Risk of Amazon rainforest dieback is higher than IPCC projects

A press release from the University of Texas at Austin:  A new study suggests the southern portion of the Amazon rainforest is at a much higher risk of dieback due to stronger seasonal drying than projections made by the climate models used in the latest report by the Intergovernmental Panel on Climate Change (IPCC). If severe enough, the loss of rainforest could cause the release of large volumes of the greenhouse gas carbon dioxide into the atmosphere. It could also disrupt plant and animal communities in one of the regions of highest biodiversity in the world.

Using ground-based rainfall measurements from the past three decades, a research team led by Rong Fu, professor at The University of Texas at Austin’s Jackson School of Geosciences, found that since 1979, the dry season in southern Amazonia has lasted about a week longer per decade. At the same time, the annual fire season has become longer. The researchers say the most likely explanation for the lengthening dry season is global warming.

“The dry season over the southern Amazon is already marginal for maintaining rainforest,” says Fu. “At some point, if it becomes too long, the rainforest will reach a tipping point.”

The new results are in stark contrast to forecasts made by climate models used by the IPCC. Even under future scenarios in which atmospheric greenhouse gases rise dramatically, the models project the dry season in the southern Amazon to be only a few to 10 days longer by the end of the century, and therefore the risk of climate change-induced rainforest dieback should be relatively low.

....“The length of the dry season in the southern Amazon is the most important climate condition controlling the rainforest,” says Fu. “If the dry season is too long, the rainforest will not survive....

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. Image courtesy of Ranga Myneni, Jian Bi and NASA.