ABOVE: Boreal forests like this one in Canada will likely see a decline in photosynthesis by the end of the century. © ISTOCK.COM, Onfokus

Plants are a highly effective carbon sink. Globally, forests absorb about 7.6 billion metric tons of carbon dioxide per year, about 1.5 times the annual emissions of the United States.

Since the 1980s, climate warming, prompted by rising levels of carbon dioxide and other greenhouse gasses in the atmosphere, has caused an increase in plant productivity in the Northern Hemisphere outside the tropics.

But new research published May 30 in Nature Climate Change estimates that summer warming in this region will begin to have the opposite effect within the next half-century, causing photosynthesis—or plant productivity—to decline. The global land carbon sink will likely take a hit as a result, the authors write. 

The researchers found that generally in the Northern Hemisphere, warmer temperatures will mean less summer plant productivity by 2070 for most regions below 60° N—the approximate latitude of Oslo, Norway, and Anchorage, Alaska. Plant productivity in Arctic areas, however, will continue rising as the temperature increases.

“It’s an interesting paper,” says Kai Zhu, an environmental scientist at the University of California, Santa Cruz, who was not involved in the research. The ability of  vegetation to capture carbon has grown over the past few decades, but whether this would continue into the future was previously unknown, he explains. The research shows that increased temperatures will eventually lead to decreased photosynthesis, which “will be bad news.” 

The study focused on the extratropical Northern Hemisphere because it is responsible for 40 percent of the planet’s terrestrial carbon sink, and the hemisphere’s productivity is strongly linked to temperature. In the tropics, water is a larger driver of productivity, so the researchers excluded this region. 

Scientists used two different ways of estimating the effects of rising summer temperatures on productivity: models that represent photosynthesis based on a variety of environmental factors, and climate models combined with extrapolations from historical observations of the thermal tolerance of wild plants. Anping Chen, a study coauthor and ecologist at Colorado State University, explains in an email that researchers rely more on the first method because it takes into account factors like elevated CO2 levels, changes in precipitation, and the acclimation of plants to warming.

Using eight Earth system models, the study found that under intermediate projected emissions conditions, about 48 percent of vegetated land in the Northern Hemisphere will experience a decline in plant productivity due to warming by 2060, rising to 78 percent by the end of this century. 

The historical observation-based approach pushed this timeframe much earlier, coauthor Brendan Rogers, an Earth system scientist at the Woodwell Climate Research Center in Massachusetts, tells The Scientist. Under the best-case scenario among possible outcomes, that method predicted that the temperature will exceed the optimum for productivity earlier than 2030 under intermediate emissions in more than 67 percent of the extratropical Northern Hemisphere. 

Rogers says the Earth system model results may be too optimistic because they overestimate how well forests acclimate to climate change. The models don’t take into account factors like widespread fires, pests, and pathogens, he explains.

Whether the results mean that overall carbon in the atmosphere will increase even faster than it is now is “the 10 million dollar question,” because there are a lot of factors affecting the carbon balance, Rogers explains. But the paper is “another warning sign that all is not well with the planet,” he says. “And it's another reason to curb emissions.”