The Greenhouse Effect and Negative Externalities | 温室效应与负外部性

📚 The Greenhouse Effect and Negative Externalities | 温室效应与负外部性

The greenhouse effect is widely regarded by economists as one of the most significant examples of a negative externality, since the marginal social cost of carbon-intensive production substantially exceeds the marginal private cost paid by producers. This article explains how greenhouse gas (GHG) emissions constitute a negative production externality, why free markets over-allocate resources to carbon-intensive activities, and what policy instruments governments can use to correct this market failure.

温室效应被经济学家广泛认为是负外部性最典型的例证之一——碳密集型生产的边际社会成本远远超过生产者所支付的边际私人成本。本文将阐释温室气体排放如何构成生产负外部性，为什么自由市场会将过多资源配置于碳密集型活动，以及政府可以使用哪些政策工具来纠正这一市场失灵。

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1. The Greenhouse Effect and Economic Relevance | 温室效应与经济相关性

Greenhouse gases — carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) — trap long-wave radiation in the Earth’s atmosphere, preventing heat from escaping into space. While a certain natural level of these gases makes life possible, human activity since industrialisation has raised atmospheric CO₂ from approximately 280 ppm to over 420 ppm.

温室气体——包括二氧化碳（CO₂）、甲烷（CH₄）和一氧化二氮（N₂O）——将长波辐射截留在地球大气层中，阻止热量逃逸到太空。虽然一定水平的自然温室气体使得生命成为可能，但自工业化以来，人类活动已将大气二氧化碳浓度从约 280 ppm 推升至 420 ppm 以上。

Nearly every production and consumption activity — burning fossil fuels in electricity generation, manufacturing, transport and agriculture — releases emissions that accumulate globally. The resulting climate change generates damage: rising sea levels, more frequent extreme weather events, loss of biodiversity, and falling agricultural yields in vulnerable regions.

几乎所有的生产和消费活动——发电、制造、交通运输和农业中的化石燃料燃烧——都会释放温室气体并在全球积累。由此引发的气候变化造成诸多损害：海平面上升、极端天气事件更加频繁、生物多样性丧失，以及脆弱地区农业产量下降。

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2. Defining Negative Externalities | 负外部性的定义

A negative externality occurs when a production or consumption decision imposes an uncompensated cost on a third party who is not involved in the transaction. Because the producer or consumer does not have to pay for this damage, the market price does not reflect the true social cost of the activity.

负外部性是指生产或消费决策对未参与交易的第三方造成未获补偿的成本。由于生产者或消费者无须为该损害付费，市场价格无法反映该活动的真实社会成本。

• Production externalities: a factory releasing pollutants into a river affects downstream fishing communities. The firm pays wages, raw material and capital costs, but not the cost of cleaning the river or the lost fishing income.

  生产外部性：工厂将污染物排入河流，影响了下游渔民生计。企业支付工资、原材料和资本成本，却无须支付河流清理费用或渔民损失的收入。

• Consumption externalities: smoking in a public space exposes passive smokers to health risks; driving a petrol car contributes to local air pollution for pedestrians.

  消费外部性：在公共场所吸烟令被动吸烟者承受健康风险；驾驶燃油汽车为行人带来局部空气污染。

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3. The Greenhouse Effect as a Negative Externality | 温室效应作为负外部性

Climate change is the archetypal negative externality because the emitter and the victim are geographically and temporally separated. A coal-fired power plant in one country generates benefits for its own consumers, while the damage falls on people across the globe, including future generations who played no part in emitting.

气候变化是典型的负外部性，因为排放者与受害者在地理上和时间上相分离。一个国家的燃煤电厂为本地消费者产生收益，而损害却落在全球所有人身上，包括并未参与排放的后代。

The marginal external cost (MEC) of carbon dioxide is substantial. Scientists and economists estimate that each additional tonne of CO₂ imposes damage in the region of $50–$200 depending on discount rates and assumptions about climate sensitivity. Yet this damage is not part of any market transaction, so producers do not factor it into their supply decisions.

二氧化碳的边际外部成本（MEC）相当可观。科学家和经济学家估计，每增加一吨 CO₂ 造成的损害约为 50–200 美元，具体取决于折现率和对气候敏感性的假设。然而，这笔损害不包含在任何市场交易之中，因此生产者不会将其纳入供给决策。

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4. Market Failure: Marginal Private Cost and Marginal Social Cost | 市场失灵：边际私人成本与边际社会成本

The greenhouse effect illustrates a classic market failure through the divergence between marginal private cost (MPC) and marginal social cost (MSC). For a firm producing electricity from coal:

温室效应通过边际私人成本（MPC）与边际社会成本（MSC）之间的背离，完美地展示了经典的市场失灵。以燃煤发电企业为例：

MSC = MPC + MEC

MPC corresponds to the firm’s supply curve — it includes coal, labour, capital and transport costs. MEC captures the climate damage caused by each additional unit of output. Adding the two gives the supply curve for society as a whole.

