美国国家公共电台 NPR As Carbon Dioxide Levels Rise, Major Crops Are Losing Nutrients

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DAVID GREENE, HOST:

The amount of carbon dioxide in the air is rising because of human activity. And NPR's Merrit Kennedy reports that scientists around the world are trying to understand how more CO2 is going to impact the plants we eat.

MERRIT KENNEDY, BYLINE¹: Plants need carbon dioxide to grow, but its effects on them are complicated. At a government lab outside of Washington, D.C., are rows of controlled chambers³ that look kind of like industrial refrigerators. Scientists are using them to test how plants react to different levels of CO2. Lewis Ziska, who's a plant physiologist⁴ with the U.S. Department of Agriculture, walks me over to a set of these chambers with a crop important to many of us, coffee.

LEWIS ZISKA: This is a sense of the coffee that's growing in there now.

KENNEDY: He swings open the door. It's really bright to mimic⁵ the sun. Green coffee plants stand in neat rows. The CO2 levels in here are lower than they are today. They correspond to levels from about 250 years ago.

ZISKA: CO2 around the pre-industrial age.

KENNEDY: Across the hall, we can see a possible glimpse of the plants' future. Here, there's a chamber² with plants growing at CO2 levels projected for the end of this century.

So we're seeing coffee that look like they're a little bit taller in the high CO2 area versus⁶ the lower CO2 area.

They've all been growing for the same amount of time, but the high CO2 coffee plants are bigger. The extra CO2 seems to be making them grow faster. Scientists have noticed that in many kinds of plants, higher CO2 levels produce bigger crops. That sounds like a good thing, but there's a problem. Take rice, for example. Ziska teamed up with an international group of scientists to study whether high CO2 levels had an effect on the rice's nutritional⁷ value.

ZISKA: Was it changing not just how the plant grew, but the quality of the plant?

KENNEDY: They tested how 18 different kinds of rice responded to CO2 levels that are projected by the end of the century, and the effect was clear. They observed decreases in protein, iron and zinc⁸, and four important B vitamins. Higher CO2 lead to less nutritious⁹ rice. And it's not just rice. The scope of this could be much bigger. Harvard's Sam Myers, who studies the impact of climate change on nutrition, tested CO2's impact on the protein, iron and zinc of a number of crops.

SAM MYERS: Most of the food crops that we consume showed these nutrient¹⁰ reduction.

KENNEDY: The effects varied¹¹. He says wheat, soybeans and field peas showed significant declines in nutritional value. Maize¹² and sorghum¹³ were less affected¹⁴. These studies are enough to raise concerns about how this is going to impact human health, he says.

MYERS: Under what circumstances would this be a big problem?

KENNEDY: Around 600 million people get more than half of their calories from rice. Myers says the people most at risk will be the ones who are highly dependent on a single crop and are struggling to get enough nutrients¹⁵.

MYERS: We're likely to see really significant health impacts from these nutrient changes.

KENNEDY: Going forward, it's going to be important to figure out whether there are other things that can help people who are at risk of nutrient deficiency. Naomi Fukagawa is the director of the USDA's Beltsville Human Nutrition Research Center. If this is indeed found to negatively impact people's health...

NAOMI FUKAGAWA: What we need to then know is, what else do we have as part of their diet that's culturally sensitive that can make up for those differences?

KENNEDY: Scientists are trying to understand why exactly higher CO2 causes plants to lose nutritional value. Finding answers to these questions is going to be crucial as CO2 levels continue to rise. Merrit Kennedy, NPR News, Washington.