Image caption Most regions show a link between temperature rises and declines in crop yields

Shifts in the climate over the past three decades have been linked to a 5.5% decline in global wheat production, a study has suggested.

A team of US scientists assessed the impact of changes to rainfall and temperature on four major food crops: wheat, rice, corn and soybeans.

Climate trends in some countries were big enough to wipe out gains from other factors, such as technology, they said.

The findings have been published in the online edition of the journal Science.

"We focused on those four crops because they make up the bulk of calories consumed today," said research leader David Lobell from Stanford University.

"There are already clear changes going on in most agricultural regions in terms of weather, and they have effects on food production that are sizeable," he told the Science podcast.

"But in terms of temperature, we see that North America seems, oddly enough, to be exhibiting no real trend at all over the past 30 years.

"Whereas places like Europe, China and Brazil - pretty much the rest of the world, in terms of major agricultural production - have seen remarkable warming."

When the team assessed rainfall data, there were as many areas receiving more precipitation as were experiencing a decline.

"There seems to be no global trend at all," observed Professor Lobell.

Food for thought

The team carried out a large statistical analysis that tried to isolate the effects of temperature and precipitation on crops, independent of all other factors such as changes in technology and land management.

Image caption Wheat and corn are the staple crops that are most affected by changes in temperature

"We can see how much these variables affect crops... for example, for a crop like wheat, a degree (Celsius) of warming on a global average translates to about a 5% loss in production."

Professor Lobell said the study only referred to past relationships, as extrapolating the findings to predict future trends would require a number of assumptions to be made.

"In particular, you have to assume how non-linear the response will be and how different the crops of tomorrow will be from the crops of today," he said.

He added that the study focused on historical data in order to strengthen confidence in the existing projections.

"I think it is very clear that climate is not the predominant driver of change over long periods of time in crop production.

"Across the board, you see crop yields going up over the past 30 years, but the question is how much is climate modified (and) what would have happened if the climate was not changing.

"In some countries, we see that climate has only affected things by a few percent. In other countries, we see that yields would have been rising twice as fast.

"On a global average, we see that wheat production would be about 5% higher if we had not seen the warming since 1980. We see about the same for maize or corn.

"Yet for rice and soybean, we actually find that production is about the same as if climate had not been trending."