Global warming has already reduced the global yields of key crops, say scientists.

Maize and wheat production have been 3.8 and 5.5 per cent lower, respectively, than they would have been without the temperature rises associated with climate change since the 1980s, according to the statistical analysis.

Rice and soya yields have dropped in some parts of the world and risen in others, so overall the warming has not changed their net global production.

Linking climate change to food prices for the first time, the scientists, led by David Lobell of Stanford University, United States, have shown that these losses have probably led to at least a six per cent rise in food prices between 1980 and 2008.

The news comes as the UN this week (3 May) revised upwards its population prediction for the planet - to 10.1 billion by 2100.

'Without successful adaptation, and given the persistent rise in demand for maize and wheat, the sizeable yield setback from climate change is likely incurring large economic and health costs,' said the team, whose work was published in Science yesterday (5 May).

The team developed two models of crop productivity using data from countries around the world. Both models included complex factors such as the increases in yield from technological advances in farming, but one included the actual increase in global temperatures between 1980-2008, while the other kept the temperature constant at 1980 levels.

For maize, warming was linked with a reduced yield of around eight per cent in Brazil and seven per cent in China, but an increase of about one per cent in India. In Africa, there were significant yield drops in Egypt, Mozambique and Uganda, but substantial increases, linked to temperature drops, in Kenya, Tunisia and Zambia.

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