Amazon Forest Drought Corrects Global Warming Model

A prominent global climate model predicts that the Amazon forest would begin to "brown down" after just a month of drought and eventually collapse as the drought progressed, but according to the University of Arizona (UA), drought-stricken regions of the Amazon forest grew particularly vigorously during the 2005 drought.

This counterintuitive finding contradicts expectations. Scott R. Saleska of the University of Arizona, who is the author of the new research paper, says that it is a very interesting and surprising result. "Instead of 'hunkering down' during a drought as you might expect, the forest responded positively to drought, at least in the short term."

Researchers already knew the Amazon forest takes advantage of annual dry seasons to absorb sunlight, which is unimpeded by clouds, and grow. The UA scientists and some other researchers had conducted previous research using satellite data in combination with field measurements and learned that intact Amazon forest increases photosynthesis, actually "greening up," during the dry season.

Current global climate models predict that the Amazon forest would quickly cut back photosynthesis when a drought starts. That slowdown in plant growth would create what is known as a positive feedback loop -- as the forest processes less and less photosynthesis, it removes less and less carbon dioxide from the atmosphere. According to the model, the CO2 ordinarily sequestered by growing trees would remain in the atmosphere, increasing global warming and further accelerating the forest's decline and additional CO2-fueled warming.

Saleska, a UA assistant professor of ecology and evolutionary biology, and his colleagues at the UA and at the University of São Paulo in Brazil used data from two NASA satellites to discover that in the 2005 drought undisturbed Amazon forest flourished as rainfall levels plummeted.

The 2005 drought reached its peak coinciding with the start of the Amazon's annual dry season that extends from July through September. This was expected to have a significantly negative affect on forest growth. In reality, satellite images showed that in many of the areas hit by drought during the dry season the canopy of the undisturbed forest became significantly greener. This indicates increased--not decreased--photosynthetic activity.

UA study co-author Kamel Didan says, "The forest showed signs of being more productive. That's the big news."

"A big chunk of the Amazon forest, the southwest region where the drought was severest, reacted positively," said Didan, a NASA-EOS MODIS associate science team member.

Didan, an associate research scientist in the UA's department of soil, water and environmental science, further said, "No one had looked at the observations that are available from satellites...We took the opportunity of the most recent drought, the 2005 drought, to do so."

The UA team is the first to examine how the forest responded to a drought. The severe 2005 drought and the detailed, long-term observations from two NASA satellites -- one that maps the greenness of vegetation, one that measures rainfall in the tropics -- provided the necessary data to study how the Amazon forest actually responds to a major drought.

The researchers used the month-to-month maps of changes in vegetation status across the Amazon that are available from the Moderate Resolution Imaging Spectroradiometer (MODIS), which is carried by the Terra satellite and launched in 1999. Further, observations of rainfall in the Amazon were collected from the Tropical Rainfall Measuring Mission spacecraft that was launched in 1997.

The satellites provided seven to nine years of observations that allowed the scientists to map typical rainfall and greenness conditions during non-drought years. These maps were then compared to data from the same months during the 2005 drought. The researchers then surprisingly found that the areas of the Amazon's intact forests that had received below-normal rainfall in 2005 also had above-average greenness.

The UA research team's findings suggest that the current global warming model represents the opposite of what actually occurs in the Amazon forest, at least in the short-term.

If this is so, then the actual model is that drought-induced upsurgance of photosynthetic activity and forest growth would dampen global warming, rather than accelerate it.

It is believed that during the 2005 drought, Amazon forest trees flourished by tapping into water deep in the forest soil. Additionally, to maintain or increase growth, trees must take up carbon dioxide from the atmosphere, thus drawing down levels of atmospheric CO2. The negative feedback loop--more photosynthesis activity, more CO2 removed--thus created would slow down global warming produced by greenhouse gases.

Saleska points out that in evolutionary terms, the forest's resilience in the face of a single drought year makes sense. During the cyclical El Nino years, occurring every four to eight years, the Amazon forest receives significantly less rain than average without browning down.

But, the limit of the forest's resiliency is still unknown. For this reason Saleska says, "But if you take away enough water for long enough, the trees will die."

The study, "Amazon Forests Green-up during 2005 drought," is online in the current issue of Science Express, the early-online version of the journal Science. The paper will be published in the October 26, 2007, issue of Science.

Saleska and Didan's co-authors are Alfredo Huete, UA professor of soil, water and environmental science and NASA-EOS MODIS science team member, and Humberto Ribeiro da Rocha of the department of atmospheric science at the University of São Paulo in Brazil. The research was funded by NASA.

Mari N. Jensen, "Amazon forest shows unexpected resiliency during drought," University of Arizona.