From Dr Andreas Vogel and Dr Gernot Wagner.

Sir, You argue against imposing any form of carbon border tariffs by invoking “slippery slope” reasoning (“The greening of globalisation”, editorial, January 4): such tariffs, you say, could result in tit-for-tat restrictions. Possible, but most reactionary tariffs would and probably should be struck down by the World Trade Organisation. Yet the WTO would likely allow carbon tariffs combined with domestic caps or taxes, for good reasons. They provide a unilateral solution to an inherently global and complex problem without violating trade rules.