained by a river and its tributaries. It is an open system with inputs (precipitation), outputs (evapotranspiration and river discharge) and internal stores (interception, surface storage, soil water, channel storage). The water balance equation links these components:
流域是由河流及其支流排水的陆地区域。它是一个开放系统，具有输入（降水）、输出（蒸散发和河流流量）和内部储存（截留、地表储存、土壤水、河道储存）。水平衡方程将这些组成部分联系起来：
Precipitation = Evapotranspiration + Runoff + Change in Storage
降水 = 蒸散发 + 径流 + 储存变化
Key processes include infiltration, throughflow, percolation and groundwater flow. Soil type, relief, vegetation and land use control the speed of these flows. Urbanisation often reduces infiltration and increases surface runoff, raising flood risk.
关键过程包括下渗、侧向流、渗透和地下水流。土壤类型、地形、植被和土地利用控制这些流动的速度。城市化通常减少下渗并增加地表径流，提高洪水风险。
4. The Carbon Cycle: Stores and Fluxes | 碳循环：储存与通量
Carbon is stored in five major reservoirs: lithosphere (sedimentary rocks and fossil fuels), oceans, soil organic matter, atmosphere and biosphere. The largest store is the lithosphere, containing over 99% of Earth's carbon, but most of it is locked in slow-cycling carbonate rocks.
碳储存在五个主要库中：岩石圈（沉积岩和化石燃料）、海洋、土壤有机质、大气和生物圈。最大的储存是岩石圈，含有超过99%的地球碳，但其中大部分被锁定在缓慢循环的碳酸盐岩中。
The fast carbon cycle involves exchanges of carbon between the atmosphere, oceans, plants and soils over days to centuries. Fluxes include photosynthesis, respiration, decomposition, ocean-atmosphere exchange and combustion.
快速碳循环涉及碳在大气、海洋、植物和土壤之间从几天到几个世纪的交换。通量包括光合作用、呼吸作用、分解、海洋-大气交换和燃烧。
| Store | 储存 | Approximate size (PgC) | 约大小 (PgC) |
|---|---|
| Sedimentary rocks | 沉积岩 | 100,000,000 |
| Deep ocean | 深海 | 38,000 |
| Fossil fuels | 化石燃料 | 4,000 |
| Soil | 土壤 | 1,500 |
| Atmosphere | 大气 | 860 |
| Biosphere | 生物圈 | 560 |
5. Carbon Transfers and Transformations | 碳转移与转化
Photosynthesis fixes CO₂ into organic matter, while respiration and decomposition return it to the atmosphere. In the oceans, CO₂ dissolves, forming carbonic acid, then bicarbonate and carbonate ions:
光合作用将CO₂固定为有机质，而呼吸作用和分解将其返回大气。在海洋中，CO₂溶解，形成碳酸，然后形成碳酸氢根和碳酸根离子：
CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2H⁺
This chemical equilibrium controls ocean acidity and the biological pump. Phytoplankton take up carbon in surface waters; their shells sink, exporting carbon to the deep ocean and seafloor sediments over long timescales.
这种化学平衡控制着海洋酸度和生物泵。浮游植物在表层水中吸收碳；它们的壳体下沉，在长时间尺度上将碳输出到深海和海底沉积物中。
On land, carbon moves through litterfall, root respiration and microbial decomposition. Net primary productivity (NPP) is the balance between carbon gained by photosynthesis and lost by plant respiration.
在陆地上，碳通过凋落物、根系呼吸和微生物分解移动。净初级生产力（NPP）是光合作用获得的碳与植物呼吸损失的碳之间的平衡。
6. Feedback Loops | 反馈回路
A feedback loop occurs when a change in one part of a system causes a further change that either amplifies or dampens the original change. Positive feedback amplifies change, while negative feedback restores balance.
