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Without wishing to unnecessarily prolong the correspondence arising from Channel 4's The Great Global Warming Swindle, Peter Kellow's 'incontrovertible' observation (Letters, AJ 19.04.07) with regard to the impact of methane produced by livestock on global warming requires correction.

Although all greenhouse gases have different strengths and lifetimes, meteorologists from the BBC weather centre have estimated that carbon dioxide and methane contribute approximately 60 per cent and 20 per cent respectively of the 'enhanced greenhouse effect'.

More than half of the latter is attributed to human activity, notably farming - a proportion of which is, of course, produced by livestock. Thus, the reality of methane's impact on global warming is rather different to Kellow's 80 per cent figure.

It is still of significance, especially when related to the removal of rainforest to make way for livestock, but the real impact is only in single percentage figures.

Professor Colin Porteous, Glasgow, Scotland

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