Barely one month after the Copenhagen climate change summit which failed to live up to the expectations of many developing nations including Nigeria, indications are rife that the increasing level of carbon dioxide emissions in the atmosphere pose serious danger to some staples like cassava on which millions of African people depend.

This is as the outcome of last month's Copenhagen's climate change summit failed to arrive at a binding protocol but a mere accord devoid of commitments by attending nations.

Already, recent studies by Australian scientists have shown that the crops become more toxic and produce smaller yields in environments with higher carbon dioxide levels and more droughts.

The findings, presented recently at a conference in Glasgow, Scotland, underscored the need to develop climate-change-resistant cultivars to feed rapidly growing human populations, said Ros Gleadow of the Monash University in Melbourne.

Gleadow's team tested cassava and sorghum under a series of climate change scenarios, with particular focus on different CO2 levels, to study the effect on plant nutritional quality and yield

Both species belong to a group of plants that produce chemicals called cyanogenic glycosides, which break down to release poisonous cyanide gas if the leaves are crushed or chewed. Around 10 per cent of all plants and 60 per cent of crop species produce cyanogenic glycosides.

The team grew cassava and sorghum at three different levels of CO2; just below today's current levels at about 360 parts per million (ppm) in the atmosphere, at about 550 ppm and about double at 710 ppm.

Current levels in the air are just under 390 ppm, around the highest in at least 800,000 years and up by about a third since the start of the Industrial Revolution.

"What we found was the amount of cyanide relative to the amount of protein increases," Gleadow told Reuters from Glasgow, referring to cassava. At double current CO2 levels, the level of toxin was much higher while protein levels fell.

The ability of people and herbivores, such as cattle, to break down the cyanide depends largely on eating sufficient protein.

Anyone largely reliant on cassava for food, particularly during droughts, would be especially at risk of cyanide poisoning.

While it was possible to use processing techniques to reduce the level of toxin in the cassava leaves, it was the 50 per cent or greater drop in the number of tubers that caused most concern, Gleadow said.

About 750 million people in Africa, Asia and Latin America rely on cassava as a staple. The starchy tubers are used to make flour and the plant is ideal in dry regions because of its hardy nature.

But the good news was that the levels of toxin in the tuber did not increase with CO2, unlike the edible leaves.

"The downside of that is that we found the plant didn't grow nearly as well," she said, adding that "there's been this common assumption that plants will always grow better in a high carbon dioxide world. And we've now found that these plants grew much worse and had smaller tubers". At the 550 ppm level, the problem was not as serious and this meant scientists had a bit of breathing space.

"We've got 20 to 30 years to develop cultivars, which is going to be absolutely essential because by then about one billion people will probably be reliant on cassava," she said.

Gleadow's group looked at a type of sorghum commonly fed to cattle in Australia and Africa and found it became less toxic at the highest carbon dioxide level. But under drought conditions, leaf toxin levels rose.

She said her team was looking at creating mutations to get rid of the toxin response to droughts.

Her words: "If we're going to adapt in the future to a world with twice today's carbon dioxide, we need to understand how plants are working, how they are responding and what cultivars we need to develop."

Her team plans to carry out additional research in Mozambique and study other tropical crops such as taro.