New study reveals rice harvests affected by global warming

Posted: 30 June 2004

Global warming could severely diminish rice yields, warns a new study from the Philippines.

Scientists at the International Rice Research Institute (IRRI) collected 25 years of climate data from a weather station in the Philippines and examined 12 years of rice yields from nearby fields to discover how changes in temperature can have an adverse affect on yields. The close proximity of the weather station and rice fields allowed the researchers to closely relate varying performance to varying temperatures.

Farmers cultivating rice, the Philippines. Photo: FAO
Farmers cultivating rice, the Philippines. Photo: FAO
Farmers cultivating rice, the Philippines© FAO
The study, reported in the Proceedings of the National Academy of Sciences journal, found that rice yields at the research site declined by 15 per cent for every 1ºC rise in average daily temperature.

With temperatures projected to rise globally by 1.5 to 4.5ºC in the coming century - or 3 to 9 times more than in the past century - global warming poses a serious threat to rice harvests that have so far kept in step with population growth, warn the scientists.

Night sweats

The study found that the 0.35ºC rise in average maximum daytime temperature since 1979 had little effect on production. However, the rise in average minimum nighttime temperatures, which rose by 1.13ºC, had a measurable effect, clearly suppressing rice yields in the seasons in which they occurred.

There are thousands of varieties of rice in the world. Photo: CGIAR/IRRI
There are thousands of varieties of rice in the world. Photo: CGIAR/IRRI
There are thousands of varieties of rice in the world. Photo © CGIAR/IRRI
The IRRI researchers attributed the rise in temperatures to increased greenhouse gas emissions - the cause of global warming - and its records are consistent with warming trends elsewhere in the Philippines and globally.

Researchers found that yields fell by 10 per cent for every 1ºC increase in average nighttime minimum temperature. Because the increase in night temperature was three times greater than the increase in daytime temperature, rice yields declined by 15 per cent for every 1ºC increase in daily average temperature.

Scientists believe that night temperatures effect rice yields by forcing rice plants to put more energy into respiration rather than growth, though more detailed studies are needed to verify this hypothesis.

"The results of this study highlight the need for greater fundamental understanding of the effects of night temperature on physiological processes governing crop growth and yield development," conclude the authors. "The challenges now is to get the financial resources needed to develop new rice varieties that can still yield well despite any increase in temperatures in rice growing areas," added lead author Dr Peng.

Predicting food security

The team believe that the findings are important for predicting the effects of global warming on food security, especially since rice is the staple diet for more than half of humanity (more than 3 billion people).

In Asia, where 90 per cent of all rice is grown and consumed, more than 2 billion people obtain 60-70 per cent of their calories from rice. More than half of the world's 1.3 billion poor are Asian rice farmers and consumers, and Asia's poorest - urban slum dwellers and rural landless - still spend up to 40 per cent of their income on rice.

The poor suffer first and foremost when meagre harvests cause rice shortages and price hikes like those now making headlines in China. The country's recent rice woes demonstrate that ongoing food security cannot be taken for granted - even in one of the world's most dynamic economies.

Harvest shortfalls induced by global warming are likely to be much more widespread and persistent. Left unchecked, they could undermine global food security and political stability, warn the researchers.

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