The Amazon rainforest, which covers a staggering 550 million hectares, is losing its ability to soak up carbon dioxide from the air, according to the results of a huge long-running study. Over the past 2 decades, trees in the Amazon have been dying at an increasing rate, rendering the massive jungle a weaker absorber, or “sink,” of CO 2 , which plants take up during photosynthesis, forest ecologists report. The unexpected finding itself isn't evidence of impending catastrophe, but it highlights the unpredictability of the effects of climate change and a warming planet.

Forests play a crucial role in maintaining the global carbon budget. Worldwide, they suck up 2.4 billion metric tons of carbon each year, with the massive Amazon absorbing a quarter of that total. In principle, climate change should help forests take up even more carbon dioxide, as warmer temperatures and increased levels of the gas should encourage trees to grow. Northern forests in both so-called boreal zones and savannas are absorbing more carbon each year.

Roel Brienen, a forest ecologist at University of Leeds in the United Kingdom, and colleagues wanted to know how the changing climate is affecting the Amazon, the world's largest tropical rainforest. So they analyzed plots of the jungle that spread across a network spanning eight South American countries and eventually included 321 plots—the biggest such tree-monitoring network in the tropics. Scientists have regularly monitored the growth of the trees in those sites since 1983, measuring the diameters of trees and calculating their total biomass.

Analyzing data through 2011, the study shows that the rate of tree growth has remained flat in the past decade. But the fraction of trees dying each year is increasing, the researchers report online today in Nature. As a result, the average hectare of Amazon forest gains less than 1 ton of biomass each year—less than one-half the rate in the early 1990s.

The finding clashes with the predictions of prominent models, Brienen says. What's most "surprising," he says, is that the increase in tree mortality occurred in healthy years, including years before major droughts in 2005 and 2010.

One possible reason for the rising tree mortality is a paradoxical effect of the higher concentration of CO 2 in the air, which generally encourages tree growth. "Trees in the forest are growing faster, but faster growth makes the trees die younger," Brienen says. That's because faster growing trees tend to invest less energy in defenses against disease and produce wood that is less dense. So they may be more susceptible to sickness or falling over. But how rising temperatures, drought, and the makeup of species in the jungle may each be affecting the forest health is still unclear, he says.

The "important" finding raises concerns that "increases in mortality could, if sustained over a long time, [lead] to degradation of the forest” with fewer old-growth trees, says Steven Wofsy, a geochemist at Harvard University who did not participate in the research. But, Wofsy says, the use of lidar and a close look at the makeup of different tree species could better explain what is actually happening.

Policymakers should pay attention to the Amazon's shrinking role as a global carbon sink, says Oliver Phillips, a forest ecologist at Leeds and co-author on the paper. "Forests are doing us a huge favor, but we can't rely on them to solve the carbon problem," he says. "Instead, deeper cuts in emissions will be required to stabilize our climate."