Scientists generally predict that crop yields could fall in a warmer world—though higher atmospheric CO2 by itself should raise yields, as plants find it easier to extract CO2 from the air to make carbohydrates. (Related: "Cornfields Could Yield Less by Midcentury.")

The effect climate change might have on the nutritional value of crops, as opposed to their yield, has been even murkier. Previous studies have given conflicting results.

In the largest study yet, Samuel Myers of Harvard University and colleagues report that the CO2 levels expected in the second half of this century will likely reduce the levels of zinc, iron, and protein in wheat, rice, peas, and soybeans. Some two billion people, the researchers note, live in countries