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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.
如果你问一位经济学家，某种稀缺物品的价格应该是多少，他们通常会说，要看人们愿意为此付出多少钱。
They will go on to say that the best way to establish that willingness is through markets.
他们还会说，建立这种意愿的最佳方式是通过市场。
There are various systems that price carbon dioxide that way.
有各种各样的系统以这种方式为二氧化碳定价。
But they do not provide the same answers.
但它们提供的答案并不相同。
And nor do they tally with what economists think might actually be the right answer.
而且它们也不符合经济学家认为的正确答案。
To most people the cost of emitting a tonne of carbon dioxide appears to be nothing.
对大多数人来说，排放一吨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.
他们必须为燃料付费，他们必须为任何燃烧它的东西付费，但一旦它成为废气，他们就可以把它随手放掉。
In a few cases, they might even find someone to buy it—a fizzy-drink maker, say, or a dj who wants dry ice.
在少数情况下，他们甚至会找到别人来购买——比如一个碳酸饮料制造商，或者一个想要干冰的流行音乐播音员。
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，尽管排放二氧化碳可能不会让排放者付出任何代价，经济学家仍然坚持认为它们有价值，而且是负面价值。
This is because the emitted carbon dioxide does harm to the environment, almost all of which is felt by people other than the emitter.
这是因为排放的二氧化碳对环境有害，几乎所有这些影响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).
要考虑这些外部效应，就意味着要考虑从海滨财产和农业生产力的损失，到热浪造成的死亡(以及寒流避免的死亡)等一切因素。
This “social cost of carbon” is estimated through modelling.
这种“碳的社会成本”是通过建模来估算的。
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.
这些模型必须做出假设，比如应该在多大程度上贴现未来损失的影响，以及如何应对气候损害估计中的固有不确定性。
Different assumptions yield wildly different costs.
不同的假设会产生截然不同的成本。
In many places the outputs of such models are used to guide policy.
在许多地方，这些模型的结果被用来指导政策。
In America, for example, cost-benefit analyses which make use of the social cost of carbon feed into decisions about fuel standards.
例如，在美国，会将碳排放的社会成本纳入燃料标准决策的成本效益分析。
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).
其政府目前估计，每吨二氧化碳(或其他温室气体产生的温室效应)的社会成本为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.
而如果美国政府听从环境保护署的建议，该成本将增加到190美元。因为环境保护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.
在唐纳德·特朗普执政期间，如果只考虑其他美国人的成本，成本则降至5美元。