反馈回路发生在系统某一部分的变化引起进一步变化，从而放大或抑制原始变化时。正反馈放大变化，而负反馈恢复平衡。
Example of positive feedback: rising temperatures thaw permafrost, releasing methane and CO₂, which further warm the climate. Example of negative feedback: higher CO₂ concentrations stimulate plant growth, which removes more CO₂ from the atmosphere.
正反馈示例：温度升高使永久冻土融化，释放甲烷和CO₂，进一步使气候变暖。负反馈示例：较高的CO₂浓度刺激植物生长，从而从大气中去除更多CO₂。
In the water cycle, warmer air holds more moisture, which can increase cloud cover. Clouds can reflect solar radiation (cooling, negative) or trap longwave radiation (warming, positive), making cloud feedbacks uncertain.
在水循环中，较暖的空气含有更多的水汽，这会增加云量。云可以反射太阳辐射（冷却，负反馈）或吸收长波辐射（增温，正反馈），使云反馈变得不确定。
7. Natural Variability: ENSO and Wildfires | 自然变率：ENSO与野火
The El Niño Southern Oscillation (ENSO) is a periodic change in Pacific Ocean circulation that alters rainfall and drought patterns worldwide. During El Niño, the eastern Pacific warms, trade winds weaken and rainfall shifts eastward, causing drought in Indonesia and Australia but floods in Peru.
厄尔尼诺-南方涛动（ENSO）是太平洋环流的周期性变化，改变全球降雨和干旱模式。在厄尔尼诺期间，东太平洋变暖，信风减弱，降雨向东移动，导致印度尼西亚和澳大利亚干旱，但秘鲁洪水。
Wildfires are a natural part of many carbon cycle systems. They release stored carbon rapidly into the atmosphere and can switch a forest from a carbon sink to a carbon source for years. In boreal forests, fire frequency is increasing with climate change, creating a positive feedback loop.
野火是许多碳循环系统的自然组成部分。它们迅速将储存的碳释放到大气中，并可能使森林在数年内从碳汇变为碳源。在北方森林中，火灾频率随着气候变化而增加，形成正反馈回路。
8. Human Impacts on the Cycles | 人类活动对循环的影响
Human activity has altered both cycles at unprecedented rates. Fossil fuel combustion transfers carbon from the slow geological cycle to the fast atmospheric cycle, increasing atmospheric CO₂ from about 280 ppm in pre-industrial times to over 420 ppm today.
人类活动以前所未有的速度改变了两个循环。化石燃料燃烧将碳从缓慢的地质循环转移到快速的大气循环，使大气CO₂从工业化前的约280 ppm增加到今天的420 ppm以上。
Land-use change, especially deforestation and afforestation, changes carbon storage and water flows. Deforestation reduces evapotranspiration, lowers rainfall recycling, and releases carbon from biomass and soil. Urbanisation increases impermeable surfaces, speeding up runoff and reducing groundwater recharge.
土地利用变化，尤其是森林砍伐和植树造林，改变碳储存和水流。森林砍伐减少蒸散发，降低降雨再循环，并从生物量和土壤中释放碳。城市化增加不透水表面，加速径流并减少地下水补给。
Agriculture also impacts both cycles: irrigation depletes groundwater stores, while fertiliser use and livestock increase nitrous oxide and methane emissions. Dam construction traps sediment and organic carbon, altering riverine fluxes.
农业也影响两个循环：灌溉消耗地下水储存，而化肥使用和牲畜增加氧化亚氮和甲烷排放。大坝建设截留沉积物和有机碳，改变河流通量。
9. Case Study: Amazon Rainforest | 案例研究：亚马逊雨林
The Amazon rainforest is a critical carbon sink and a major driver of regional hydrology. Its trees store around 80-120 billion tonnes of carbon, and the forest recycles approximately 50-70% of rainfall through evapotranspiration, maintaining a humid regional climate.
亚马逊雨林是一个关键的碳汇和区域水文的主要驱动因素。其树木储存约800-1200亿吨碳，森林
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