One of the silver linings of climate change, some have argued, is that high carbon dioxide levels will mean increased crop yields, which will, in turn, be good for combating global hunger (the logic, I suppose, being that if we’re frying fifty years from now, at least we won’t be hot and hungry). But some underpublicized studies, reported this month in Nature, cast a long shadow on this sunny assertion. (Sorry! It looks like the the article is subscription only, so I’ll be as descriptive as possible.)

In the 1980s, Bruce Kimball, a soil physicist with the USDA in Arizona, began conducting scientific experiments simulating a high-CO2 environment (using a system called "free air carbon dioxide enrichment," or FACE). He found that crop yields were elevated — plants imbibing large quantities of CO2 had more starch and more sugar in their leaves than those on a normal carbon diet. But because they also took up less nitrogen from the soil, they made less protein.

High-CO2 plants, one theory goes, skimp on nitrogen uptake because with so much CO2 around, they no longer need as much protein machinery to "fix" this gas into sugar. Kimball, however, said he wasn’t too worried about the reduced nutrition. "As far as crops go, I think higher CO2 is definitely a benefit. Yes, a little less nutrition than before, but we get more food … The farmer in the future would have to be sure and apply ample fertilizers to keep protein quality up,” he said. But others aren’t so sanguine.

Arnold Bloom, a plant biologist at the University of California in Davis, thinks that the reduced nitrogen levels seen in high-CO2 conditions is not just the result of plants needing less protein. He believes they actually become less able to absorb nitrates from the soil — in which case, dumping extra fertilizer on fields won’t be of much help.

So just how much of a protein difference are we talking about? Measuring bread flour, Herbert Weiser, head of cereal proteins at the German Research Centre for Food Chemistry, found that normal-CO2 wheat produced flour with 9.1 grams of protein per 100 grams, while high-CO2 wheat produced flour with only 7.8 grams of protein per 100 grams — about a 14 percent difference. While this won’t amount to much in the average American diet (or that of most developed nations, for that matter, where protein intake is far greater than necessary), it could certainly have an impact on the developing world.

It’s not just humans, of course, that consume plants. A nitrogen-impoverished plant world will have effects all along the food chain, from the grubs that feed on corn to the birds that eat those grubs to the snakes that eat those birds (you get the picture).

Meanwhile, other FACE research has shown that crops grown in high-CO2 environments have diminished calcium and zinc levels (between 10 and 20 percent). The take-home message? In a warming world, quality of nutrition will probably go down, even as quantity of food goes up.