A NASA-led study has shown that a part of the Amazon rainforest twice the size of California is still suffering from a “megadrought” that began in 2005. Researchers cited this and damage due to drought recurrences in the Amazon during the past decade as evidence that the rainforest may face “large-scale degradation due to climate change.”

The study looked at satellite microwave radar data from 2000 to 2009, measurements of rainfall from NASA’s Tropical Rainfall Measuring Mission, and moisture content from the rainforest canopy from the Seawinds scatterometer on NASA’s QuikScat satellite.

During the summer of 2005, over 270,000 square miles of old-growth forest in the Amazon experienced “extensive, severe drought.” This megadrought caused changes in the forest canopy, including possible dieback of branches and tree falls. Though rainfall levels recovered in the years after the drought, much of the damage to the forest canopy remained until the next drought in 2010.

“The biggest surprise for us was that the effects appeared to persist for years after the 2005 drought,” said Yadvinder Malhi, co-author of the study at the University of Oxford. “We had expected the forest canopy to bounce back after a year with a new flush of leaf growth, but the damage appeared to persist right up to the subsequent drought in 2010.”

The study shows that around 30% of the Amazon basin’s total forest area was affected by the 2005 drought. Almost half of the entire Amazon rainforest was affected by the 2010 drought. The drought rate in the area has been abnormally high during the past decade. Research has shown that rainfall over the southern Amazon rainforest fell by nearly 3.2% from 1970 to 1998.

Malhi and his colleagues attribute recent Amazonian droughts to long-term warming of tropical Atlantic sea surface temperatures.

“In effect, the same climate phenomenon that helped form hurricanes Katrina and Rita along U.S. southern coasts in 2005 also likely caused the severe drought in southwest Amazonia,” said Sassan Saatchi, leader on the research at NASA’s Jet Propulsion Laboratory (JPL). “An extreme climate event caused the drought, which subsequently damaged the Amazonian trees.

“Our results suggest that if droughts continue at five- to 10-year intervals or increase in frequency due to climate change, large areas of the Amazon forest are likely to be exposed to persistent effects of droughts and corresponding slow forest recovery. This may alter the structure and function of Amazonian rainforest ecosystems.”

(Image courtesy NASA/JPL-Caltech)