A new study has suggested that the yields of rice, which is a major food source, will drop in many areas in the future, as a result of increasing global warming.
According to a report in New Scientist, the study is a combination of 80 different studies on rice and climate change, reviewed by Elizabeth Ainsworth of the University of Illinois, Urbana-Champaign.
The study indicates that in regions where the average daily temperatures are expected to rise above 30ºC, rice yields will start to fall off, and the impact will get worse as the temperature increases.
Though the drop in yield caused by rising temperatures can be counteracted by the boost to photosynthesis provided by the increased levels of carbon dioxide driving climate change, that effect is not strong enough to counteract the stress plants suffer at high temperatures.
Harvests will also be reduced by rising ground-level ozone concentrations, which are caused by nitrogen oxides (NOX) from power stations that catalyse the formation of ozone in warm and sunny conditions.
Ainsworth's review found that ozone concentrations of around 60 parts per billion, which have already being recorded on farms in China and the United States, cause yields to drop by 14%.
According to Ainsworth, new varieties of rice, bred to tolerate high ozone and increased temperatures, are urgently needed.
She points out that tropical regions need these varieties most, as temperatures there are already close to the maximum that traditional types of rice can withstand. And these many of those areas, including parts of Africa, already suffer regular food shortages.
"This won't affect the planet equally," said Ainsworth. "In places where the demand for food is already too great, things are going to get worse," she added.
Ainsworth's study, together with her previous field experiments, have all but wiped out early hopes that increased carbon dioxide might be enough to overcome the other factors and boost yields.
"Considering that we're likely to see an increase in population, if one doesn't see an increase in yields that's worrisome," said Daniel Taub of Southwestern University in Georgetown, Texas, US.