From David Powlson, Rothamsted Research, and Peter Cox, Chris Jones and Richard Betts, Hadley Centre for Climate Research
David Bellamy correctly points out one positive result of global warming (22 May, p 30). The extra carbon dioxide in the atmosphere will cause increased plant production and, through their roots, plants will bury “lots more organic humus in the soil” thus improving the soil.
So far, so good – there is published evidence for this. Unfortunately he ignores other published evidence indicating that higher temperatures will cause soil microbes to decompose that humus faster. Published work using the Hadley Centre model suggests that by the middle of this century, faster decomposition will far outweigh the effect of increased plant growth. So the world’s soils will almost certainly not benefit. In addition, the extra carbon dioxide released from humus in soil will further accelerate climate change.
Bellamy also ignores the strong probability that climate change will cause more droughts in many continental interior regions. This will exacerbate malnourishment in poorer regions of our world – the exact opposite of his claim. And the rise in sea level expected to result from climate change will cause flooding of low-lying areas, such as parts of Bangladesh, which are vitally important as both home to millions of people and also as land for food production.
Despite what Bellamy says, carbon dioxide is a greenhouse gas and is the strongest single contributor to global warming. Its positive effect on plant growth will not continue indefinitely and is not sufficient to counteract the climate change it causes. Nor does extra carbon dioxide in the atmosphere remove the need for chemical fertilisers – in many parts of the world lack of phosphate or nitrogen are major causes of malnourishment, and climate change will certainly not overcome this. In fact, shortage of these nutrients (or water) may prevent the potentially positive effect of extra carbon dioxide on crop growth occurring at all.
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We hesitate to disagree with Bellamy, who has done so much to interest the wider public in science and vital environmental issues. But public prominence carries responsibility. In this case he is only giving one part of the story, which taken alone is highly misleading and encourages complacency.
Harpenden, Hertfordshire, UK, and Exeter, Devon, UK
