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A new study co-authored by BU Professor of Earth & Environment Ranga Myneni suggests the southern portion of the Amazon rainforest is at a much higher risk of dieback—the widespread death of forest species—due to climate change than previously expected. 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 most bio-diverse regions in the world.
Using ground-based rainfall measurements from the last three decades, the researchers 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. According to the study, the most likely explanation for the lengthening dry season is global warming.
“The rainforests survive the dry season with their deep root systems tapping water deep in the ground,” says Myneni. “This adaptation will be impaired by frequent, severe and prolonged dry seasons and may cause tree mortality. This has been observed prominently during the 2005 drought.”
The new results are in stark contrast to forecasts made by climate models used by the Intergovernmental Panel on Climate Change (IPCC). Even under future scenarios in which atmospheric greenhouse gases rise dramatically, the IPCC models project the dry season in the southern Amazon to be only a few days to ten days longer by the end of the century, and therefore the risk of climate change-induced rainforest dieback to be relatively low.
Myneni and his colleagues’ report appears this week in the journal Proceedings of the National Academy of Sciences.
“The length of the dry season in the southern Amazon is the most important climate condition controlling the rainforest,” says Rong Fu, co-author and professor at The University of Texas at Austin’s Jackson School of Geosciences. “If the dry season is too long, the rainforest will not survive.”
To see why the length of dry season is such a limiting factor, imagine there is heavier than usual rainfall during the wet season. The soil can only hold so much water and the rest runs off. The water stored in the soil at the end of the wet season is all that the rainforest trees have to last them through the dry season. The longer the dry season lasts, regardless of how wet the wet season was, the more stressed the trees become and the more susceptible they are to fire.
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: First, it makes it harder for warm, dry air near the surface to rise up and freely mix with cool, moist air above. And second, it blocks cold front incursions from outside the tropics that could trigger rainfall. The climate models used by the IPCC do a poor job representing these processes, which might explain why they project only a slightly longer Amazonian dry season, says Fu.
The Amazon rainforest normally removes the greenhouse gas carbon dioxide from the atmosphere, but during a severe drought in 2005, it released 1 petagram of carbon (about one tenth of annual human emissions) to the atmosphere.
“The more severe 2010 drought impacted twice the forested area than the 2005 drought and could have likely resulted in substantial carbon loss from the forests,” says Myneni, who has previously studied these droughts with NASA satellite sensor data.
Myneni and his colleagues estimate that 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.
Some scientists have speculated that the combination of longer dry seasons, higher surface temperatures and more fragmented forests due to ongoing human-caused deforestation could eventually convert much of southern Amazonia from rainforest to savanna.
Earlier studies have shown that human-caused deforestation in the Amazon can alter rainfall patterns. But the researchers didn’t see a strong signal of deforestation in the pattern of increasing dry season length. The dry season length increase was most pronounced in the southwestern Amazon, while the most intense deforestation occurred in the southeastern Amazon.
Because the northwestern Amazon has much higher rainfall and a shorter dry season than the southern Amazon, the researchers think it is much less vulnerable to climate change.
The co-authors of this study include Rong Fu, Lei Yin, Robert Dickinson, Lei Huang and Sudip Chakraborty at The University of Texas at Austin’s Jackson School of Geosciences; Wenhong Li at Duke University; Paola A. Arias at Universidad de Antioquia in Colombia; Katia Fernandes at Columbia University’s Lamont-Doherty Earth Observatory; Brant Liebmann at the National Oceanic & Atmospheric Administration (NOAA); Rosie Fisher at the National Center for Atmospheric Research; and Ranga Myneni at Boston University.
This work is supported by the National Science Foundation (AGS 0937400) and NOAA Climate Program Office Modeling, Analysis, Prediction and Projection Program (NA10OAAR4310157) and the NASA Earth Science Division.