New study says IPCC projections are too conservative
By Summit Voice
FRISCO — In a classic case of climate disruption, research shows that the dry season in southern Amazonia has lengthened by about one week per decade since 1979. Parts of the region may not be able to support rainforest vegetation much longer. A big forest die-back could trigger the release of large volumes of the greenhouse gas carbon dioxide into the atmosphere, a team of scientists warned this week.
The changes could disrupt plant and animal communities in one of the regions of highest biodiversity in the world, said University of Texas professor Rong Fu, who led the team of scientists.
“The dry season over the southern Amazon is already marginal for maintaining rainforest,” said Fu, with the University of Texas at Austin’s Jackson School of Geosciences. “At some point, if it becomes too long, the rainforest will reach a tipping point,” she said.
The risk of a large die-back due to seasonal drying is much higher than projected by the latest assessment from the Intergovernmental Panel on Climate Change.
The ground-based measurements from the past three decades also show that the annual fire season has become longer. The researchers say the most likely explanation for the lengthening dry season is global warming.
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 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,” Fu said. “If the dry season is too long, the rainforest will not survive.”
No matter how much rain falls during the wet season, the soil can only hold so much moisture, so that becomes the limiting factor for rainforest persistence, as trees become more stressed and susceptible to fire as the dry season gets longer.
The researchers concluded the most likely explanation for the lengthening dry season in the southern Amazon in recent decades is human-caused greenhouse warming, which inhibits rainfall by making it harder for warm, dry air near the surface to rise and freely mix with cool, moist air above. Second, greenhouse gas warming 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, according to 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. Fu and her 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.
“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.
Some scientists have speculated that the combination of longer dry seasons, higher surface temperatures and more fragmented forests resulting from 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, Fu and others think it is much less vulnerable to climate change.
Fu’s co-authors at The University of Texas at Austin’s Jackson School of Geosciences are Lei Yin, Robert Dickinson, Lei Huang and Sudip Chakraborty. The team also includes Wenhong Li at Duke University; Paola A. Arias at Universidad de Antioquia in Colombia; Kátia Fernandes at Columbia University’s International Research Institute for Climate and Society; 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 the NOAA Climate Program Office Modeling, Analysis, Prediction and Projection Program (NA10OAAR4310157).
Filed under: Environment, global warming, climate and weather, Drought, greenhouse gases Tagged: | global warming, IPCC, Intergovernmental Panel on Climate Change, Amazon Rainforest, Climate disruption