Carbon dioxide is a heat-trapping greenhouse gas linked to automobile exhaust and other fossil fuels. Some scientists expect the level of carbon dioxide in the atmosphere to significantly rise over the next few decades.

A gradual increase in the earth's temperature is feared to have many harmful effects, including melting glaciers, raising sea levels and destroying some wildlife habitats.

"If you're looking for a positive spin on rising carbon dioxide levels, it's that agricultural production in some areas is bound to increase," Curtis said. "Crops have higher yields when more carbon dioxide is available, even if growing conditions aren't perfect."

But while there may be more food, it may not be as nutritious, Curtis said.

"The quality of the food produced by the plant decreases, so you've got to eat more of it to get the same benefits," Curtis said. "Under the rising carbon dioxide scenario, livestock - and humans - would have to increase their intake of plants to compensate for the loss."

Curtis and other researchers pulled together data from 159 similar studies from the past two decades to determine the effects of climate change on plant reproduction. They analyzed the ways plants respond to carbon dioxide through flowers, fruits, fruit weight, number of seeds, and the plant's capacity to reproduce.

Individual crops varied in their response to higher carbon dioxide levels.

Rice was the most responsive with its seed production increasing an average of 42 percent. Soybeans showed a 20 percent increase in seed, followed by wheat with 15 percent, and corn with 5 percent, Curtis said.

Even though seed size increased, the amount of nitrogen in the seeds didn't. Nitrogen levels fell by an average of 14 percent across all plants except cultivated legumes, such as peas and soybeans, the research showed.

For example, the total number of seeds in wheat and barley plants increased by 15 percent, but the amount of nitrogen in the seeds declined by 20 percent.

"That's bad news," Curtis said. "Nitrogen is important for building protein in humans and animals. If anything, plant biologists want to boost the levels of nitrogen in crops."