The answer, according to researchers at UC Davis, is: Probably! Since the late 1980s, researchers have run experiments — like this one in Arizona — that pipe carbon dioxide into crop fields to simulate what happens if fossil-fuel use continues unabated. Now, Nature published the answer from the first field study: Get ready for fast-growing foodstuffs with less nutritional value.
Testing levels of carbon dioxide that are "anticipated during this century," the authors of the report write, "Food quality will suffer." The reason, they explain, is because the carbon dioxide enters wheat and occupies space that would otherwise be occupied by nitrogen. This is a problem because plants use nitrogen to make protein. It's not just wheat: The change occurs in crops like potatoes and barley, too. The researchers point out it's a problem Big Ag could manage with more fertilizer (surprise), but this basically has no upside — it's costly, could contaminate groundwater, and actually increases greenhouse gases.