A new study revealed that the Amazon is gradually losing its capacity to absorb atmospheric carbon dioxide.

The findings suggest the Amazon rainforest is absorbing only half of the two billion tons of CO2 per year it was taking in during the 1990s, the University of Leeds reported. 

The findings come from a 30-year survey of the South American rainforest that was the most extensive to date. For decades the Amazon rainforest was a major "carbon sink," meaning it absorbed more CO2 than it released. Climate sinks are crucial for the environment because they are believed to help slow down climate change and ease the greenhouse effect, but the death of a large number of trees has reduced the rainforest's efficiency. Amazonian tree mortalities are believed to have increased by as much as a third since the 1980s. 

To make the findings a team of researchers analyzed 321 forest plots across the Amazon and identified and measured 200,000 trees. They determined that when carbon dioxide first started to increase in the atmosphere it boosted photosynthesis, leading to a "growth spurt" in Amazon trees, but there appear to have been later consequences.

"With time, the growth stimulation feeds through the system, causing trees to live faster, and so die younger," said study co-author Professor Oliver Phillips, from the university's School of Geography

Regardless of the causes behind the increase in tree mortality, this study shows that predictions of a continuing increase of carbon storage in tropical forests may be too optimistic. The observed phenomenon could also be linked to noticeably higher temperature and droughts in the Amazon.

"Climate change models that include vegetation responses assume that as long as carbon dioxide levels keep increasing, then the Amazon will continue to accumulate carbon. Our study shows that this may not be the case and that tree mortality processes are critical in this system," said Roel Brienen, from the school of geography at the University of Leeds.

The findings were published in a recent edition of the journal Nature.