As the amount of atmospheric carbon dioxide increases, trees grow more vigorously—but that boost fades when they experience drought and higher temperatures that are also associated with rising CO2 levels.

So concludes a study published (with open access) last month in the journal New Phytologist.

In essence, the researchers found that, at least for the Aleppo pines that they studied, the harmful effects of increased CO2 canceled out the beneficial effects. (The Aleppo pine is a Mediterranean species adapted for warm, dry conditions.)

Researchers grew Aleppo pines in a laboratory where they could control levels of CO2. Photo from Plant Ecophysiology Lab, Karlsruhe Institute of Technology

In laboratory conditions providing twice the current level of atmospheric CO2, the trees thrived, growing 40% larger than trees bathed in "normal" levels of CO2. But the growth advantage of increased CO2 essentially disappeared when the researchers introduced stress from drought and higher temperatures—which are also predicted to occur in the real world as a result of CO2 levels.

Ever since Swedish physical chemist Svante Arrhenius first concluded in 1895 that human activity was making the world warmer, some scientists have argued that rising CO2 levels are, on the whole, beneficial. (Arrhenius was one of them.)

In 2015, for example, The Global Warming Policy Foundation, a United Kingdom-based organization that is skeptical of the dangers of climate change, published a paper titled Carbon Dioxide: The Good News. It concluded that, because of increased carbon dioxide in the atmosphere, "the earth is greener, farms are more productive, and the planet can support both a larger biomass and more human beings."

The paper's author, Indur M. Goklany, is a senior adviser in the US Interior Department's Office of Policy Analysis. The New York Times reported earlier this month that he has been instructing department scientists to add language to the agency's scientific reports saying that the continuing increase in atmospheric CO2 "is beneficial because it 'may increase plant water use efficiency' and 'lengthen the agricultural growing season.'"

The authors of the new study say their research contradicts that sunny outlook.

"Overall, the impact of the increased CO2 concentration on stress reactions of the trees was rather moderate," said Nadine Ruehr, an author of the study and head of the Plant Ecophysiology Lab at Karlsruhe Institute of Technology in Karlsruhe, Germany. She was quoted in a KIT news release.

"With increasing heat and drought, it decreased considerably," she said. "From this, we conclude that the increasing CO2 concentration of the atmosphere cannot compensate the stress of the trees resulting from extreme climate conditions."