It involves placing a limit on the volume of emissions that can be produced around the country each year, and then auctioning tradable permits to pollute.

这个策略设置了全国每年可排放污染气体的总量限额，然后以拍卖的方式出售排污许可。

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Under the ETS, they are required to buy tradable permits for a gradually rising portion of their emissions: this year the EU will give the airlines permits to cover 85% of them.

ECONOMIST: Planes and pollution