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Rainforests in South America have survived three previous extreme global warming events in the past, suggesting that they will survive a projected 2-6 degree rise in temperatures over the coming century, reports a study published in the Annual Review of Earth and Planetary Science.
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The research, published by Carlos Jaramillo and Andrés Cárdenas of the Smithsonian Tropical Research Institute (STRI) in Panama, reviewed some 3,800 published estimates of temperature over the past 120 million years and compared them to the existence of tropical plants in the fossil record. They found that tropical rainforest plant diversity actually increased during previous global warming episodes, including the mid-Cretaceous period 120 million years ago when temperatures in the South American tropics rose 9-12 degrees (5-7°C), the Paleocene-Eocene thermal maximum 55 million years ago when tropical temperatures rose by 5-9 degrees (3-5°C) in less than 10,000 years, and another period of warming 53 million years ago.
Jaramillo and Cárdenas said that a concern raised by other tropical researchers — that increased incidence of drought in the tropics will be detrimental to tropical forests — may be overblown, citing research by STRI scientist Klaus Winter that some tropical trees can endure short-term exposure to temperatures up to 122-127 degrees (50-53°C). The researchers added that trees use less water when CO2 concentrations increase.
Jaramillo and Cárdenas paint a far rosier picture than some other researchers working in the region. Studies based on climate model suggest that large extents of the Southern Amazon could be increasingly vulnerable to die-off from severe droughts driven by rising temperatures in the tropical Atlantic Ocean and worsened by large-scale deforestation. Although rainfall appears to be on the rise in the Western Amazon, the Southern Amazon has seen the two worst droughts on record during the past decade. A study published last December showed that trees across large blocks of the Amazon are failing to recover as expected after extended dry periods.
“If droughts continue to occur at 5—10-year frequency, or increase in frequency, large areas of Amazonian forest canopy likely will be exposed to the persistent effect of droughts and the slow recovery of forest canopy structure and function,” wrote Sassan Saatchi and colleagues in the journal Proceedings of the National Academy of Sciences. “In particular, areas of south and western Amazonia have been shown to be affected severely by increasing rainfall variability in the past decade, suggesting that this region may be witnessing the first signs of potential large-scale degradation of Amazonian rainforest from climate change.”
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