很多人认为，削减我们排放到大气中的二氧化碳是个好主意。但对于怎样做才是正确，却没有多少人有明智的见解。

A lot of people think it a good idea to reduce the amount of carbon dioxide we pump into the atmosphere. But not many have sensible ideas about the correct way to do that.

最近，英国《金融时报》的一位投稿人断言："个人碳交易计划比征收碳排放税更公平、更有效，因为它能迅速大量削减消费量。"他显然是被那些二氧化碳搞糊涂了。除非知道会征什么税，或是配额的多少，否则就不可能计算出配给制是否会比税收促成更多的减排量。

One contributor to the FT recently asserted that, "An individual carbon trading scheme is more equitable and effective than carbon taxation as it reduces consumption quickly and dramatically." All that carbon dioxide has clearly addled his thinking. It isn't possible to work out whether rationing would reduce emissions more than a tax until you know what the tax might be, or how generous the ration.

在谈到削减碳排放时，"削减多少"与同等重要的"如何削减"是不同的问题。明智的选择介于征收碳排放税和某种污染许可交易安排之间。（政界人士喜欢在自己喜欢的措施上大把花钱；这符合他们的本性，却让环境方面的进步更难以实现。）对于任何水平的环境税，你都能通过使用许可配额，得到同样的碳排放价格和减排量。

When it comes to reducing carbon emissions, the question "How much?" is separate from the equally important question "How?" The sensible choice is between a carbon tax and some kind of scheme to trade pollution permits. (Politicians prefer to bung cash at favoured initiatives; it's in their nature, but makes environmental gains harder to come by.) And for any level of environmental tax you can get the same carbon price and emissions reduction by using a permit quota instead.

这并不意味着两种体系是等同的。一个区别是系统的管理成本。英国皇家艺术促进协会(Royal Society for the Encouragement of Arts)个人碳排放许可量研究项目负责人马特o普雷斯科特(Matt Prescott)描绘了一幅令人兴奋的图景：可以将碳排放许可量便宜地充入信用卡。而我认为进行税收管理要更简单些。另一个区别是收入的去向。税收将让钱直接流入征税的政府。然后主要就取决于政府如何使用了：如果更多地分配给穷人，那么税收就比个人碳排放许可量更具进步性。

That doesn't mean the two systems are equivalent. One difference is the cost of administering the system. Matt Prescott, the director of a Royal Society for the encouragement of Arts (RSA) research project into personal carbon allowances, paints an exciting picture of cheaply loading your carbon permits on to a credit card. I think it would be simpler just to administer a tax. A second difference is where the revenue goes. A tax directs cash to the government levying it. A lot then depends on how the revenue is used: if it's distributed more heavily towards the poor, taxes are more progressive than a personal carbon allowance.

许可制会使最先拥有许可的人变得富有：如果是通过拍卖发放许可证，那就又是政府；如果每个人的配额相同，那就是社会中的每一个人。目前的流行做法是向高污染公司发放许可证，这便于收买能源游说团体，但其它就没什么用处了。

Permits enrich whoever starts with the permit: the government again, if the permits are auctioned off; or each person in society, if everybody gets an equal ration. The current vogue is to give permits to highly polluting corporations, which is handy for buying off the energy lobby but otherwise not much use.

第三个区别可能最重要，但却很少有人讨论。碳税给了我们碳排放价格方面的确定性，但却没有排量方面的。可交易的许可制度给了我们排量方面的确定性，却没有价格方面的。

A third difference is perhaps the most important but rarely discussed. A carbon tax gives us certainty about the price of carbon but not the quantity of emissions. A tradable permit scheme gives us certainty about the quantity of emissions, but not the price.

因此，问题在于哪方面的不确定最具破坏性。比如说，如果我们征税，希望能减排15%，结果只减少了5%；而按照目前可交易的许可制度，我们本来预计碳排放价格是每吨25英镑，实际却是每吨75英镑，那是不是意味着，征税的破坏性没有排放权制度那样严重？

The question, then, is where the uncertainty is most damaging. Say we impose a tax, hoping for a 15 per cent reduction in emissions but getting only a 5 per cent fall. Is that less serious than a tradable permit scheme where we expected a carbon price of ?25 a tonne but got a price of ?75 a tonne?

的确如此。明年排量的不确定性并不十分令人担心，因为气候变化不是由明年发生的事造成的，而是由工业革命以来排放量的日积月累造成的。如果我们定错了税率，导致一年的排量过多，那是很容易修正的。另一方面，经济更容易受到短期震荡的影响：如果许可配额定错了，哪怕只有一年，结果也可能造成持久性损害。

It is indeed. The uncertainty about next year's emissions is not very worrying, because climate change is caused not by what happens next year, but by the accumulation of emissions since the industrial revolution. If we get the tax wrong and emit too much one year, it will be easy to fix. The economy, on the other hand, is more vulnerable to short-term shocks: get the permit quota wrong, even for one year, and you could cause lasting damage.

有些环境学家现在叫嚷着说，我是个白痴，没有努力去拯救地球，他们错了。你越害怕气候变化，税收或配额的力度就应越强。我的意思是，无论做出何种决定，使用税收的风险总是较小。政府付出了如此大的努力在好事上征税，比如储蓄和开支。如果它缺乏勇气对坏东西征税、以求改变，那将是一种耻辱。

Those environmentalists currently yelling that I am an idiot and am not committed to saving the planet have missed the point. The more you fear climate change, the more stringent the tax or quota should be. What I am saying is that whatever you decide about that, it is less risky to use a tax. The government expends so much effort taxing good things, such as saving and spending money. It would be a shame if it lacked the courage to tax something bad for a change.