Global temperature increases could cause significant reductions in yields of rice — the staple food for more than half of the world’s population — according to just-released research available online at the Science and Development Network.
Scientists have published direct evidence that rising night-time temperatures associated with global warming can cause rice yields to fall. The study, conducted at the International Rice Research Institute in Manila in the Philippines, used local climate data from 1979 to 2003 and data on Philippine rice yields from the past 12 years.
Kenneth Cassman, one of the study’s authors, told SciDev.Net: “it appears that where rice is grown in the lowland tropics and subtropics of Asia, which account for more than 50% of global rice supply, rice yields would be negatively affected by increasing temperatures associated with global warming.”
The research found that rice yields decreased by more than 10% while the night-time temperatures in the dry season rose by 1,1 degrees Celsius — three times the increase in the average maximum temperature over the same period. This trend in nocturnal temperatures is consistent with data from elsewhere and is linked to increasing concentrations of greenhouse gases.
By looking at the potential influence of other factors, such as solar radiation, weeds, diseases and insect pests, the researchers were able to determine that night-time temperatures were the most likely cause for the declining rice yields.
Globally, temperatures are projected to rise by 1,5 to 4,5 degrees Celsius in the coming century — three to nine times more than in the past century. The scientists still don’t know what causes the reduced yields and this limits their ability to propose a solution. They suspect that rice plants need to spend more energy to keep growing in the warmer nights. But the changing relationship between day and night temperatures may be having a range of effects on the plants’ physiology.
According to Cassman, the trend will be difficult to overcome if it is caused by higher energy demands. Alternatively, if developmental processes are being affected, genetic improvement and crop management may combine to reduce the impacts of increasing temperatures, he says. But he warns that even these options are limited in the intensive, lowland rice systems that currently produce two or three crops each year.
Cassman points out that many interacting variables affect rice yields, which have been stagnating in countries such as China, Indonesia and the Philippines. But, he says: “It is not possible to link this stagnation to climate change because of the other interacting factors that affect yield.”
According to the International Rice Research Institute, in Asia — where 90% of all rice is grown and eaten — more than two billion people obtain 60% to 70% of their calories from rice.
Meanwhile, a dispute is raging in the letters column of the prestigious weekly journal Nature over an article earlier this year claiming that the so-called “rice intensification” process developed in Madagascar off the east coast of Africa may be able to feed the world.
Although the demand for rice is increasing, yields of the crop are levelling out despite efforts on a variety of fronts, including genetic engineering of rice strains for improved nutrition and growth. The system of rice intensification developed in the late 1980s in Madagascar works on three tenets: that seedlings should be transplanted quickly when young; that plants should be spaced widely apart; and that fields should be kept moist but not flooded. This simple method, which claims to boost yields at lower cost to farmers, is being hailed by many as a solution.
But, as Christopher Surridge reported in a March edition of Nature, although advocates of rice intensification routinely report yields of up to twice those achieved by conventional agriculture, many eminent agronomists dismiss such achievements as the result of poor record keeping and unscientific thinking.
However, in the June 24 edition of the magazine, researchers in India and The Netherlands argue that evidence exists to explain the increases.
Highlighting the role of water management and weed control, the researchers say that improved yields are “not a miracle”. A Satyanarayana has successfully — and cheaply — implemented the system of rice intensification in India, and urged the scientific community to take a closer look at the reasons for increased productivity.
Separately, Huib Hengsdijk and Prem Bindraban point out that location is everything. They say that rice intensification has the potential to help reduce hunger and poverty, as one of a range of management practices. But they stress the importance of selecting from such practices according to location-specific conditions.
Earlier research from the International Rice Research Institute found that not just rice but maize and wheat could decline by as much as a third in tropical areas as temperatures rise, due to climate change disrupting the flowering process.
“Food crops grown in the tropics are at or near their thermal limits, making it difficult for them to withstand further rises in temperature,” explained John Sheehy, a crop ecologist at the institute. His team is looking into ways of genetically modifying strains of the crops to make them more heat tolerant.
Although climate change is likely to benefit agricultural production in some regions — temperate areas such as Siberia and Canada may experience extended growing seasons, for example — overall these gains are unlikely to offset the losses in the tropics, which will particularly impact on the world’s poorest people.
But it’s not just rice that will be affected. Your morning cup of coffee or tea could soon be at risk. An earlier study by Grid Arendal — a United Nations Environmental Programme collaborative centre in Norway — reveals that key cash crops, such as coffee and tea, will also be vulnerable to the effects of global warming over the coming decade.
New environmental maps of Uganda show that the total area for growing Robusta coffee would be dramatically reduced if local temperatures rose by just two degrees Celsius. If this happened, farmers might be forced to abandon existing plantations and seek cooler sites. Dr Svien Tveitdal, managing director of Grid Arendal, said “only higher areas would remain, as the rest would become too hot to grow coffee”. — SciDev.Net