Sowing False Hopes
With an amazing determination to see a silver lining to the dark cloud of climate change, some scientists are now predicting that an atmosphere rich in CO2 may actually benefit agricultural production by acting as a kind of airborne fertiliser.
The basic theory – ‘CO2 fertilisation’ – is sound. All life on Earth is carbon-based. CO2 can act as a fertiliser because plants synthesise the glucose they need to grow from sunlight, water and CO2 (photosynthesis). Because atmospheric CO2 acts like a ‘fertiliser’, many commercial greenhouses artificially raise CO2 levels to accelerate plant growth.
There is a catch, however. While higher levels of CO2 can significantly stimulate crop yields in enclosed spaces like greenhouses, the effect in the real world is significantly less impressive.
The results of studies in controlled environments during the 1970s and 80s suggested that CO2 fertilisation could offset crop losses due to climate changes. But newer data is more equivocal. For instance, studies of soybean plants grown in enclosed environments show a 28-30 per cent increase in yield when the atmosphere contains 550 parts per million CO2. But in open-field experiments (known as FACE, or Free Air CO2 Enrichment) with the same atmospheric CO2 concentrations, this drops to an increase of between 13 and 15 per cent. In fact, overall FACE studies suggest that yields from open fields are only half those of closed chambers.
In addition, increases in yield are not universal. Indeed, Pramod Aggarwal, head of environmental sciences at the Indian Agricultural Research Institute in New Delhi and a co-author of the food, fibre and forestproducts section of the IPCC’s latest report, says the idea that CO2 fertilisation will offset weather-related losses ‘is a Western point of view’ that ignores much of the developing and underdeveloped world.
According to Aggarwal, high CO2 levels are beneficial only when crops are adequately fertilised, irrigated and protected from pests, so food crops in tropical climates will derive less – if any – yield benefit.
William R. Cline, a senior fellow jointly at the Peterson Institute for International Economics and the Center for Global Development in Washington, DC, agrees. His research, summarised in his recent book Global Warming and Agriculture, suggests that as atmospheric CO2 levels rise the world’s overall agricultural productivity will decline by between three and 16 per cent by 2080 – even with the effect of CO2 fertilisation.
If climate change continues on its current trajectory, Cline argues, the US, Canada, most of Europe and Russia could see agricultural gains, but India, Pakistan, most of Africa and most of Latin America would be hit very hard. In the developing world, he estimates India’s decline in agricultural production could be between 29 and 38 per cent, while crop yields in Sudan and Senegal could decline by more than 50 per cent. Not all rich countries escape either: in a high-CO2 world Australia could see yields declining between 16 and 27 per cent.
But overly optimistic predictions about crop growth with CO2 fertilisation also ignore disturbing data on plant biology.
As described in Nature article ‘The Other Greenhouse Effect’, published in August 2007, research is beginning to show that levels of plant nutrients go down as CO2 goes up. Researchers have observed significantly lowered levels of protein, especially wheat gluten (an effect that reduces baking quality), lowered trace mineral content, lowered vitamin C in potatoes and lowered calcium in soybeans (potentially problematic for those who substitute soya milk for cows’ milk).
Why the nutritional quality of plants should decline in a CO2-rich environment is still poorly understood. One theory is that while a CO2 enriched atmosphere triggers the plant to produce higher levels of carbohydrates it does not boost the plants’ ability to take relatively more nutrients form the roots.
Plants exposed to higher levels of CO2 also CO2 fertilisation appear to develop narrower stomata, through which they exhale water vapour in a process known as transpiration. Some studies suggest that transpiration decreases by around 23 per cent in high-CO2 environments. This makes plants more drought-resistant, but it also means that fewer waterborne nutrients are taken up by the roots.
In addition to reducing the micronutrient content of plants, rising CO2 levels have been found to drive up the production of plant non-nutrients such as tannins and phenolics – ‘poisons’ that enhance plant defences against their would-be consumers. Their potential effect on plant-eaters is not well understood.
Humans aren’t the only animals to eat plants and the effect of lower nutritional quality of plants is not only a vegetarian issue. Grazing livestock eat plants and we eat the livestock. So as nutrient levels decline the full range of what we eat is affected. Research at Kansas State University recently found that the grass grown in high-CO2 conditions was less digestible, with lower nitrogen and protein content, and concluded that these changes were dramatic enough to reduce the growth rate of cattle grazing on prairie, even if the animals continued to eat the same amount of grass. Wild animals’ nutritional status and growth will also be affected.
Plants grown in a high-CO2 atmosphere may also be more vulnerable to the pests that exist at the other end of the food chain. One of the studies cited in the Nature article found that certain types of plant pests thrive on the CO2-fertilised soybean leaves, laying eggs that destroy the following year’s crop.
Conventional agricultural science suggests we can overcome problems of atmospheric CO2 in plants simply by using more fertilisers or pesticides, or by giving livestock more synthetic nutrients in their diets. This, of course, assumes the oil on which fertilisers and pesticides is based will continue to flow. It also ignores the terrible damage that the continued usage of fertilisers and pesticides does to the soil – the true source of good food right along the food chain.
Far from being a boon to agriculture, rising CO2 levels are likely to exacerbate the problems of hidden malnutrition in the developed world, while placing poor countries on the road to the devastating famines they have previously known. This is one silver lining we should do our utmost to avoid.
Pat Thomas is Editor of the Ecologist.
- This article appeared in the March 2008 edition of the Ecologist. Not available online.
