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 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 dry season over the southern Amazon is already marginal for maintaining rainforest,” says Rong Fu, co-author and professor at The University of Texas at Austin’s Jackson School of Geosciences 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 ten days longer by the end of the century and therefore the risk of climate change-induced rainforest dieback should be relatively low.
The 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 Fu. “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.
Note to Reporters: A preprint of the article is available to journalists on the following secure reporters-only web site: http://www.eurekalert.org/pio/pnas.php
About Boston University—Founded in 1839, Boston University is an internationally recognized private research university with more than 30,000 students participating in undergraduate, graduate, and professional programs. As Boston University’s largest academic division, the College and Graduate School of Arts & Sciences is the heart of the BU experience with a global reach that enhances the University’s reputation for teaching and research. In 2012, BU joined the Association of American Universities (AAU), a consortium of 62 leading research universities in the United States and Canada.
Author contact for the study:Professor Ranga Myneni
Professor Ranga Myneni | Newswise
Air pollution leads to cardiovascular diseases
21.08.2018 | Universitätsmedizin der Johannes Gutenberg-Universität Mainz
Waste in the water – New purification techniques for healthier aquatic ecosystems
24.07.2018 | Eberhard Karls Universität Tübingen
There are currently great hopes for solid-state batteries. They contain no liquid parts that could leak or catch fire. For this reason, they do not require cooling and are considered to be much safer, more reliable, and longer lasting than traditional lithium-ion batteries. Jülich scientists have now introduced a new concept that allows currents up to ten times greater during charging and discharging than previously described in the literature. The improvement was achieved by a “clever” choice of materials with a focus on consistently good compatibility. All components were made from phosphate compounds, which are well matched both chemically and mechanically.
The low current is considered one of the biggest hurdles in the development of solid-state batteries. It is the reason why the batteries take a relatively long...
New design tool automatically creates nanostructure 3D-print templates for user-given colors
Scientists present work at prestigious SIGGRAPH conference
Most of the objects we see are colored by pigments, but using pigments has disadvantages: such colors can fade, industrial pigments are often toxic, and...
Scientists at the University of California, Los Angeles present new research on a curious cosmic phenomenon known as "whistlers" -- very low frequency packets...
Scientists develop first tool to use machine learning methods to compute flow around interactively designable 3D objects. Tool will be presented at this year’s prestigious SIGGRAPH conference.
When engineers or designers want to test the aerodynamic properties of the newly designed shape of a car, airplane, or other object, they would normally model...
Researchers from TU Graz and their industry partners have unveiled a world first: the prototype of a robot-controlled, high-speed combined charging system (CCS) for electric vehicles that enables series charging of cars in various parking positions.
Global demand for electric vehicles is forecast to rise sharply: by 2025, the number of new vehicle registrations is expected to reach 25 million per year....
17.08.2018 | Event News
08.08.2018 | Event News
27.07.2018 | Event News
21.08.2018 | Ecology, The Environment and Conservation
21.08.2018 | Life Sciences
21.08.2018 | Power and Electrical Engineering