Retorts

Science and the Greenhouse Effect

Your meteorological correspondents [August, 1995] put up an impressive defence of the Intergovernmental Panel on Climate Change (IPCC), and of Government Policy, but they do not appear to have read the latest report of the IPCC Climate Change 1994.

Although claims have been made that this report does not alter the essential conclusions of the previous reports, this is not entirely true.

To begin with, the report shows that sulphate aerosols, produced in association with the combustion of fossil fuels, cause a significant cooling effect which can counteract the calculated warming caused by greenhouse gases. There is at present much uncertainty attached to the calculations of the magnitude of aerosol cooling, but some estimates consider that it could completely cancel out the greenhouse effect.

Then, the build-up of greenhouse gases in the atmosphere is much less than has been previously supposed. The earlier IPCC reports claimed that carbon dioxide was increasing by 1.8ppmv (parts per million, by volume) a year, 0.5% a year. The 1994 report now gives 1.5 ppmv/yr (0.42%/yr), based on the period 1980-89, but the most recent figure, for 1993, is 1.2ppmv/yr, 0.34%/yr. At this rate it would take 300 years to double the carbon dioxide in the atmosphere. If the predicted increase in temperature from such a doubling of 1.5^oC to 4.5^oC is adjusted for the cooling effect of sulphate aerosols, we may now be facing a rise of no more than 1^oC per century.

The rate of increase of methane in the atmosphere has been falling steadily for ten years, and appears to be reaching a steady value. This means that methane is unlikely to make further future contributions to the greenhouse effect. Many computer climate models have assumed an "effective" carbon dioxide increase of 1% a year, leading to a doubling in 70 years, partly based on an assumed large increase in methane, which, it now appears, is unlikely to happen.

The 1994 report increases the "Global Warming Potential" of methane, so that in some cases it might be preferable to consider methane reductions as an alternative to carbon dioxide reductions.

Your correspondents seem pleased that models are now capable of simulating the current climate, but they do not seem to realise that if a model is to be used to predict a future climate it must be shown to be capable of predicting past climates; in other words it must simulate past climates as well as current climates. So far there is no model which has been validated in this sense, so that none of the current models can be used with confidence to project into the future.

The latest information from the IPCC shows that the earlier expectations of large temperature increases (as much as 3^oC) by the end of the next century are definitely wrong. The most we can expect from increases in carbon dioxide during the next century is around 1^oC, an amount which might not even be noticed in the midst of other influences on the climate. Our policies should be adjusted in line with this much reduced threat.

Dr Vincent Gray, Wellington