Eminent climatologists support the opinion of the Intergovernmental Panel on Climate Change (IPCC) that rising global average temperature is due to anthropogenic CO2 emissions. A number dispute this. This paper is written by a physical scientist aiming to understand the controversy for himself. An elementary account is given of the physical theory of the Earth's average temperature so as to introduce the principal parameters: solar irradiance; earth's albedo; physics of the factors determining the emissivity of the earth. The rise in mean global temperature observed intermittently over the last one hundred years has markedly slowed over the last ten. This discrepancy with the prediction of accepted models of a secular change of climate, coupled with the known uncertainties concerning the effects of clouds and of water vapour cast doubt on the accuracy of the models. The lead position of CO2 in forcing a change of climate is hence questioned. The apparent neglect of man-made changes in the distribution of water is emphasised. The uncertainties regarding the behaviour of water vapour and cloudiness must be resolved if the cited causes of any climate change are to be fully reliable.

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