Climate change is likely to alter rainfall patterns in some of the world's most important food crop growing areas over the next 20 years, a study has found.

By 2040, up to 14% of land dedicated to wheat, maize, rice and soy will be permanently drier than in 1986, 2005 while 31% will be wetter, according to the research.

A number of regions are already experiencing a climate re-set, with rainfall patterns different to what they were a generation ago, say scientists.

The rapid speed of the change means that many farmers are going to have to act quickly to adapt, they warn.

The four crops studied together account for about 40% of global calorie intake and are vital to feeding the world's growing population.

Researchers used computer simulations of low to high greenhouse gas emission scenarios to predict "time of emergence" (TOE) of permanent precipitation changes.

When a TOE is reached, changes in rainfall pattern become fixed, reflecting a continuing altered climate.

Regions expected to be heading for a drier future include south-western Australia, southern Africa, south-western South America, central Mexico and the Mediterranean.

Wetter areas include Canada, Russia, India and the eastern United States.

Many wheat producers are likely to experience drier conditions, including Australia, Algeria, Morocco, South Africa, Mexico and Spain, said the researchers.

Lead author Dr Maisa Rojas, from the University of Chile, said: "These are definitely countries that will need to think rather quickly what they'd like to do with their wheat production.

"What we're predicting are probably conservative years for time of emergence.

"Detectable precipitation changes are of course not only important for agriculture, but for water resource management more in general, so our results are relevant to other sectors as well."

Areas of predicted reduced rainfall in the northern hemisphere currently grow 11% of the world's wheat 8% of its maize, said the scientists.

In those countries, 62% of wheat and 69% of maize production would potentially be affected by a scenario of high greenhouse gas emissions.

Some parts of the world were already making the transition to new rainfall climates, the study found.

A handful of regions had acquired entirely new rainfall "regimes" compared with the average for the 30 years spanning 1986 and 2005, which provided the study's baseline.

These included Russia, Norway, Canada, parts of the US east coast.

Co-author Dr Julian Ramirez-Villegas, from the International Centre for Tropical Agriculture, said: "Farmers growing crops in those areas are going to experience significantly different conditions than what they are used to.

"They're going to be completely outside their normal historical environments and many farmers are already struggling with historic variability."

The study found that quick action to curb emissions, in line with the 2015 Paris Agreement, could delay the changes or reduce the size of affected areas.

But even a rapid cut in emissions would not prevent altered rainfall patterns becoming established in most of the regions studied.

The findings are reported in the journal Proceedings Of The National Academy Of Sciences.