Barring nuclear wars, pandemics and cosmic accidents, there will be about 9.3 billion people in the world in 2050 but they will eat food enough for 13 billion at today's nutritional levels.

Many people, having risen through the economic development curve, will consume diets far higher in protein: in the cases of China and India, three to five times higher than today. To meet this, global food output must rise by 110 per cent, says the UN Environment Program.

The UN's Food and Agriculture Organisation thinks this is technically feasible. The unanswered question is whether it is sustainable. Key indicators suggest it is not.

For the first time in history, urban demand for water is outpacing farm demand, as city users outbid irrigators. By 2050, cities will consume half the world's fresh water, reducing that available for food production by one-third. Worldwide, ground water is running out, especially in regions where it is used to grow food. By 2025, water scarcity may cause an annual reduction of 350 million tonnes of food: almost the same as losing today's entire global rice harvest.

About 1.2 billion hectares, or 10per cent of the world's arable area, is affected by serious degradation. 300 million hectares is unusable for farming. There is a continuing loss of five million to 10 million hectares a year.

Eighty per cent of the remaining arable area is degraded to some degree. While not seen as a limit to global food production, soil loss is a significant constraint in India, Africa, the Middle East and parts of Asia. It is expected to worsen because of climate change.

Large areas of coastal seas and lakes are unfit for aquaculture because of sediment, nutrient and pesticide contamination from the land. This is on top of the decline in global catches due to overfishing.

There has been a shift to the production of biofuels, characterised by critics as "the rich burning the food of the poor". Ethanol production will equal 1 per cent of world oil consumption by 2010. Wherever they are grown, biofuels displace food crops.

The FAO says fertiliser supplies are ample and growth of up to 2per cent a year is within the world's capacity to supply. However, the US Department of Agriculture notes that the world suffers a net loss of 55million tonnes of nutrients a year, chiefly by erosion, pointing to a significant nutrient imbalance.

A new barrier to a sufficient harvest in 2050 is the knowledge drought: a worldwide decline in agricultural research and development, especially in production research.

Finally, there is climate. Drought conditions may affect up to half the planet's land surface by the second half of this century, says Britain's Hadley Centre for Climate Prediction. The Consultative Group on International Agricultural Research expects a 50 per cent decline in Indian wheat production - equal to about 7 per cent of the global crop - due to drying.

Two-thirds of the wars in the past 15 years have been, at root, about shortages of food, land and water, and the desire of various ethnic, religious or political groups to control resources, as the Oslo Peace Research Institute has pointed out. If food shortages in key regions intensify, conflicts, genocides and refugee waves are liable to multiply, creating instabilities that will topple governments, favouring terrorism and insurgency.

Far and away the greatest challenge facing humanity this century is the necessity to double global food production with less land, less water, less nutrients, less science and in frequent drought.

Australia has been a leader in the global effort that doubled food output last time around, when the Club of Rome initiated the green revolution in the 1960s. Despite popular misconceptions, we are leaders at managing drought, in water-efficient farming, in sustainable agricultural systems and in fisheries and landscape management. Some of our previous scientific firepower is still intact.

While adapting to greenhouse gases is urgent, it isn't anything near as urgent as finding ways to double the world harvest using far fewer resources. Climate change will be the least of our worries if large parts of humanity starve. It is fair to say that this is an issue about which governments are totally complacent. They have yet to see 21st century agricultural research for what it is: defence spending.

The world is ripe for another wake-up call, and who better than Australia - skilled at confronting the harsh realities of drought, uncertain climate, pests, disease and lack of nutrients - to administer it?

First, of course, we need to put our own effort in order and reverse the disastrous decline in public good research on food production. 

Julian Cribb is adjunct professor of science communication at the University of Technology, Sydney. He edits Australian R&D Review and ScienceAlert.com.au