General Comments On Longlived Greenhouse Gases

In 1994, Ravishankara and Lovejoy wrote that the release of any long-lived species into the atmosphere should be viewed with great concern [15]. They noted that the CFCs, with relatively 'short' lifetimes of ca. 100 a, have had a disastrous effect over a relatively short period of time, ca. 30 50 a, on the ozone layer in the stratosphere that protects humans from harmful UV radiation. However, following implementation of international treaties (e.g., Montreal, 1987 [16]) it is now expected that the ozone layer will recover within 50 100 a [17]. At present, there are no known undesired chemical effects of low concentrations of perfluorocarbons such as CF4 and SF6 in the atmosphere. However, their rapidly increasing concentrations (ca. 7% per annum for SF6) and their exceptionally long lifetimes (thousands, not hundreds of years) means that life on earth may not be able to adapt to any changes these gases may cause in the future. They suggested that all such long-lived molecules should be considered guilty, unless proven otherwise. If SF6 is perceived potentially to be the major problem of this family of molecules, inert, dielectric gases with lower GWP values could be used as substitutes for SF6 in industrial applications; ring-based perfluorocarbons, such as cyclic-C4F8 and cyclic-C5F8 are possibilities [18]. However, the simplest, possibly naive, suggestion is that humans should not put up into the atmosphere any more pollutants than are absolutely necessary. The worldwide debate just starting, probably 50 a too late, is what constitutes 'absolutely necessary'.

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