An atmospheric valentine for you

Published February 1, 2001

With Valentine’s Day fast approaching, we thought we’d stop and smell the carbon dioxide-enhanced roses. What fate does forsythia face? Dare we hope for daffodils? Will the wisteria whither? Three recent studies focus on flora in a world of elevated atmospheric CO2.

In the first, Niu and colleagues grew yellow and primrose pansies in greenhouses with various temperatures, light levels, and atmospheric CO2 concentrations. They found that a 600 ppm increase in carbon dioxide increased the total vegetative dry weights of the plants from 10 percent to 30 percent and flower size by 4 percent to 10 percent. Their research shows that as CO2 concentrations continue to rise in the years to come, pansies will increase growth and increase the production of flowers.

In another study, scientists from the University of South Carolina grew flowering camphorweed at natural (400 ppm) and elevated (700 ppm) atmospheric CO2 concentrations. But rather than sit back and enjoy the plants’ pretty yellow blossoms, the researchers systematically defoliated 25 percent of the leaves each week.

Johnson and Lincoln found that the CO2-enriched plants had a substantially higher rate of photosynthesis and an earlier and more abundant yield of flower biomass. They concluded that camphorweed “plants can compensate reproductively for defoliation, and that CO2 enrichment has a positive effect on reproductive output that is unrelated to defoliation or plant size.”

The final piece comes from James Bunce, a prolific scientist and frequent subject of Greening Up attention. Bunce grew dandelions (the bane of many a gardener, but a flower nonetheless, and a staple salad ingredient on many a Depression-era table) in a field site with atmospheric CO2 concentrations maintained at 350 ppm and 525 ppm. Measurements showed that the photosynthesis of the dandelions was stimulated by higher levels of carbon dioxide and that higher temperatures further increased the stimulation.

But at the coldest temperatures, the photosynthetic response to elevated carbon dioxide was significantly higher that what Bunce had anticipated. He concluded that “one result is that the stimulation of photosynthesis by elevated [CO2] can be larger than expected at low temperatures. It is premature to conclude that the relative stimulation of photosynthesis by rising atmospheric [CO2] will necessarily be less in cool climates or at cool times of year.”

Enjoy that bouquet this Valentine’s Day, and rest assured that the flowers of today and those of the future will flourish in higher atmospheric carbon dioxide concentrations.

Robert C. Balling Jr., Ph.D. is director of the Laboratory of Climatology at Arizona State University and coauthor of The Satanic Gases.


Bunce, J.A. 2000. Acclimation of temperature of the response of photosynthesis to increased carbon dioxide concentration in Taraxacum officinale. Photosynthesis Research, 64, 89-94.

Johnson, S.L., and Lincoln, D.E. 2000. Allocation responses to CO2 enrichment and defoliation by a native annual plant Heterotheca subaxillaris. Global Change Biology, 6, 767-778.

Niu, G., et al., 2000. Day and night temperatures, daily light integral, and CO2 enrichment affect growth and flower development of pansy (Viola x wittrockiana). Journal of the American Society of Horticultural Science, 125, 436-441.