Climate Change Inhibiting Amazon Rainforest's Ability to Absorb Greenhouse Gases, Study Suggests

The southern region of the Amazon rainforest faces a much greater risk of dieback due to seasonal drying than stated by the latest Intergovernmental Panel on Climate Change (IPCC) report, a new study published in the Proceedings of the National Academy of Sciences suggests.

Since 1979, the dry season in southern Amazonia has increased by roughly a week every 10 years even as the annual fire season has expanded, according to researchers from the University of Texas at Austin's Jackson School of Geosciences.

"The dry season over the southern Amazon is already marginal for maintaining rainforest," team leader Rong Fu said in a statement. "At some point, if it becomes too long, the rainforest will reach a tipping point."

The IPCC's recent report, in contrast, projected the dry season would be a maximum of 10 days longer by the end of the century -- a forecast that would place the risk of climate change-induced rainforest dieback relatively low.

Human-triggered greenhouse warming, which inhibits rainfall, is likely to blame, Fu and her colleagues said. It does this through two mechanisms. First, it makes it more difficult for warm, dry air near the surface to rise and mix with cool, moist air. Second, it keeps cold fronts from outside the tropics that could trigger rainfall from entering.

The climate models IPCC used poorly account for these processes, Fu said, which she believes is the likely reason for the different outcomes.

Under normal conditions, the Amazon rainforest removes carbon dioxide from the atmosphere. During a severe drought in 2005, however, it released 1 petagram -- equivalent to roughly one-tenth of annual human emissions -- into the atmosphere. Should the dry season continue to expand, Fu warns that years like 2005 may become the norm rather than the exception.

"Because of the potential impact on the global carbon cycle, we need to better understand the changes of the dry season over southern Amazonia," Fu said.