By Rob Taylor

CANBERRA, Sept 25 (Reuters) - Climate change, not war or terrorism, will be the century's biggest security challenge with China unlikely to be able to feed its vast and growing population as a result, Australia's top policeman has warned.

With predictions climate shift could slash agricultural output in Asia, and bring more droughts and flooding, climate refugees "in their millions" may be on the move within decades, Australian Federal Police Commissioner Mick Keelty said.

China, its population tipped to reach 1.5 billion from the current 1.3 billion by 2030, had particular challenges, Keelty said, although instability would hit the entire Asia-Pacific.

"We could see a catastrophic decline in the availability of fresh water. Crops could fail, disease could be rampant and flooding might be so frequent that people, en masse, would be on the move," Keelty said in a speech late on Monday.

"Even if only some and not all of this occurs, climate change is going to be the security issue of the 21st century."

Climate shift and a growing population mean China could face a food shortfall of 100 million tonnes by 2030, the country's top meteorological official said recently.

Temperature rises of up to 3 percent, rising sea levels and shrinking glacial runoff is expected to reduce runoff into major Chinese river arteries like the Yangtze and Pearl, while land for grain and rice production could be reduced by 30 per cent.

"In their millions, people will look for new land and they'll cross borders to do it," Keelty said.

"The existing cultural tensions may be exacerbated as large numbers of people undertake a forced migration."

Police, Keelty said, would struggle to cope with the impact of global warming and should be involved in the regulation of emerging carbon trading schemes, which could open the door to corruption on a massive scale.

Australia, along with the United States, has refused to ratify the Kyoto Protocol, which requires 36 industrial nations to cut greenhouse emissions, arguing the pact would unfairly impact its energy export-reliant economy.

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