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Scientists' predictions that rising levels of atmospheric carbon dioxide will boost crop yields have been too optimistic, according to a study published today (30 June) in Science.

It says the effect is likely to be only about half as strong as previously thought.

Researchers have long reported that most major crops grow faster and need less water when more carbon dioxide (CO2) is available.

It was thought that this 'fertilisation effect' might offset the negative effects on crops — such as increased temperature and reduced soil moisture — that climate change is expected to bring.

But the new study, led by Stephen Long of the University of Illinois at Urbana-Champaign, United States, points out that these conclusions are mostly based on research done in greenhouses or controlled-environment chambers in labs and fields.

Long's team surveyed results from a more realistic approach known as Free-Air Concentration Enrichment (FACE), which involves releasing CO2 just above the crops in open fields without interfering with other environmental conditions.

They concluded that the CO2 level expected by 2050 would only increase crop yields by about half what was predicted.

Long told SciDev.Net that the new results suggest that "the damaging effects of rising temperature and decreased soil moisture will not be offset by the fertilisation effect of rising CO2".

The researchers note that for some tropical crops such as maize and sorghum the fertilisation effect might not even apply because they do not use CO2 for growth in the same way that most other plants do.

Commenting on the research in the same issue of Science, David Schimel of the US National Center for Atmospheric Research says the findings "may move impacts on agriculture higher up on the list of pressing concerns about climate change".

Atmospheric CO2 concentrations have risen from 260 parts per million (ppm) 150 years ago to 380 ppm today, and are expected to rise to 550 ppm by 2050 because of human activities.

Link to full paper in Science

Reference: Science 312, 1918 (2006)

Link to full commentary by David Schimel