Sir

In your Special Report 'Cutting out the chemicals', you discuss the possible shift of regulatory control of hydrofluorocarbons (HFCs) from the UN Framework Convention on Climate Change to the Montreal Protocol (Nature 457, 518–519; 2009). Since then, amendments to the protocol have been proposed that would establish HFC phase-down schedules, in parallel with similar provisions in US climate legislation — as mentioned in your News story 'Climate burden of refrigerants rockets' (Nature 459, 1040–1041; 2009). However, although a bridge may be under construction to address the problem of HFCs, there is another gap that requires action.

Neither treaty controls emissions of the remaining chlorofluorocarbons and other ozone-depleting substances that HFCs were designed to replace. Besides the damage they cause to stratospheric ozone, these are also greenhouse gases that are up to 11,000 times more potent than carbon dioxide. Large quantities of chlorofluorocarbons produced before the phase-out deadlines remain in use in older appliances and buildings. These 'banks' are capable of releasing into the atmosphere the equivalent of 18 gigatonnes of carbon dioxide (see the International Panel on Climate Change and the Technology and Economic Assessment Panel's Safeguarding the Ozone Layer and the Global Climate System, 2005, at http://tinyurl.com/ndtd9z), about one-third of which will be emitted by 2015 unless recovered and destroyed under proper incentives.

Policy-makers have an immediate opportunity to prevent hundreds of millions of tonnes of greenhouse-gas emissions every year. Possible strategies could include: counting banked ozone-depleting substances as controlled greenhouse gases; allowing greenhouse-gas offsets for certified destruction of banked ozone-depleting substances; setting rigorous standards and protocols to quantify and verify projects for destruction of ozone-depleting substances; and establishing incentives for replacement technologies that avoid substitution of ozone-depleting substances with other greenhouse gases.

Recognizing these banks as greenhouse gases in domestic legislation and international agreements would spur carbon markets to finance cost-effective collection, as well as transport and destruction of unwanted ozone-depleting substances. In the interim, regional and state cap-and-trade schemes could take comparable steps to mobilize projects, permanently removing a major threat to ozone and climate, and accelerate the transition to advanced technologies. There is only a narrow window to address this opportunity, but the timing is right for action.