Stunting Growth: Ozone will trim plants’ carbon-storing power

Increases in low-altitude ozone predicted for the upcoming century will stifle the growth of vegetation in many regions, causing planet-warming carbon dioxide to build up in Earth’s atmosphere more quickly than had been expected, a new model suggests.

GROWING PROBLEM. Top map shows average ozone concentrations June through August today; bottom map predicts concentrations in the year 2100. Estimated increases in ozone could suppress vegetation and thereby exacerbate global warming. Sitch et al.

Low-altitude ozone—as opposed to the planet-protecting layer of the gas in the stratosphere—forms when the sun’s ultraviolet radiation stimulates reactions between gases such as nitrogen oxide, carbon monoxide, and methane. Preindustrial concentrations of ground-level ozone typically ranged between 15 and 30 parts per billion (ppb), says Stephen Sitch, an ecologist at the Hadley Centre for Climate Prediction and Research in Wallingford, England. Now, emissions from cars, power plants, and other industrial sources routinely boost ozone in many regions of the world to well over 40 ppb.

Such ozone concentrations could easily damage a plant’s leaves and stifle its growth, says Sitch. Most current climate models include the planet-warming effect of low-altitude ozone, a relatively weak greenhouse gas, he adds, but previous predictions haven’t included warming due to ozone stifling the growth of vegetation. In an upcoming Nature, he and his colleagues attempt to calculate that effect. They also improved on current estimates of ozone damage to plants by including the effect of ozone concentrations below 40 ppb, says Sitch.

“I totally agree with this concept,” comments David F. Karnosky, a plant geneticist at Michigan Technological University in Houghton. “This [new technique] is a large improvement over previous [climate-change] models,” he adds.

According to some current projections, low-altitude ozone will exceed 40 ppb in almost all areas of the world by 2100, says Sitch. Moreover, many regions—including western Europe, eastern North America, Brazil, and southeast Asia—will see ozone concentrations above 70 ppb, he notes.

The researchers simulated the climate from 1901 to 2100, using data from the 20th century to calibrate the model. In 1901, the world’s plants absorbed enough carbon dioxide from the atmosphere each year to store about 115 billion metric tons of carbon, Sitch and his colleagues estimate. A simulation with rising carbon dioxide but unchanging ozone concentrations predicted that plants would soak up about 200 billion metric tons of carbon annually by the year 2100.

When the researchers allowed ozone concentrations to rise as expected, however, the projected damage to vegetation reduced carbon absorption to only about 170 billion metric tons each year. The warming due to ozone-reduced plant growth would thus rival the warming from ozone’s greenhouse effect, says Sitch.

The team’s new simulations are “a useful start,” says Mike Ashmore, an environmental scientist at the University of York in England. However, he notes, scientists have only limited data about how various plants—especially tropical ones—are damaged by ozone exposure.

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