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The social cost of carbon is notional, if sometimes consequential.
碳排放的社会成本是名义上的，尽管有时会产生重大影响。
The costs charged in carbon-pricing schemes are real.
碳定价方案的成本是真实存在的。
Such systems typically place a cap on emissions from a certain sector, and then distribute (often by auction) permits to emit that are equal to that cap.
这类制度通常会对某一行业设定排放上限，然后分配(通常通过拍卖)与该上限相等的排放许可。
Firms then trade permits in a “compliance market”.
然后，企业在“合规市场”上交易许可证。
Economists like these market-based “cap and trade” schemes because they discover the firms most willing to make cuts.
经济学家喜欢这些以市场为基础的“总量控制与交易”计划，因为它们发掘了最愿意减排的企业。
That spreads the burden in an efficient way and lowers the total cost of keeping emissions below the cap.
这以一种有效的方式分散了负担，降低了将排放控制在上限以下的总成本。
But even when efficiently spread, the total cost is something which most of the governments experimenting with such schemes have wanted to keep low: the average price charged in the world’s emissions-trading systems is about $20.
但是，即使能有效地分散成本，大多数试验此类计划的政府也想要将总成本保持在较低水平:世界排放交易系统的平均价格约为20美元。
The imf estimates that for Paris-compliant decarbonisation the price per tonne would have to reach $280 on all emissions by 2050.
国际货币基金组织估计，要实现符合《巴黎协定》要求的脱碳，到2050年，所有排放的碳价必须达到每吨280美元。
That, the fund drily notes, “might be politically unpalatable in many countries, despite carbon pricing’s effectiveness”.
国际货币基金组织讽刺地指出，“尽管碳定价具有有效性，但这可能在政治上令许多国家心中不快”。
A third way to establish a price is to find people willing to be paid not to emit, thus “offsetting” the emissions of those who do.
第三种确定价格的方法是，找到那些愿意有偿不排放的人，从而“抵消”其他人的排放。
This has various practical drawbacks and two fundamental flaws.
这有各种实际缺陷和两个根本缺陷。
One is that offsetting is voluntary; no one has to do it.
其一，抵消是自愿的; 没人非得这么做。
The second is that offset emissions are still emissions.
第二，抵消排放仍然是排放。
They still warm the planet.
它们仍然在使地球变暖。
CDR avoids the second problem.
二氧化碳脱除避免了第二个问题。
If a tonne of carbon dioxide is removed from the atmosphere in one place at the same time as another is emitted somewhere else, the harm done is pretty much zero.
如果把一吨二氧化碳从一个地方的大气中移除，同时在其他地方排放另一吨二氧化碳，那么造成的危害几乎为零。
Unfortunately, the costs of removal are currently a lot higher than the estimates of the social cost of carbon favoured by governments or the prices charged in cap-and-trade schemes; they outstrip offset prices over a hundred-fold.
不幸的是，目前清除碳排放的成本远远高于政府所青睐的碳排放社会成本估计，或者高于限额与交易计划所收取的价格; 它们比抵消价格高出100多倍。
The idea of a market where the cost of emitting carbon dioxide is the price you have to pay to have it removed is very appealing.
在市场中，排放二氧化碳的成本等于你必须支付的去除二氧化碳的价格，这种想法非常有吸引力。
Actually creating one will be very hard.
实际上首创这一举动将是非常困难的。
