Scientists found that during the summer of 2005, more than 700,000 sq km, or 70 million hectares of pristine, old-growth forest in southwestern Amazonia experienced an extensive, severe drought.
This megadrought caused widespread changes to the forest canopy that were detectable by satellite.
While rainfall levels gradually recovered in subsequent years, the damage to the canopy persisted all the way to the next major drought, especially involving the older, larger, more vulnerable canopy trees, the journal Proceedings of the National Academy of Sciences reports.
About half the forest affected by the 2005 drought -- an area the size of California -- did not recover by the time NASA's QuikScat stopped gathering global data in November 2009 and before the start of a more extensive drought in 2010, according to a NASA statement.
These results, together with observed recurrences of droughts every few years and associated damage to the forests in southern and western Amazonia in the past decade, suggest these rainforests may be showing the first signs of potential large-scale degradation due to climate change.
An international research team led by Sassan Saatchi of NASA's Jet Propulsion Lab, California, analyzed more than a decade of satellite microwave radar data collected between 2000 and 2009 over Amazonia.
"The biggest surprise for us was that the effects appeared to persist for years after the 2005 drought," said study co-author Yadvinder Malhi of the University of Oxford, UK.
"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."
Researchers attribute the 2005 Amazonian drought to the long-term warming of tropical Atlantic sea surface temperatures.
"In effect, the same climate phenomenon that helped form hurricanes Katrina and Rita along the southern coasts of the US in 2005 also likely caused the severe drought in southwest Amazonia," Saatchi said.