Scientists' attempts to identify the main sources of greenhouse gases continue to throw up surprises. Only last year, nylon production was pinpointed as a major emitter of nitrous oxide. The global warming effect of this source in the UK is equivalent to 4% of that from its carbon dioxide emissions (ENDS Report 206, pp 26-7 ).
Now Dean Abrahamson of the University of Minnesota has claimed that the Intergovernmental Panel on Climate Change has failed to spot the contribution made to global warming by two fluorocarbons, CF4 and C2F6.
Other than a small, but hitherto unidentified, natural source of CF4, the two gases are known to be produced only during primary aluminium smelting. Both are emitted as by-products during the electrolysis of alumina.
The few available estimates indicate that annual global emissions of CF4 amount to 28,000 tonnes, and of C2F6 about 3,200 tonnes. These are very small in comparison to emissions of CO2 and most other greenhouse gases. However, both fluorocarbons absorb infra-red radiation much more strongly than CO2. On a molecule-for-molecule basis, according to Abrahamson, their global warming potential is about 10,000 times greater than that of CO2 over a period of 100 years.
Another important feature of both gases is their extremely long residence time in the atmosphere - more than 10,000 years. This means that their contribution to global warming is essentially permanent, Abrahamson points out.
His calculations indicate that the small quantity of CF4 and C2F6 emitted per tonne of aluminium produced is equivalent in global warming terms to 15-20 tonnes of CO2. To this can be added at least 22 tonnes of CO2 per tonne of aluminium if the electricity used by a smelter is produced by a conventional coal-fired power station, and another 1.5-2.2 tonnes of CO2 per tonne of metal produced during the reduction of alumina in the smelter.
World aluminium production is nudging 15 million tonnes per year. Abrahamson's figures suggest that the global warming effect of the greenhouse gases emitted in the process is at the very least half as large as that of the UK's CO2 emissions.