Extra CO2 means less-nutritious rice

ASteve Rissing

study published last month shows that rice plants grow well under conditions of increased atmospheric carbon dioxide expected later this century. Those plants, however, produce less-nutritious rice than what’s grown today.

Two billion of the 7.3 billion people alive today depend on rice as a major food source. For them, this is a big deal, even if they don’t know about the change — or our policymakers deny the underlying science.

Even the most ardent climate-change deniers accept that the amount of carbon dioxide in the atmosphere, emitted by burning fossil fuels, has increased in the past 50 years.

Carbon dioxide comes out of your car’s tailpipe when you drive a fossil-fueled car, or from your flue when you heat your home with fossil fuels.

In the middle of the past century, carbon-dioxide levels stood at 310 parts per million (ppm) of our atmosphere. The average last month was 411 ppm.

Climate deniers put a positive spin on this increase in carbon dioxide by calling it “plant food.” Like most distortions of scientific insights, there’s a grain of truth in that word play.

Plants form sugars — high-energy molecules — from carbon dioxide and water using sunlight energy. If carbon dioxide is “plant food,” then so is water. We use carbon dioxide and water to extinguish fires precisely because they are low-energy molecules. They are not food in the sense we normally use the term, even for plants.

Plants store sunlight energy using carbon dioxide and water as they synthesize sugars and other molecules made from those sugars.

The deniers say: More carbon dioxide “food,” bigger plants.

Not so fast. Carbon dioxide is hardly a complete diet for plants.

Plants get carbon dioxide from the air. The rest of their “diet,” to extend the analogy, comes from soil. Unlike atmospheric carbon dioxide, those soil components are not increasing. Indeed, increasing levels of atmospheric carbon dioxide appear to reduce absorption of soil minerals and other compounds necessary for robust plant growth and human food production.

Most plants, especially those we farm, face a delicate balance between absorbing carbon dioxide from the air and losing water in the process. Plant leaves are covered with tiny pores, stomates, that open, permitting entry of carbon dioxide but also resulting in evaporation of water.

This flow of water from the soil through the roots, into the plant and out stomates draws minerals and essential compounds such as nitrates, an organic form of nitrogen, into plants. Those nitrates provide the sub-units for plant-based proteins and vitamins that we and our farm animals eat.

The new study, published in Science Advances, shows that plants will experience a carbon-dioxide high later this century, but they also will produce grain with less protein, vitamins and minerals than crops grown today.

The authors of the study emphasize that 600 million people, primarily in Southeast Asia, derive more than 50 percent of their calories from rice.

The deterioration of the nutritional quality of rice in an environment with increased atmospheric carbon dioxide will stress these already at-risk populations even more.

Climate deniers offer these people carbs when they need — but won’t get — proteins, vitamins and minerals.