If the world doesn't act now, it faces a catastrophic global food shortage by mid-century.

As negotiators sat down this week for another hard round of bargaining at the climate change summit in Cancun, Mexico, new warnings emerged of potential catastrophe ahead.

Researchers reported in Science magazine that almost every part of the world's oceans have been damaged by human activity, magnified by a significant rise in water temperatures and predicted more would come.

Another study, by the ARC Centre of Excellence for Coral Reef Studies, warned of a potential mass extinction as the number of ocean dead zones - waters starved of oxygen - increase at an accelerating pace.

Yet another, released at Cancun by the Tyndall Centre for Climate Change Research, pointed to the increasing likelihood of frightening changes to rainfall, water supplies, weather systems, sea levels and crop harvests by the end of the century.

In Canberra, Professor Julian Cribb, one of Australia's most distinguished science writers, warns that the dangers facing humankind extend beyond the already-alarming projections of climate scientists, threatening the lives of hundreds of millions of people.

"The world has ignored the ominous constellation of factors that now make feeding humanity sustainably our most pressing task - even in times of economic and climatic crisis," Cribb writes in a disturbing new analysis of the state of the planet.

Cribb's book, The Coming Famine: The Global Food Crisis and What We Can Do to Avoid It, identifies a complex and interconnected confluence of factors driving the world towards a global emergency that experts predict will peak by the middle of this century.

"It is arriving even faster than climate change," he writes.

Cribb and the battery of experts he cites say there remains time to forestall catastrophe, identifying a range of measures that could be put in place, but warn that the world has barely begun to understand the ramifications of what it now faces.

The main drivers of the looming crisis include population growth that will reach 9.2 billion by 2050 (most in the poorer nations), rising demand for protein foods such as meat, milk, fish and eggs, and a global food requirement by 2050 that will be up to double today's, outstripping growth in food output.

Fuelling this will be growing shortages of water and productive land - 25 per cent of land is already so damaged it can barely yield food - declining nutrients, high energy costs, over-fishing, decimation of coral reefs and ocean food chains, declining agricultural research, climate change, and economics, politics and trade policies that distort world markets.

War and its appalling flow-ons could also be a consequence - in the past decade conflict has been triggered by land disputes and corrupt land distribution, environmental degradation, famine and water resources.

More, and possibly worse, could be in store. British defence analysts have predicted that rising populations, declining resources and climate change will increase the risk of food price spikes and shortages, water scarcities in volatile regions, mass displacement cause by climate or resource scarcities, a possible collapse in fish stocks, and greater risk of civil wars, intercommunal violence, insurgency, pervasive criminality and widespread disorder.

The Intergovernmental Panel on Climate Change also said flooding of coastal communities could trigger an even more alarming prospect: "Armed conflict between nations over resources, such as the Nile and its tributaries, is likely and nuclear war is possible."

The strain on global water supplies is enormous and growing, under pressure from irrigation to meet rising food demand, the explosion of cities, demand for higher-quality diets as economies grow, extraction exceeding natural rates of recharge, ignorance over the real state of water reserves, pollution, salinity and acidity, desertification, and poor management, farming practices and infrastructure.

The International Water Management Institute predicts that by 2050, more than two billion people will face severe water scarcity, and a further five billion moderate shortages.

Land is also running short. Global agricultural financier Rabobank estimates that the area of food production has shrunk from 0.45ha a person in the 1960s to 0.23ha, and will fall further, to about 0.18ha, by 2050.

New land is increasingly scarce, while productive land is being lost through accelerating soil degradation faster than new areas can be opened up - and much of the new land that is turned to farming has poor soil, requiring massive inputs of fertiliser and energy or drainage.

More land will be lost to rising sea levels, and at greater risk from salinisation, more and fiercer storms, worse flooding by rivers held back by rising seas: in the vital Bay of Bengal alone, a 40cm sea-level rise would inundate 11 per cent of its coastal lands, ripe out one-sixth of Bangladesh's rice harvest and displace 13 million people.

Researchers further warn that oceans - and the life they support - are under growing threat from acidity that Britain's Royal Society says is now irreversible in our lifetimes. It will require tens of thousands of years to restore ocean chemistry to pre-industrial revolution conditions.

While Cribb says co-operation, sharing of knowledge, and the subordination of national pride, greed and fear could stave off the worst, failure to work to guarantee the global food supply would mean catastrophe: "If through our neglect or abuse of resources it fails, each of us will bear the consequences."

Straight from the horse's mouth

Digging into a mountain of caviar, sea urchin roe, succulent Kyoto beef, rare conger eels, truffles and fine champagne, the leaders of the world's richest and most powerful countries shook their heads over soaring grocery prices in the developed world and spreading hunger in Africa, India, and Asia.

Over an 18-course banquet prepared for them by 60 chefs, the eight global potentates declared: "We are deeply concerned that the steep rise in global food prices coupled with availability problems in a number of developing countries is threatening global food security. The negative impacts of this recent trend could push millions more back into poverty." (Statement issued after the July 2008 meeting of the Group of Eight nations in Hokkaido, Japan.)

- From Julian Cribb's book The Coming Famine: The Global Food Crisis and What We Can Do To Avoid It