Climate change is expected to drive declines in average global crop yields, but new research shows that shifts in temperature and soil moisture will also increase year-to-year variability in staple crop yields - making food production more volatile and threatening food security.

As lead author Dr Jonathan Proctor from Canada's University of British Columbia explains, "farmers and the societies they feed don't live off of averages - they generally live off of what they harvest each year".

"A big shock in one bad year can mean real hardship, especially in places without sufficient access to crop insurance or food storage."

For every degree of warming, the study estimates year-to-year variability in crop yields will increase by 7% for corn (maize), 19% for soybeans and 10% for sorghum.

Under a 2°C warmer growing season, the researchers project that weather-induced yield losses which are 1-in-100-year occurrences today (2015-2035) will instead happen every 49 years for maize, 25 years for soybeans and 54 years for sorghum.

"Rates increase to once every 33 years for maize, 14 years for soybeans, and 40 years for sorghum under 3◦C of warming," they write.

Current policies in place around the world are projected to result in about 2.7°C of warming above pre-industrial levels by 2100.

But under the Intergovernmental Panel on Climate Change's (IPCC) Shared Socioeconomic Pathway-8.5 high-emission scenario, in which greenhouse gas emissions rise steadily -- doubling by 2050 and more than tripling by the end of the century -- maize and soybean growing seasons would likely warm by 4.3°C and sorghum growing seasons by 3.2°C on average by 2100.

"Under these hotter conditions, a 1-in-100-year loss occurs every 17 years for maize, 8 years for soybeans, and 28 years for sorghum," the authors write.

"Maize production is the largest of any grain; soybean is a globally important source of protein and calories for livestock and humans; and sorghum is the fifth most produced grain globally and an important food crop in parts of Africa, Central America, and South Asia."

Proctor and his colleagues paired a crop model with Earth system simulations to explore the impacts of expected climate change-driven shifts in the variability of temperature and soil moisture on crop yields.

They identified "increased covariance of temperature and water stresses as a substantial and previously unquantified driver of future increases in yield variance."

In other words: "A double whammy of heat and dryness, increasingly arriving together," explains Procter.

"If you're hydrated and go for a run your body will sweat to cool down, but if you're dehydrated you can get heatstroke. The same processes make dry farms hotter than wet ones."

As hotter weather dries out soil, lower soil moisture in turn makes heatwaves worse by allowing temperatures to rise more quickly. This leads to increased crop yield variance because both hot and dry conditions damage yields.

The authors say their results illustrate a concerning relationship where climate-driven decreases in average crop yield are accompanied by wilder swings in yield variability, which then combine to "substantially increase the frequency of weather-induced negative yield shocks".

To build resilience, the authors call for urgent investment in heat- and drought-resistant crop varieties, improved weather forecasting, better soil management and stronger safety nets, including crop insurance.

But the most reliable solution is to cut emissions driving global warming, they say.

"Not everyone grows food, but everyone needs to eat. When harvests become more unstable, everyone will feel it," says Procter.