From Slate:

The Food and Agriculture Organization predicts that the global population will increase by 2.3 billion between now and 2050. This demographic explosion, intensified by an emerging middle class in China and India, will require the world's farmers to grow at least 70 percent more food than we now produce. Making matters worse, there's precious little arable land left (PDF) for agricultural expansion.

Barring a radical rejection of the Western diet, skyrocketing demand for food will have to be met by increasing production on pre-existing acreage. No matter how effectively we streamline access to existing food supplies, 90 percent of the additional calories required by midcentury will have to come through higher yields per acre.

For more, see: Organic Crops Alone Can't Feed the World