Seasonal Drying In The Amazon Has Greater Impact Than Previously Thought

redOrbit Staff & Wire Reports – Your Universe Online

New research suggests that the southern part of the Amazon Rainforest faces a higher risk of dieback (the gradual dying of plant shoots beginning at the tip) as a result of seasonal drying than reported by climate models used by the most recent Intergovernmental Panel on Climate Change (IPCC).

Furthermore, University of Texas at Austin Jackson School of Geosciences professor Rong Fu and her colleagues report that severe loss of rainforest could result in the release of massive amounts of carbon dioxide into the atmosphere, while also disrupting plants and animals in one of the regions of highest biodiversity on Earth.

Fu’s team reviewed 30 years of ground-based rainfall measurements and found that since 1979, the dry season in the southern part of the rainforest has increased by about one week per decade. During this same period, the annual fire season has also increased in length, and the researches blame global warming for the longer dry season.

“The dry season over the southern Amazon is already marginal for maintaining rainforest. At some point, if it becomes too long, the rainforest will reach a tipping point,” Fu said in a statement. “The length of the dry season in the southern Amazon is the most important climate condition controlling the rainforest. If the dry season is too long, the rainforest will not survive.”

The new paper, which is published this week in the journal Proceedings of the National Academy of Sciences (PNAS), contradict forecasts made by the climate models used in the IPCC report. Those models project the southern Amazonia dry season to increase by no more than 10 days by the end of the century, even under scenarios that predict dramatic increases in greenhouse gas levels. By those calculations, there should be a low risk of rainforest dieback caused by climate change.

“The researchers say the most likely explanation for the lengthening dry season in the southern Amazon in recent decades is human-caused greenhouse warming, which inhibits rainfall in two ways,” the university explained. “First, it makes it harder for warm, dry air near the surface to rise and freely mix with cool, moist air above. And second, it blocks cold front incursions from outside the tropics that could trigger rainfall.”

According to Fu and her associates, the climate models used by the IPCC do not do an adequate job representing the processes that result in prolonged dry seasons, potentially explaining why they foresee just a slight increase in those periods.

Typically, the rainforest removes carbon dioxide from the atmosphere, but during a severe drought eight years ago, it released a significant amount of the greenhouse gas, and the researchers believe that the phenomenon could occur again if the annual dry season continues to grow longer.

In fact, “if dry seasons continue to lengthen at just half the rate of recent decades, the Amazon drought of 2005 could become the norm rather than the exception by the end of this century,” the university said.

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

Researchers from Duke University, the Universidad de Antioquia in Colombia, Columbia University’s International Research Institute for Climate and Society, the National Oceanic & Atmospheric Administration (NOAA), the National Center for Atmospheric Research (NCAR) and Boston University were also involved in the study. Their work was sponsored by the National Science Foundation (NSF) and the NOAA Climate Program Office Modeling, Analysis, Prediction and Projection Program.

Image Below: During the 2005 and 2010 droughts, satellites detected decreased vegetation greenness — or a lower Normalized Vegetation Index (NDVI) — over the southern Amazon rainforest (orange and red regions). NDVI is derived from MODIS instruments on NASA’s Terra and Aqua satellites. Credit: Image courtesy of Ranga Myneni, Jian Bi and NASA.