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Global temperature increases could cause significant reductions in yields of rice — the staple food for over half of the world's population — according to research released this week.

Scientists have published 'direct evidence' that increased night-time temperatures associated with global warming can cause rice yields to fall. The study, conducted at the International Rice Research Institute (IRRI) in the Philippines, used local climate data from 1979 to 2003 and data on Philippine rice yields from the last 12 years.

The research found that rice yields had decreased by more than ten per cent while the night-time temperatures in the dry season rose by 1.1 º Celsius — three times the increase in 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 ºC in the coming century — three to nine times more than in the past century.

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 per cent of global rice supply, rice yields would be negatively affected by increasing temperatures associated with global warming."

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 IRRI, in Asia – where 90 per cent of all rice is grown and eaten — more than two billion people obtain 60-70 per cent of their calories from rice.

Reference: Proceedings of the National Academy of Sciences of the USA 101, 9971 (2004)

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