RICE is the principal food for over 60 percent of mankind. It is particularly important to Asia where over half of the world's population lives. Another bad news that global warming brings is that rice yields may greatly be reduced.

Even modest rises in global temperatures will drive down rice production in the region, where millions of poor people depend on it as a staple food, according to a report published in the Proceedings of the National Academy of Sciences.

Researchers from the United States, the Philippines and the Rome-based Food and Agriculture Organization (FAO) analyzed six years of data from 227 irrigated rice farms in China, India, Indonesia, the Philippines, Thailand and Vietnam.

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Looking at the impact of rising daily minimum and maximum temperatures on irrigated rice production between 1994 and 1999, the team found that the main culprit in cutting rice yields was higher daily minimum temperatures.

In fact, rising temperatures during the past 25 years have already cut the yield growth rate by 10 to 20 percent in several locations. The loss in production is expected to get worse as temperatures rise further towards the middle of the century, the report said.

"We found that as the daily minimum temperature increases, or as nights get hotter, rice yields drop," said Jarrod Welch, lead author of the report and graduate student of economics at the University of California, San Diego.

"Up to a point, higher daytime temperatures can increase rice yield, but future yield losses caused by higher nighttime temperatures will likely outweigh any such gains because temperatures are rising faster at night."

While the exact reason for this has not been proven, some scientists believe that rice plants are having to respire more as nights become warmer, resulting in the plants spending more energy without being about to photosynthesize.

"Global warming is more disastrous to the agricultural industry of the Philippines and its neighboring Asian countries than in other parts of the world," observed Dr. David Street of the US Argonne National Laboratory some years back.

A study done by the Laguna-based International Rice Research Institute (IRRI) showed that rice plants could benefit from higher levels of carbon dioxide in the atmosphere. However, an increase in temperature up to four degrees Celsius caused by global warming would "nullify any yield increase."

Another limiting factor in rice production is water. An estimated 40 percent of agricultural products and 60 percent of the world's grain are grown on irrigated land. "Agriculture is by far the biggest consumer of water worldwide," IRRI said. For instance, to raise a ton of rice, you need a thousand gallons of water.

"In a warmer world, we will need more water - to drink and to irrigate crops," said the London-based Panos Institute. "Water for agriculture is critical for food security," points out Mark W. Rosegrant, a senior research fellow at the Washington-based International Food Policy Research Institute (IFPRI).

"The link between water and food is strong," said Lester R. Brown, president of Earth Policy Institute, also based in Washington, D.C. "We drink, in one form or another, nearly four liters of water per day. But the food we consume each day requires at least 2,000 liters to produce, 500 times as much."

An earlier study, done by the Manila-based Asian Development Bank, said rice production in the Philippines will decline by as much as 75 percent if the country is not quick enough to adapt to and put in place safeguards against climate change.

The Philippines, according to the study, will increasingly see more rains "for most of this century." The western part of Southeast Asia is predicted to become hotter. Decline in rice production will start in 2020, it said.

"Global warming is an environmental threat unlike any other the world has faced," Christopher Flavin wrote in his book, Slowing Global Warming: A Worldwide Strategy. "While human activities during the past century have damaged a long list of nature systems, most of these problems are local or regional in scope and can be revered in years to decades if sufficient effort is exerted."

"The global warming is very simple," said Dr. Robert Watson, chair of the Nobel-winning United Nations Intergovernmental Panel on Climate Change (IPCC). "We are increasing emissions of greenhouse gases and thus their concentrations in the atmosphere are going up."

Greenhouse gases produce the greenhouse effect, which traps heat near the earth's surface, maintaining a relative constant temperature. However, many human activities increase the amount of greenhouse gases in the atmosphere. As concentrations increase, the temperature of the earth also rises.

Carbon dioxide is the primary greenhouse gas. Since the beginning of the Industrial Revolution an estimated 350 billion tons of carbon dioxide have been released through the burning of fossil fuels. Other greenhouse gases are chlorofluorocarbons, methane, nitrogen compounds, and ozone.

Fresh data has shown that greenhouse gas emissions have grown by an average of 3.5 percent a year from 2000 to 2007. That's "far more rapid than we expected" and more than three times the 0.9 growth rate in the 1990's, according to Chris Field, coordinating lead author of the IPCC report.

Unknowingly, rice production is also a contributor to the global warming problem. Methane emissions from flooded rice paddies contribute to global warming just as coal-fired power plants, automobile exhausts and other sources do with the carbon dioxide they spew into the atmosphere.

"An estimated 19 percent of the world's methane production comes from rice paddies," said Dr. Alan Teramura, botany professor at the University of Maryland in the United States. "As population increase in rice-growing areas, more rice, and more methane, is produced."

"Methane emissions are unique to rice," said Reiner Wassmann, a climate change specialist at IRRI. "If Asian countries are exploring possibilities to reduce greenhouse gas, they have to look at rice production. I'm not saying it's the biggest source, but in Asia it's a source that cannot be neglected."