Rice in a pressure cooker

Flooded rice paddies paint the definitive landscape of Asia. Shimmering across endless deltas and cascading down terraced hillsides, they seem eternal as the Orient. In reality, rice farming continually evolves in response to changing agricultural markets and technologies. Pressure on labour causes some farmers to broadcast seed, instead of transplanting seedlings. Scarcer water supplies lead some farmers to plant rice in damp but unflooded fields, or rotate rice with dry-field crops that require less water, earn more profit, or both. Global warming, increasingly recognised as the mother of all drivers of change, is expected to accelerate these water-saving trends. But what reduced flooding means for the long-term sustainability of rice lands, or for their greenhouse gas emissions, is mostly a matter of educated guesses.

To directly measure the effects of land-use change, thereby improve climate change models, and coordinate rice breeding for future climates, the International Rice Research Institute (IRRI) has proposed establishing a consortium for rice in a changing climate. An initial planning workshop in March at IRRI's research campus in the Philippines attracted scientists from 11 countries, who heard Robert Zeigler, director general of the Institute, pledge US$2 million toward an estimated 5-year budget of $20-25 million.

Flooded rice - productive, but under threat

	credit: Peter Fredenburg

Rice farming is at a crossroads. Irrigation expansion took off in the 1960s, allowing farmers to extend rice cultivation beyond the rainy season and double-crop much more land. New, quickly maturing Green Revolution rice cultivars made triple-cropping possible for the first time ever. And flooded tropical and subtropical fields that produce rice year-round are remarkably stable, productive and reliable. The 1963-vintage Long-Term Continuous Cropping Experiment at IRRI found, for example, that soil organic matter increases in flooded fields, even when farm workers cart away, three times a year, all above-ground crop residues along with tons of grain. "Well-managed irrigated rice ecosystems are masterpieces of ecological vitality and sustained productivity," observes Roland J. Buresh, who manages the experiment.

Multiple cropping of rice has been central to making an ever larger Asian population, the best fed in history, and so making a monkey of Malthus. But for how much longer? Drought driven by global warming is perhaps the foremost threat and certainly the most familiar. Pumping costs and, in peri-urban areas, competition for water already drive farmers to stretch supplies by adopting alternate-wetting-and-drying techniques and dry-field "aerobic" rice varieties. Costly water combined with market opportunities, notably rising demand for animal feed, encourage farmers to rotate rice with maize. In the Red River delta of northern Vietnam, rice farmers have moved on from winter maize to potatoes. The catch is that rice lands removed from continuous flooding typically suffer fading productivity because of nutrient mining, declining soil organic matter and aggravated pest pressure. And their greenhouse gas emissions appear to increase.

Other risks to rice

Rising temperature poses other threats: flowering in extreme daytime heat causes sterility, and high night-time temperatures hinder grain filling. Stretches of Asia's best rice deltas may drown under rising sea levels. Even the expected fertilising effect of elevated atmospheric carbon dioxide is a roll of the dice when it comes to weed control.

"Augment one resource for one crop and eight weed species, and chances are a weed will benefit most, especially as the crop has less genetic diversity," comments Lewis Ziska of the USDA Agricultural Research Service.

David Dawe, an agricultural economist with FAO, points out that rice farmers increasingly earn more of their income from off-farm activities and crops other than rice, adding that the main cause of rice harvest shortfalls in El Nino years is farmers' reluctance to plant rice in the teeth of an event forecast. If global warming conjures the prospect of El Nino-like weather every year, how many Asian rice farmers will simply quit?

Tracking rice systems in transition

The consortium proposal includes setting up three supersites measuring about 20 hectares each on change-prone rice lands in the Philippines, southern China and northern India. One section of each site will continue under double-cropped, flooded rice. Another will grow rice with water-saving techniques. Another will rotate rice and maize, and yet another maize and soybean. Researchers will monitor greenhouse gas emissions section by section and more broadly, as well as other factors. Data from the supersites, the first to track agricultural lands in transition, will help refine predictive models on water availability, land use and crop productivity. Modeling results will inform agricultural policy and technology needs, including new cultivars, for mitigating and coping with climate change.

Written by Peter Fredenburg