MPC 对应企业的供给曲线——包含煤炭、劳动、资本和运输成本。MEC 则反映每增加一单位产出所造成的气候损害。将两者相加即得到整个社会的供给曲线。

Consequently, the free-market equilibrium produces output Q₁ where MPC = marginal benefit (MB), which exceeds the socially optimal output Q* where MSC = MB. The price in the free market, P₁, is too low relative to true social cost P*. The triangle between Q₁ and Q* represents deadweight welfare loss to society.

因此，自由市场均衡产量 Q₁ 出现在 MPC = 边际收益（MB）处，高于社会最优产量 Q*（MSC = MB）。自由市场价格 P₁ 相对于真实社会成本 P* 过低。Q₁ 与 Q* 之间的三角形面积即为社会的无谓福利损失。

The overproduction of carbon-intensive goods means the free market allocates too many resources to activities that generate emissions, and too few to clean alternatives and climate adaptation — a misallocation of scarce resources.

碳密集型商品的过度生产意味着自由市场将过多资源配置给产生排放的活动，而配置给清洁替代能源和气候适应的资源则不足——这是稀缺资源的配置失误。

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5. The Social Cost of Carbon | 碳的社会成本

The social cost of carbon (SCC) is the dollar value of the present and future damages caused by one additional tonne of CO₂ emitted today. It is an attempt to attach a monetary price to the MEC discussed above.

碳的社会成本（SCC）是指今天多排放一吨 CO₂ 所造成的当前及未来损害的美元价值。它是试图为上述 MEC 赋予货币价格的做法。

• Components include: reduced agricultural productivity, heat-related mortality, coastal flooding damage, forced migration, and ecosystem collapse.

  组成部分包括：农业生产力下降、高温关联死亡、沿海洪水损害、被迫移民以及生态系统崩溃。

• Estimation difficulty: the SCC requires assumptions about discount rates (how much we value future generations), the rate of climate sensitivity, and the economic valuation of non-market goods such as biodiversity.

  估算困难：SCC 需要关于折现率（我们有多重视后代）、气候敏感速率以及生物多样性等非市场物品经济估值的一系列假设。

Despite uncertainty, the SCC serves as a benchmark for policy: if the carbon price in a tax or trading scheme equals the SCC, the market outcome will approximate the socially optimal level of emissions.

尽管存在不确定性，SCC 仍作为政策基准：如果碳税或交易机制中的碳价等于 SCC，市场结果将趋近于社会最优排放水平。

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6. Policy Solution 1 — Carbon Taxation | 政策方案一：碳税

A carbon tax is a Pigouvian tax designed to internalise the externality. The optimal tax is set equal to the marginal external cost at the socially optimal output level, i.e. t = MEC. This raises the firm’s private marginal cost until it coincides with marginal social cost.

碳税是一种庇古税（Pigouvian tax），旨在将外部性内部化。最优税率应设定为社会最优产量下等于边际外部成本的水平，即 t = MEC。这会使企业的边际私人成本上升，直至与边际社会成本重合。

New MPC = Original MPC + t = MSC

The carbon tax has several strengths. It creates a continuous incentive for firms to reduce emissions through innovation, because every unit of output avoided saves the tax payment. It raises public revenue, which can be used to reduce distortionary taxes (a “double dividend”) or fund green research and development.

碳税具有若干优势。它为企业的减排创新提供了持续激励，因为每避免一单位产出就节省相应的税款。碳税还能增加公共财政收入，可用于降低扭曲性税收（”双重红利”）或资助绿色研发。

However, carbon taxes face problems. Measuring the true MEC is highly uncertain, so setting the correct tax rate is difficult in practice. A tax is highly visible and politically unpopular, because it raises the price of electricity, petrol and domestic heating. It can also be regressive: poorer households spend a larger share of their income on energy-intensive goods, so they suffer disproportionately unless the revenue is redistributed.

然而，碳税也面临诸多问题。真实 MEC 的衡量高度不确定，因此现实中难以设定正确的税率。碳税非常显眼并且在政治上不受欢迎，因为它抬高电价、油价和家庭供暖价格。碳税还可能是累退的：较贫困家庭将更大比例的收入用于能源密集型商品，因此除非政府将收入再分配，否则他们会承受不成比例的负担。

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7. Policy Solution 2 — Cap-and-Trade Systems | 政策方案二：总量管制与交易制度

Cap-and-trade, the approach underlying the EU Emissions Trading System (EU ETS), works by setting a legal ceiling on total emissions and distributing or auctioning tradable permits to firms. Firms with low marginal abatement costs can cut emissions and sell surplus permits; firms with high abatement costs buy additional permits instead.

总量管制与交易制度是欧盟排放交易体系（EU ETS）所采用的方案。该制度对总排放量设定法定上限，并向企业分配或拍卖可交易的排放配额。边际减排成本较低的企业可以削减排放并出售多余配额；边际减排成本较高的企业则购买额外配额。

In equilibrium, the permit price equals the marginal abatement cost across all firms, so emissions are reduced at the lowest possible total cost. This static efficiency is the scheme’s main economic attraction.

在均衡状态下，配额价格等于所有企业统一的边际减排成本，因此排放削减以最低

Published by TutorHao | Economics Revision Series | aleveler.com

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