Amazon confusion: new research shows forest is resilient to drought, but is this the whole picture?

Jeremy Hance
March 15, 2010

A drought that happens once in a hundred years had little negative or positive effect on the Amazon rainforest according to a NASA funded study in Geophysical Research Letters.

"We found no big differences in the greenness level of these forests between drought and non-drought years, which suggests that these forests may be more tolerant of droughts than we previously thought," said Arindam Samanta, the study's lead author from Boston University.

Employing NASA MODIS satellite data the study measured the greennesss of the Amazon over the last decade, including 2005 when the rainforest experienced drought conditions that only occur every hundred years or so.

Patchwork of legal forest reserves, pasture, and soy farms in the Brazilian Amazon. Photo by Rhett A Butler.
"This new study brings some clarity to our muddled understanding of how these forests, with their rich source of biodiversity, would fare in the future in the face of twin pressures from logging and changing climate," said Boston University Prof. Ranga Myneni, senior author of the new study.

The study shows results quite different from the 2007 IPCC report which stated that 40 percent of the Amazon rainforest was threatened by climate change. While the IPCC has received some criticism for the statement, which cites a WWF study, prominent tropical ecologists have stated that the IPCC was largely correct in its assessment given information at the time.

Thomas Lovejoy, biodiversity chair at the Washington DC-based Heinz Center for Science, Economics and the Environment, and chief biodiversity adviser to the president of the World Bank, recently told Tierramerica that the Amazon rainforest was "very close to a tipping point".

Citing a World Bank report that looks at the synergistic effects of deforestation, climate change, and fires, Lovejoy said that the "tipping point for the Amazon is 20 percent deforestation". Currently 17-18 percent of the Amazon has been lost. The strength of this study is that it didn't focus on a single threat, but looked at the combination of impacts currently barraging the ecosystem.

Another tropical forest expert, Simon Lewis of Leeds University, told the BBC earlier this year that "it is very well known that in Amazonia, tropical forests exist when there is more than about 1.5 meters of rain a year, below that the system tends to 'flip' to savannah."

He added: "indeed, some leading models of future climate change impacts show a die-off of more than 40 percent Amazon forests, due to projected decreases in rainfall."

Dr. Jose Marengo, a Brazilian National Institute for Space Research climate scientist and member of the IPCC, disagreed, saying that, "the way that the WWF report calculated this 40 percent was totally wrong, while [the new] calculations are by far more reliable and correct."

According to the WHRC, the above map is "a product of our ongoing drought monitoring effort from Oct. 2005, the worst month we have in our record going back to 1995. It shows moisture stored in the soil which is available for use by plants, what we call 'Plant-Available Water' or 'PAW', expressed as a percentage of the total water-holding capacity of the top 10m of soil at any given point; %PAW is one of the strongest indicators we have of severity of drought and of forest susceptibility to fire. Courtesy of WHRC.
Consensus is not yet clear on this issue. Another study published in Science in 2007 stated that forests would actually do better in drought conditions, because they would receive more sunlight from a cloudless sky. However, the results of this study have not been able to be reproduced.

Given the differing opinions, more research may be needed before consensus can be reached on how the Amazon will react to climate change, especially in light of other threats such as deforestation.

Citation: Samanta, A., S. Ganguly, H. Hashimoto, S. Devadiga, E. Vermote, Y. Knyazikhin, R. R. Nemani, and R. B. Myneni (2010), Amazon forests did not green‐up during the 2005 drought, Geophys. Res. Lett., 37, L05401, doi:10.1029/2009GL042154.

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Jeremy Hance (March 15, 2010).

Amazon confusion: new research shows forest is resilient to drought, but is this the whole picture? .