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IT IS a convenient argument against trimming carbon dioxide emissions: as CO₂ levels in the atmosphere rise, plants will grow faster and bigger, locking up more of the gas and keeping levels in check. But a study on the likely environmental impact of CO₂ emissions has revealed a fatal flaw 鈥� plant growth can actually drop if temperature and moisture levels creep up as well.

Atmospheric CO₂ levels have risen more than 30 per cent in the past century and further increases are almost certain. Previous studies have shown that more CO₂ means faster photosynthesis, and hence faster plant growth, which ought to strip more CO₂ out of the atmosphere. But most of these studies looked at the effect of CO₂ alone. When researchers factored in other influences on plant growth such as water or nitrogen, the results were ambiguous.

To get a definitive answer, Christopher Field and his team of ecologists at Stanford University in California set up a three-year study. They investigated how changes in CO₂ levels affected the growth of natural grassland when accompanied by other anticipated environmental changes such as higher rainfall, temperature and nitrogen levels in the soil.

The study was carried out in a protected reserve called the Jasper Ridge in Stanford. Field鈥檚 team marked out plots of land and sealed them in enclosures so that they could control the environmental conditions in each plot. First they raised just the CO₂ levels, from the ambient levels of about 380 parts per million to 680 parts per million.

As expected, plant growth rates jumped, with an increase of 32 per cent in the third year of the experiment. But in plots where they changed temperature as well as water and nitrogen levels, raising the CO₂ levels actually had the reverse effect. In these plots, net plant growth fell by an average of more than 9 per cent when CO₂ levels were raised, compared with plots where CO₂ was kept at the ambient level (Science, vol 298, p 1987).

So why does increasing CO₂ levels make the plants grow less? 鈥淲e honestly don鈥檛 know what the mechanism is,鈥� says Field. But the researchers鈥� hunch is that bacteria and fungi in the soil somehow fare better under the elevated CO₂ levels and outcompete plants for nutrients such as potassium or phosphorus.

鈥淚t鈥檚 a really excellent study,鈥� says Peter Reich, a global climate change expert at the University of Minnesota in St Paul. He says it is surprising that increasing CO₂ can have such a strong damping effect when combined with other environmental factors.

But Reich is worried that scientific funding policies are hampering such long-term studies in real ecosystems. 鈥淲e can鈥檛 be comfortable that we really know what the combination of things we are doing to the globe will lead to,鈥� he says.