The National Climate Assessment, released yesterday by the U.S. Global Change Research Program, warns that “by mid-century, when temperature increases are projected to be between 1.8°F and 5.4°F and precipitation extremes are further intensified, yields of major U.S. crops and farm profits are expected to decline.”

But extreme weather and temperatures won’t be the only threats to crops: New research shows that the nutrition of those crops may decline. Several crop species grown in an environment that had atmospheric carbon dioxide concentrations like those expected in 2050 had lower levels of two important nutrients, zinc and iron, researchers report today in Nature.

Around two billion people worldwide already suffer from deficiencies in these nutrients, and the research shows that climate change will exacerbate this global public health problem. The scientists didn’t dance around the issue, bluntly titling their paper “Increasing CO 2 threatens human nutrition.”

The international group of researchers, led by Samuel S. Myers of Harvard University, gathered data from seven experimental locations in Japan, Australia and the United States where crops were grown under natural conditions except with additional carbon dioxide pumped into the air. Six crop species—rice, wheat, corn, soybeans, field peas and sorghum—were raised with carbon dioxide levels of 546 to 586 parts per million in the air surrounding them.

For context, the current atmospheric carbon dioxide concentration hovers around 400 ppm, a significant increase from the pre-industrial level of about 280 ppm.

Wheat, rice, field peas and soybeans exposed to these high levels of atmospheric carbon dioxide all had lower levels of zinc and iron when grown under conditions of elevated carbon dioxide. Wheat, rice and field peas also had lower levels of protein.

All four crops are C3 plants. There are three basic types of photosynthesis—C3, C4 and crassulacean acid metabolism (CAM, which isn’t used by any of the plants in this study). In C3 plants, photosynthesis happens throughout the entire leaf. In contrast, C4 plants have special anatomy that lets them conduct photosynthesis inside their cells. These plants can concentrate and hold carbon dioxide within their cells, so the amount of the gas in the air doesn’t matter so much. But C3 plants appear to be more sensitive, the researchers found.

Just how the elevated carbon dioxide works to lower nutrient levels isn’t clear at this point. But the finding is worrisome because billions of people worldwide depend on C3 crops to get the majority of their dietary zinc and iron.

Declining nutrition, however, is only one worry when it comes to climate change. The impacts on agriculture are much wider.

In a simplistic view of climate change, more carbon dioxide is often thought of as a positive for agriculture. After all, plants need carbon dioxide to grow. And as the world warms, that opens up new areas to agriculture.

But reality is not so simple. Excess carbon dioxide has actually been shown to reduce some crop yields. There’s no guarantee that there will be good soil and enough water for agriculture in those newly opened-up areas. And climate change is even now affecting plant diseases, helping to spread coffee rust, for instance, and driving up the price of your morning cup of joe.

Already, “there have already been detectable impacts on production due to increasing temperatures,” notes the National Climate Assessment, “Climate change is affecting the American people now,” and the “choices we make will affect our future and that of future generations.”

Science may be able to help. The Nature study notes, for instance, that it might be possible to breed varieties of rice that are less vulnerable to higher levels of carbon dioxide. And scientists have been breeding crop varieties that are drought-tolerant, which could help people to grow these plants in areas that are otherwise unsuitable for agriculture.

Exactly how climate change will play itself out and affect the world’s food supply and public health is difficult to predict, though, the Nature researchers note. There are too many variables in the global equation, including how humanity will respond to the climate change challenge. “We expect many surprises,” they write.