Carbon goes green

Let’s check off the things we’ve heard about carbon dioxide: Humans exhale it; oil and coal emit it. It’s a combination of a carbon atom and two oxygen atoms. It’s a greenhouse gas. It’s accumulating in the atmosphere, contributing to rising global temperatures. It’s making the planet greener.

Oh, wait. You hadn’t heard the green part? If not, it’s not for lack of science. Multiple recent studies show more clearly than ever how carbon’s “fertilization effect” is affecting plant growth on a global scale, increasing foliage, and making plants in dry environments more water efficient. Scientists say models have vastly underestimated carbon’s role in greening. We know little about the long-term effects, but many could be positive.

To a plant, carbon dioxide is food. But the gas has a counterintuitive effect on a plant’s metabolism: The more CO2 in the air, the less water a leaf needs.

A 2011 study of Florida plants and a July 2013 study of forests around the world saw this odd effect in action. In the first study (published in PNAS), an analysis of Florida foliage and leaves buried in peat estimated to be 150 years old showed a trend of increased water efficiency over time. In the second (in Nature), forest carbon and water data from the past two decades showed trees becoming more water efficient. “We went through every possible hypothesis of what could be going on,” said researcher Trevor Keenan, who now works at Macquarie University in Australia. “The only phenomenon that could cause this type of shift in water-use efficiency is rising atmospheric carbon dioxide.”

The efficiency effect is most striking in arid environments, such as deserts, savannahs, and dry woodlands. When plants in dry ground sense an increase in CO2 and become more water efficient, they respond by shooting out new leaves to take advantage of the situation.

In another study this summer, published in Geophysical Research Letters, Australian researchers used satellite images of global foliage cover to estimate the growth between 1982 and 2010. Although foliage around the world increased 9 percent, the greening in warm, dry environments was much higher—14 percent.

Increased rainfall drove much of the foliage growth, but CO2 also played a major role. After isolating the effects of changes in rainfall, humidity, temperature, and land use, the researchers estimated that warm, dry areas were 11 percent greener than they would have been had CO2 remained constant since 1982.

The long-term effects of this greening are hard to predict, but scientists speculate it could prevent soil erosion, increase plant life in deserts, and increase forest cover. On the other hand, it could increase fuel for forest fires, and provoke unexpected changes in habitats, requiring wildlife—or farmers—to adjust ways of living.

At least now you’ve finally heard about it.

Money trees

Tree leaves that contain microscopic particles of gold could be flagging hidden deposits beneath. Scientists publishing in Nature Communications in October confirmed that eucalyptus trees growing above gold deposits suck up tiny amounts of the precious metal through their roots—which may grow more than 100 feet deep—then deposit it within their leaves. —D.J.D.