Droughts and floods stoked by global warming threaten to destabilize China's grain production, the nation's top meteorologist has warned, urging bigger grain reserves and strict protection of farmland and water supplies.

Extreme weather damage can now cause annual grain output in the country to fluctuate by about 10 to 20 percent from longer-term averages. China is the world's biggest grain producer.

But with global warming intensifying droughts, floods and pests, the band of fluctuation in annual production could widen to between 30 and 50 percent, Zheng Guoguang, head of the China Meteorological Administration, wrote in a new essay. He did not say how long it might be before that could happen.

A stretch of especially bad weather for farming conditions could be disastrous for the world's most populous nation, Zheng wrote in the latest issue of Seeking Truth (Qiushi), an official magazine run by the Central Committee of the Communist Party of China, which was published on Tuesday and reached subscribers yesterday.

"If extreme climatic disasters occur twice or more within five years - for example, major drought over two or three years - then the impact on our country's economic and social development would be incalculable," wrote Zheng, who has a part in developing China's climate change policies.

Zheng is a member of a leading small group charged with developing the government's policies on climate change.

His warning appeared days before governments gather in Copenhagen seeking to forge the framework of a new agreement on fighting global warming. China will be a crucial player in those talks. Last week, the government announced emissions goals for the next decade.

A vast developing country with a farming population of about 750 million, China is also one of the nations most vulnerable to global warming, wrote Zheng. He urged greater attention to adapting to unstoppable shifts in temperatures, rainfall and extreme weather.

China should make a priority of "reducing the impact of global warming on the country's food security, and strengthening the capacity of agriculture to withstand climatic risks," wrote Zheng.

China's grain production has recently reached record levels, despite damage from droughts, floods and frost. In 2008, the country enjoyed a fifth straight year of bumper harvests, with grain output at a record 525 million tons.

Citing previously published research, Zheng wrote that by 2030, China's crop productivity could be 5 to 10 percent lower than it would be without global warming.

While rising temperatures may extend potential growing times and areas for some crops, especially in northeast China, the accompanying rise in evaporation rates is likely to reduce water supplies, undercutting any increases in crop yields, wrote Zheng.

Without adequate adaptive measures, in the second half of the century, wheat, rice and corn production could fall by as much as 37 percent of recent averages, he wrote, citing earlier research.

The government should focus on expanding domestic grain reserves, protecting farmland, developing water-saving technology for farms, and boosting farmers' productivity, he wrote.