Higher global temperatures forecast for the rest of this century will lead to major disruptions in the world food supply unless farmers can adapt to the changing climate, U.S. researchers say.

Growing-season temperatures in the tropics and subtropics are more than 90 per cent likely by the end of the 21st century to exceed the most extreme seasonal temperatures from the past century, scientists at the University of Washington and Stanford University said.

Based on previous record hot seasons, agriculture in those regions will likely suffer, potentially impacting the half of the world's population that lives there, the researchers wrote in the journal Science.

The findings were based on observational data from past heat waves and output from 23 global climate models used by the Intergovernmental Panel on Climate Change in its assessment of global warming.

The UN-led Intergovernmental Panel on Climate Change already sounded the alarm on global warming in 2007, predicting by the end of the 21st century that global temperatures will rise by an average of 1.8 to 4.0 degrees Celsius.

But when discussing the impact of these changes, the IPCC reports mostly focused on how changing temperatures in the subtropics would alter precipitation, leading to drought in many regions.

University of Washington scientist David Battisti and Stanford University's Rosamond Naylor, writing in the Friday issue of Science, say changes in temperature alone can also have an impact on crop yields.

It cites unusual heat waves in Ukraine in 1972 and France in 2003 as examples of how rising temperatures can impact world agriculture.

The 1972 temperature rise in the Ukraine — about 2 to 4 degrees Celsius above long-term norms — led to a spike in the price of wheat from $60 US to $228 US per tonne in international markets between the first quarters of 1972 and 1974.

Heat, not dry weather, caused crop trouble: authors

The authors note that while much of the attention at the time was focused on drought, they said the precipitation that season wasn't as unusual as the temperature: about one-third of the years from 1900 to 2006 were drier than 1972.

The 2003 heat wave in Europe led to a decline in production of many of France's crops: maize production dropped 30 per cent from the previous year while fruit harvests were down 25 per cent and wheat harvests fell 21 per cent.

The authors also looked at the decades-long drought in the Sahel region south of the Sahara desert in Africa, where food supply problems remained even after rain returned to some regions in the past fifteen years.

Changes to temperatures in subtropical and tropical regions will likely be more jarring to local crops, they say, because they can cause agricultural droughts as opposed to meteorological droughts; that is, even when it rains, temperatures are so high that evaporation and runoff from hard soils robs food crops of much-needed water.

Food supply and price in both France and Ukraine eventually returned to normal in part because temperatures returned to normal, and also because international markets balanced shortages in one region with surpluses in another. But the future, the scientists say, may be very different if every nation is experiencing climate change.

"If the growing season temperatures by the end of the 21st century remain chronically high and greatly exceed the hottest temperature on record throughout much of the world … then global food security will be severely jeopardized unless large adaptation investments are made," they wrote.