Many of the pesticides on which the crop increases have depended are losing their effectiveness, as the pests acquire more resistance.
A key constraint is water. The 17% of cropland that is irrigated produces an estimated 30-40% of all crops, but in many countries there will be progressively less water available for agriculture.
Many of these are poor countries, where irrigation can boost crop yields by up to 400%. There are ways to improve irrigation and to use water more effectively, but it's not clear these can bridge the gap.
In the 1990s global poverty fell by 20%, but the number of hungry people rose by 18 million. In 2003, 842 million people did not have enough to eat, a third of them in sub-Saharan Africa, according to the UN's Food and Agriculture Organization.
It may already be too late to head off major famines in the mid-part of this century. That's one of the clear implications of the latest PricewaterhouseCoopers review of the world's carbon economy, Too Late for Two Degrees?
Two degrees of global warming may not sound much to most people, but combined with extreme weather events it is the point at which the world's food supply, and grain production in particular, begins to face serious jeopardy. And if grain runs short, the first thing to go is meat. So this isn't just an issue that will hit the poor and the already hungry: it will affect everyone who eats.
The report by PwC, one of the world's largest accounting firms, is blunt: ''Now one thing is clear: businesses, governments and communities across the world need to plan for a warming world - not just 2ºC, but 4ºC and, at our current rates, 6ºC.''
''This isn't about shock tactics, it's simple maths,'' PwC partner Leo Johnson told Canadian science writer Steve Leahy. The ''simple maths'' says the world has been cutting its carbon emissions at only one-sixth of the rate needed to avoid four to six degrees of global warming. In perspective: the US would have to eliminate all its existing coal-fired power stations in eight years to meet its share of the two-degree target - and much the same applies to Australia.
Its sober assessment has been echoed by a second major report, by the World Bank, which warned that places such as India could lose up to half their grain crops under 2 degrees of warming, and Africa a third of its arable area. It links 13 of the most devastating climatic events of this decade to man-made global warming.
The reports come just before the United Nations Climate Change conference opens in Doha on Monday, already shaping as another of those dispiriting affairs where pious sentiments, platitudes and good intentions substitute for concrete, decisive action by what is already rated as the least-effective worldwide political leadership in a century.
However, the real significance of these reports is that we are now looking down the barrel at climate-induced economic shocks that will make the global financial crisis look like a modest hiccup.
What some of those shocks might look like will be discussed by scientists attending the Second Australian Earth System Outlook Conference hosted by the Australian Academy of Science in Canberra, also on Monday. They will explore some of the ''tipping points'' where systems break down and shift to a new, more perilous, state. The focus will be on the continuing disconnect between climate science and society over carbon, impacts on the poles, the oceans, the Great Barrier Reef, and food security.
Food security is vital because that's the main way most people will personally experience climate change - as huge, unexpected rises in the cost of foods they had previously taken for granted, due to drought, flood, tempest or searing temperatures. So far, it appears the world has lost about 4 per cent to 6 per cent of its major grain harvests due to global warming.
What is not generally understood, however, is that global (and Australian) food security also depends on a series of scarce resources that are becoming increasingly unaffordable to farmers: land, water, oil, fertiliser, fish, finance and technology, to name a few. Climate change is now amplifying these emerging scarcities in unpredictable ways.
For example, it is probable that key regions of the world will run out of water in the next 10-20 years - the north China foodbowl, the Indo-Gangetic plain, the US midwest and the Middle East. These could trigger mass migration by tens of millions of people as well as the outbreak of local conflicts, as numerous military studies now attest. Climate change, with its sudden, unanticipated impacts, will exacerbate water scarcity and flood destruction of food systems. The bit the world's policymakers don't seem to get is that these impacts are synergetic and cumulative, reinforcing and compounding one another. They cannot be solved by one-off ''fixes'' such as water markets, fuel subsidies or GM crops.
To put this in personal perspective, you consume each day 4.1 litres of diesel fuel, 29 kilograms of soil and 2.2 tonnes of water in the form of food. If any of these things run short, here or globally, your supermarket bill will tell you all about it. Yet governments here have quietly been dismantling Australian landcare, irrigation and agricultural science as if they did not matter.
Such complacency mirrors the attitudes of governments globally on climate change - either in preparing for the worst or in attempting to prevent it. Humanity, paralysed by an incoherent and acrimonious media debate where ideology predominates over evidence, knowledge and wisdom, is sliding helplessly towards what PricewaterhouseCoopers policy expert Jonathan Grant terms ''the carbon cliff''.
