Diet For A Healthy Planet

By Robert C. Balling Jr., Ph.D.
Arizona State University

Two predictions for the future are air-tight. One, atmospheric carbon dioxide (CO₂) concentrations will surely rise, and two, Americans will continue the trend of eating healthier foods.

Milkshakes are all but obsolete, replaced by wheatgrass frappes and lingonberry smoothies. Cafeterias and restaurants now feature extensive sprout- and tofu-laden salad bars, and the old 1960s health food co-op is being replaced by health food chain stores bigger than the average Chevy dealership.

Americans are eating lower on the food chain than ever before, and in this series of "Greening Up" entries, we will explore how their favorite healthy foods will fare in an environment of increasing atmospheric CO₂.

Before we get to the main course, let’s dip into a platter of crudités—those popular munchies that are filling more backpacks and lunchboxes than ever before. We’re talking about those colorful favorites carrots and radishes.

A literature search on CO₂ enrichment and these two popular veggies quickly identify several classic articles that bring us more terrific eco-news. As with virtually every crop on the planet, these two popular vegetables will be more abundant thanks to the ever-increasing levels of atmospheric CO₂.

For example, Idso and Kimball grew these two crunchy treats in experimental chambers in Phoenix a decade ago; some were grown at ambient CO₂ conditions near 340 ppm (ambient at the time of the experiment), others at CO₂ concentrations raised by 300 ppm. The results for plants grown with mean temperatures near 31°C showed an incredible 190 percent growth enhancement (measured as mean per-plant dry weights) for carrots and a "mere" 31 percent enhancement for radishes (Figure 1).

Figure 1. Percentage growth enhancement related to an increase of atmospheric CO₂ from 340 ppm to 640 ppm.

If you are worried that some future temperature increase will spoil your appetite, forget it. Idso and Kimball found that for both carrots and radishes, increasing temperatures actually increased the growth enhancement. When the plants were grown with mean temperatures lower than 13°C, little enhancement from CO₂ was realized. But as the mean temperature rose, the growth enhancement effects of the elevated CO₂ also rose linearly.

Said the researchers: "As the air warms, the beneficial effects of added CO₂ appear to rise dramatically, with carrots more than doubling their normal productivity above a mean air temperature of 25°C." Radishes could not quite keep up with the carrots, but who cares? We all eat the carrots first anyway!

Do any conditions ruin plants’ hunger for more CO₂? Hardly. In an article by Pfirrman and Barnes, we find that the CO₂ enhancement may be even greater when some other environmental stress is applied. These two scientists grew radishes in CO₂ levels of 350 ppm and 750 ppm. Consistent with Idso and Kimball, they reported an increase of 36 percent in mean absolute growth rate for an increase of CO₂ of 400 ppm.

But then they attempted to torture the plants by increasing atmospheric ozone concentrations by a factor of four. Under the highly-elevated ozone levels, the effects of elevated CO₂ produced a staggering radish plant enhancement of 103 percent, compared with those plants grown in elevated O₃ and ambient CO₂. High levels of ozone are undoubtedly harmful to radishes, but when CO₂ is increased, the damage is more than compensated for in the plants. Radishes grown in elevated ozone and CO₂ still showed a 20 percent increase in growth over the plants grown with ambient CO₂ and ozone levels.

Again and again, we see that elevated CO₂ provides a biological benefit. Radishes and carrots are wonderful additions to our diet—staving off illness and promoting better eyesight and overall wellness.

So do your part for a healthier planet. Next time you go to the health food store, please do not ride a bike. Take a luxury car, enrich the atmosphere with CO₂, and feel good about your impact on carrots and radishes.

References:

Idso, S.B., and B.A. Kimball, 1989, Growth response of carrot and radish to atmospheric CO₂ enrichment. Environmental and Experimental Botany, 29, 135–139.