Global population growth is looming as a bigger threat to the world's food production and water supplies than climate change, a leading scientist says.

Speaking at a CSIRO public lecture in Canberra yesterday, UNESCO's chief of sustainable water resources development, Professor Shahbaz Khan, said overpopulation's impacts were potentially more economically, socially and environmentally destructive than those of climate change.

''Climate change is one of a number of stresses we're facing, but it's overshadowed by global population growth and the amount of water, land and energy needed to grow food to meet the projected increase in population. We are facing a world population crisis.''

In the past four years, the price of rice in Thailand had risen from $A200 a tonne to $A800 a tonne, and India had banned rice exports in a bid to ensure the country had sufficient supplies of this staple food, Professor Khan said.

''It would be a mistake for Australia's governments to assume they can adapt to declining water availability within the Murray-Darling Basin by deciding staple crops like wheat and rice can be grown in other countries. We need smarter ways to improve water efficiencies so we can continue to grow those crops.''

Before taking up the UNESCO post in Paris earlier this year which involves supervising sustainable water development projects in 190 countries Professor Khan led CSIRO's irrigation systems research and was founding director of the international centre for food security at Charles Sturt University, Wagga Wagga.

The city's mayor farewelled him with a public reception, praising his passion and commitment to water reform and his role in championing rural communities. Yesterday, Professor Khan called for debate on national water reform to be ''opened up to include a genuine diversity of opinion'', claiming scientists ''are worried about being crucified'' by governments if they express dissenting views.

''Scientists are fearful, to be honest,'' he said.

Many politicians were out of touch with crucial livelihood issues facing rural Australia, particularly poverty and the loss of jobs in communities built on wealth generated by irrigated food production.

''There is a disconnect and mistrust. You have politicians and scientists from the big cities coming up with ideas and warning of painful decision, but they're not bringing the communities who will be affected into the discussion.

''In my experience, irrigators are not vandals: they're trying to make a living for their families, often faced with great hardships, and have made a lot of effort to achieve water efficiencies. We should celebrate some of the successes achieved by our farmers, because there have been stunning successes in the Murray-Darling Basin.''

Australia must also think about the future social and environmental implications of its ''population footprint''.

He said, ''It's not something that should happen by an act of God. It has to be an informed decision about geographic spread and location, about benefits for indigenous communities, for river systems and wetlands. It's a big exercise and needs to be done very carefully.''

BY ROSSLYN BEEBY
SCIENCE AND ENVIRONMENT REPORTER

22/07/2008 12:00:00 AM