On the brighter side, however, PwC clearly identifies green technology as the most likely driver of the next big global investment wave - as China and Germany are already demonstrating by their huge swing to solar. Australia, the nation with more free photons per square metre than anyone else on Earth, still lurks among the also-rans and ne'er-do-wells: what is it with us, that cloudier countries see more opportunities in sunshine?
However, it is food, not energy, that will deliver the ultimate wake-up call - to Australians, Chinese, Americans, Indians, everyone alike.
Temperatures are rising rapidly during the growing season in some of the most important agricultural countries, and a paper published several weeks ago found that this had shaved several percentage points off potential yields, adding to the price gyrations.
For nearly two decades, scientists had predicted that climate change would be relatively manageable for agriculture, suggesting that even under worst-case assumptions, it would probably take until 2080 for food prices to double.
In part, they were counting on a counterintuitive ace in the hole: that rising carbon dioxide levels, the primary contributor to global warming, would act as a powerful plant fertilizer and offset many of the ill effects of climate change.
Until a few years ago, these assumptions went largely unchallenged. But lately, the destabilization of the food system and the soaring prices have rattled many leading scientists.
“The success of agriculture has been astounding,” said Cynthia Rosenzweig, a researcher at NASA who helped pioneer the study of climate change and agriculture. “But I think there’s starting to be premonitions that it may not continue forever.”
A scramble is on to figure out whether climate science has been too sanguine about the risks. Some researchers, analyzing computer forecasts that are used to advise governments on future crop prospects, are pointing out what they consider to be gaping holes. These include a failure to consider the effects of extreme weather, like the floods and the heat waves that are increasing as the earth warms.
A rising unease about the future of the world’s food supply came through during interviews this year with more than 50 agricultural experts working in nine countries.
These experts say that in coming decades, farmers need to withstand whatever climate shocks come their way while roughly doubling the amount of food they produce to meet rising demand. And they need to do it while reducing the considerable environmental damage caused by the business of agriculture.
Agronomists emphasize that the situation is far from hopeless. Examples are already available, from the deserts of Mexico to the rice paddies of India, to show that it may be possible to make agriculture more productive and more resilient in the face of climate change. Farmers have achieved huge gains in output in the past, and rising prices are a powerful incentive to do so again.
But new crop varieties and new techniques are required, far beyond those available now, scientists said. Despite the urgent need, they added, promised financing has been slow to materialize, much of the necessary work has yet to begin and, once it does, it is likely to take decades to bear results.
“There’s just such a tremendous disconnect, with people not understanding the highly dangerous situation we are in,” said Marianne Bänziger, deputy chief of the International Maize and Wheat Improvement Center, a leading research institute in Mexico.
A wheat physiologist at the center, Matthew Reynolds, fretted over the potential consequences of not attacking the problem vigorously.
“What a horrible world it will be if food really becomes short from one year to the next,” he said. “What will that do to society?”
***By the late 1980s, food production seemed under control. Governments and foundations began to cut back on agricultural research, or to redirect money into the problems created by intensive farming, like environmental damage. Over a 20-year period, Western aid for agricultural development in poor countries fell by almost half, with some of the world’s most important research centers suffering mass layoffs.
And erratic weather began eating into yields. A 2003 heat wave in Europe that some researchers believe was worsened by human-induced global warming slashed agricultural output in some countries by as much as 30 percent. A long drought in Australia, also possibly linked to climate change, cut wheat and rice production.
In 2007 and 2008, with grain stockpiles low, prices doubled and in some cases tripled. Whole countries began hoarding food, and panic buying ensued in some markets, notably for rice. Food riots broke out in more than 30 countries.
Farmers responded to the high prices by planting as much as possible, and healthy harvests in 2008 and 2009 helped rebuild stocks, to a degree. That factor, plus the global recession, drove prices down in 2009. But by last year, more weather-related harvest failures sent them soaring again. This year, rice supplies are adequate, but with bad weather threatening the wheat and corn crops in some areas, markets remain jittery.
Experts are starting to fear that the era of cheap food may be over. “Our mindset was surpluses,” said Dan Glickman, a former United States secretary of agriculture. “That has just changed overnight.”
Forty years ago, a third of the population in the developing world was undernourished. By the tail end of the Green Revolution, in the mid-1990s, the share had fallen below 20 percent, and the absolute number of hungry people dipped below 800 million for the first time in modern history.
But the recent price spikes have helped cause the largest increases in world hunger in decades. The Food and Agriculture Organization of the United Nations estimated the number of hungry people at 925 million last year, and the number is expected to be higher when a fresh estimate is completed this year. The World Bank says the figure could be as high as 940 million.