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Climate change jeopardizing Amazon rainforest, say researchers

A new NASA-led study has found that an area of the Amazon rainforest two times the size of the state of California is still suffering from the impacts of an extreme drought that started in 2005. When combined with observed recurrences of droughts every few years and related damage to the forests in southern and western Amazonia over the past ten years, the study’s findings suggest that climate change may be jeopardizing the Amazon rainforest.

An international research team examined more than a decade of satellite microwave radar data gathered between 2000 and 2009 over Amazonia. The satellite data included measurements of rainfall from NASA’s Tropical Rainfall Measuring Mission and measurements of the water content and structure of the forest canopy from the Seawinds scatterometer on the space agency’s QuikScat spacecraft.

Researchers discovered that during the summer of 2005, more than 270,000 square miles of old-growth forest in southwestern Amazonia experienced a megadrought. The severe drought led to changes to the forest canopy that were identified by satellite. The changes suggest the gradual dying of branches and tree falls in the Amazon rainforest.

While rainfall levels gradually recovered in subsequent years, the damage to the forest canopy lasted all the way to the next severe drought, which started in 2010. According to researchers, approximately 50 percent of the forest impacted by the 2005 drought failed to recover by the time QuickScat stopped gathering global data in November 2009 and before the start of a more far-reaching drought in 2010.

Study co-author Yadvinder Malhi of the University of Oxford was amazed that the impacts of the megadrought lasted for years after the drought ended. Malhi and his colleagues thought that the forest canopy would return with new leaf growth within a year.

Recent drought in the Amazon rainforest have brought renewed attention to the vulnerability of tropical rainforests to climate change. Satellite and ground-based observations have revealed an increase in wildfires during drought years and tree die-offs after severe droughts. This is the first satellite-based assessment of the multi-year effects of these droughts across all of Amazonia.

Megadrought can lead to sustained release of carbon dioxide from decaying wood, impacting ecosystems and Earth’s carbon cycle. The researchers fault the long-term warming of the tropical Atlantic sea surface temperatures for the 2005 Amazonian drought.

Sassan Saatchi of NASA’s Jet Propulsion Laboratory said that the same climate phenomenon that assisted the formation of hurricanes Katrina and Rita along the U.S. southern coasts in 2005 may have also led to severe drought in southwest Amazonia.

Megadroughts, Saatchi added, can have long-lasting impacts on rainforest ecosystems. The team’s results revealed that if droughts continue at five- t0 10-year intervals due to climate change, sizable areas of the Amazon rainforest could be exposed to the continuous effects of droughts and equivalent slow forest recovery. Researchers worry that this could change the structure and function of Amazonian rainforest systems.

Researchers discovered that the area impacted by the 2005 drought was much larger than scientists had previously thought. Researchers contend that the “double whammy” of successive droughts (2005 and 2010) reveals a potentially long-lasting and widespread effect on forests in southern and western Amazonia.

Researchers note that the drought rate in Amazonia during the past ten years is unexampled over the past century. The area has also experienced numerous localized mini-droughts in recent years. Climate analyses for the period from 1995 to 2005 reveal a steady drop in water availability for plants in the region, suggesting a decade of moderate water stress led up to the 2005 drought.

Saatchi said that a major debate has been raging over how the Amazon rainforest responded following the megadrought. Previous studies using conventional optical satellite data produced contradictory results due to the challenge of correcting optical data for interference by clouds and other atmospheric conditions. QuikScatt’s scatterometer radar, on the other hand, was able to look through the clouds.

The study’s findings were recently published in the journal Proceedings of the National Academy of Sciences.