Water tables are now falling and wells are going dry in countries that contain half the world's people, including the big three grain producers: China, India, and the United States.  In China, water shortages have helped lower the wheat harvest from its peak of 123 million tons in 1997 to below 100 million tons in recent years. Water shortages are also making it more difficult for farmers in India to expand their grain harvest. In parts of the United States, such as the Texas panhandle and in western Oklahoma and Kansas, depletion of the Ogallala aquifer has forced farmers to return to lower-yield dryland farming.

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Perhaps the most dangerous threat to future food security is the rise in temperature.  Among crop ecologists there is now a consensus that for each temperature rise of 1 degree Celsius above the historical average during the growing season, we can expect a 10 percent decline in grain yields. When describing weather-reduced harvests, crop analysts often refer to the crop prospect when weather returns to normal. They fail to realize that with the earth's climate now in flux, there is no longer a norm to return to.

More and more in recent years, crop-withering heat waves have led to major crop losses. For a recent example, the early estimate of India's wheat harvest this year of 73 million tons dropped to 68 million tons as high temperatures during the crop's critical growth stage in January and February shrank the harvest.

The troubling constraints on grain production growth, such as spreading water shortages and rising temperatures, are making it difficult for farmers to keep up with the record growth in demand.  As a result the world grain market may become a seller's market, one where higher grain prices, like high oil prices, are an integral part of the economic landscape.

and, from the same source referencing a June 21, 2007 article by Sarit Menahem in Haaretz headlined "World Grain Stocks Fall to 57 Days of Consumption":

The price of wheat shot up 30% in less than a month, to a 12-year high of around $420 to around $600 for 5,000 bushels.

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When it comes to agricultural commodities, you can't ignore the effect of global warming. The weather is changing. Floods and droughts are not good for crops.

The U.S. is the biggest wheat exporter and its main cultivation areas are Kansas and Oklahoma.

"The last winter was a terrible one. The ground froze. A week ago the region was swept by rainstorms that turned the ground muddy and impossible to harvest," describes Ron Eichel, the chief international markets strategist at Israel Brokerage & Investments. It's still raining, too and the crops are rotting. Only 9% of the crop is rated as being in excellent condition, [emphasis added] the Kansas Agricultural Statistics Service said this week, adding that 37% is in horrible condition.  

It is to be noted China does not published grain production figures so their production is an even worse estimate than those for the US and India.

The increase in Grain Production:

has been met with an increase in population [Note the timeline is different from the previous chart!]:

and while the rate of increase is declining the total population is increasing.  Giving:

[Again, the timeline is different.]

Peak Oil plays into this in two ways.  First, as the cost rises for crude the costs of agricultural production also rises.  Oil is used, notably, as a fuel for farming machinery but it is also used as a basis for many farm input: fertilizers, pesticides, & etc.  Oil, as fuel, is also used for transportation - at all levels of the farm to consumer network - but is also in surprising ways, e.g., burned to dry the grains to acceptable moisture levels for long term storage.  Secondly, as the price of gasoline (petrol) increases the use of grain as the raw material for automobile fuel  substitutes, e.g., ethanol.  Thus, grain prices must rise to cover the rise in production costs and grain prices must rise as demand for grain stocks as fuel substitutes grows.

Global Warming will play a part, again in two ways.  As GW sets-in the current prime growing areas with shift northward.  Unfortunately, the farmers in these areas do not have the equipment, nor do they have the capital to purchase the equipment required for large field-crop grain production.  To give an idea, a lower range John Deere combination harvester has a base price of around $100,000 (US.)  The tractor, and equipment the tractor pulls around the field, runs about the same.  Ancillary equipment: dryers, wagons, grain harvesting heads, total about another $100,000.  $300,000 may not sound like a lot of money but it's beyond the financial reach of most farmers.  Secondly, as GW really starts to impact the weather it is to be expected the weather will move out of "normal" (since ~ 12,000 BP.)  This is Bad News for farmers since once a crop is in the field there's really no way -- without a massive expediture in money and effort -- to irrigate the crop; they have to depend on rain.  And rain will be a sometime thing.  Worse it has to be the right amount of rain as too much is as bad as too little and it has to be at the right times too early is as bad as too late, or never.

The good news is GW will open up new areas for grain field crop production.  The bad news is those areas lack the infrastructure required for 'taking up the slack.'  Try harvesting, storing, and transporting 10 million tons of corn (zea mays) without harvesters, grain bins, and railroad lines.  Further, if you don't do it right grain dust can catch on fire and even explode.  

Prognosis:  food prices will rise.  World hunger will increase.  Without an "activist goverment"  (alert the KlausBot!) and a whole bunch of world co-operation we can expect famine, mass migration, and a very high likelihood of regional food wars - think Rwanda with automatic weapons and machine guns. (I doubt WW III is in the cards since we don't have the oil.)  If the EU had any brains you'd be making nicey-nice with Russia since they look to be a major winner in the food production department as well as being a up & coming oil producer to boot.