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Ask an economist what something scarce should cost and they will normally say whatever someone is willing to pay for it.
如果你問一位經濟學家，某種稀缺物品的價格應該是多少，他們通常會說，要看人們愿意為此付出多少錢。
They will go on to say that the best way to establish that willingness is through markets.
他們還會說，建立這種意愿的最佳方式是通過市場。
There are various systems that price carbon dioxide that way.
有各種各樣的系統以這種方式為二氧化碳定價。
But they do not provide the same answers.
但它們提供的答案并不相同。
And nor do they tally with what economists think might actually be the right answer.
而且它們也不符合經濟學家認為的正確答案。
To most people the cost of emitting a tonne of carbon dioxide appears to be nothing.
對大多數人來說，排放一噸二氧化碳的成本似乎微不足道。
They have to pay for fuel, they have to pay for whatever burns it, but once it is an exhaust gas they can just let it go.
他們必須為燃料付費，他們必須為任何燃燒它的東西付費，但一旦它成為廢氣，他們就可以把它隨手放掉。
In a few cases, they might even find someone to buy it—a fizzy-drink maker, say, or a dj who wants dry ice.
在少數情況下，他們甚至會找到別人來購買——比如一個碳酸飲料制造商，或者一個想要干冰的流行音樂播音員。
But though the emissions may not cost the emitter anything, economists insist that they still have a value, and that it is a negative one.
但是，盡管排放二氧化碳可能不會讓排放者付出任何代價，經濟學家仍然堅持認為它們有價值，而且是負面價值。
This is because the emitted carbon dioxide does harm to the environment, almost all of which is felt by people other than the emitter.
這是因為排放的二氧化碳對環境有害，幾乎所有這些影響都由排放者以外的人感受到了。
To take into account those externalities means taking into account everything from the loss of seafront property and farm productivity to deaths caused by heatwaves (as well as those avoided in cold snaps).
要考慮這些外部效應，就意味著要考慮從海濱財產和農業生產力的損失，到熱浪造成的死亡(以及寒流避免的死亡)等一切因素。
This “social cost of carbon” is estimated through modelling.
這種“碳的社會成本”是通過建模來估算的。
Those models must make assumptions, such as how much the effects of a future loss should be discounted and what to do about the uncertainty inherent to estimates of climate damage.
這些模型必須做出假設，比如應該在多大程度上貼現未來損失的影響，以及如何應對氣候損害估計中的固有不確定性。
Different assumptions yield wildly different costs.
不同的假設會產生截然不同的成本。
In many places the outputs of such models are used to guide policy.
在許多地方，這些模型的結果被用來指導政策。
In America, for example, cost-benefit analyses which make use of the social cost of carbon feed into decisions about fuel standards.
例如，在美國，會將碳排放的社會成本納入燃料標準決策的成本效益分析。
The government currently estimates the social cost at $51 for every tonne of carbon dioxide (or for an amount of some other greenhouse gas which provides the same warming).
其政府目前估計，每噸二氧化碳(或其他溫室氣體產生的溫室效應)的社會成本為51美元。
If the administration heeded the advice of its own Environmental Protection Agency, which approaches modelling in a different way, the cost would increase to $190.
而如果美國政府聽從環境保護署的建議，該成本將增加到190美元。因為環境保護署采用不同的建模方法。
During Donald Trump’s administration, when only costs on other Americans were considered, it fell to $5.
在唐納德·特朗普執政期間，如果只考慮其他美國人的成本，成本則降至5美元。
