New research from the US has added to the weight of evidence on ways in which anthropogenic climate change (change as a result of human activity) will effect food production.

A study published last week suggests that over the next two decades there will be an increased risk of a global slowdown in yield gains for many key crop plants.

Scientists from Stanford University and the National Centre for Atmospheric Research have predicted that the risk of a major production slowdown of wheat and maize is about 20 times more significant than it would be without global warming, and suggested that international organisations should plan now to avoid food shortages.

Stanford professor David Lobell commented, "I'm often asked whether climate change will threaten food supply, as if it's a simple yes or no answer. The truth is that over a 10- or 20-year period, it depends largely on how fast the Earth warms, and we can't predict the pace of warming very precisely. So the best we can do is try to determine the odds."

Lobell and NCAR's Claudia Tebaldi used computer models of global climate, as well as data about weather and crops, to calculate the chances that climatic trends would have a negative effect on yields. They predicted that, though yields may continue to increase in some places, the effects of climate change could cut the projected rate of increase by about half of what it otherwise would be; at the same time their demand is projected to grow sharply.

The IPCC's report on the impacts of Climate Change, released in March, found that the effects of anthropogenic climate change (including higher atmospheric carbon concentrations, more erratic weather and shifting rainfall patterns) are already having an impact on yields in almost all global regions. However, yields of key crops continue to increase by about 1-2 percent each year, on average, and the FAO predicts that crop production will continue to increase over the next 20 years.

Nevertheless, in December, a study from the University of Nebraska suggested that once factors including climate change, water scarcity and growing resistance to agricultural chemicals have been factored in, predicted yield gains could disappear, leading yields to plateau or even decline in some areas in coming years; they warned that 30 percent of global cereal crops could already have reached 'peak yield'.  

The NCAR researchers suggested that, despite FAO's predictions, global demand for crops is also expected to rise rapidly during the next two decades because of population growth, greater per-capita food consumption, and increasing use of biofuels. They set out to investigate whether the effects of climate change could interfere with the ability of crop producers to keep up with demand.

Although society could offset certain climate impacts – for example by planting wheat and maize in cooler regions, if average temperatures continue to rise – such planting shifts have, so far, not occurred quickly enough to offset warmer temperatures. The authors also found little evidence that other adaptation strategies, such as changes in crop varieties or growing practices would totally offset the impact of warming temperatures.

"Although further study may prove otherwise, we do not anticipate adaptation being fast enough to significantly alter the near-term risks estimated," they wrote. NCAR scientist Tebaldi put it starkly; "Climate change has substantially increased the prospect that crop production will fail to keep up with rising demand in the next 20 years."

"We can't predict whether a major slowdown in crop growth will actually happen, and the odds are still fairly low," said Tebaldi. "But climate change has increased the odds to the point that organizations concerned with food security or global stability need to be aware of this risk."

Climate change and air pollution will combine to curb food supplies

The authors of separate research, published on Sunday by the Massachusetts Institute of Technology (MIT), warned that, although many studies have shown the potential for global climate change to cut food supplies, these have mainly ignored the interactions between increasing temperature and air pollution – specifically ozone pollution, which is known to damage crops.

The MIT researchers looked in detail at global production of four leading food crops – rice, wheat, maize, and soy – that account for more than half the calories humans consume worldwide. They warned that the combinatory effects of heat and pollution will affect yields of different crops in different ways: For example, wheat is very sensitive to ozone exposure, while maize is much more adversely affected by heat.

Colette Heald, who conducted the research, said that, in the United States, tougher air-quality regulations are expected to lead to a sharp decline in ozone pollution, mitigating its impact on crops. But in other regions, the outcome "will depend on domestic air-pollution policies." Heald added that, "An air-quality cleanup would improve crop yields."

Ozone pollution can be tricky to identify, Heald said, because its damage can resemble other plant illnesses, producing flecks on leaves and discoloration. The MIT professor added that, in some areas, heat and pollution can interact; heat can increase the production of ozone in some cases, which could lead to an echo-chamber effect. The MIT team found that 46 percent of damage to soybean crops that had previously been attributed to heat may well actually be caused by increased ozone.

Heald's team called for more action on pollution-control measures, which could make a major dent in the expected crop reductions associated with climate change and pollution.