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A study confirms an area of the Amazon rainforest twice as big as California still suffers from the effects of a megadrought that began in 2005, NASA said.

The study results suggest the rainforests may be showing the first signs of potential large-scale, long-term degradation due to climate change, the space agency reported Thursday.

An international research team led by scientists from NASA's Jet Propulsion Laboratory in Pasadena, Calif., analyzed more than a decade of satellite microwave radar data collected between 2000 and 2009 over Amazonia. The data included measurements of rainfall and the moisture content and structure of the forest canopy conducted by NASA satellites.

More than 270,000 square miles of pristine, old-growth forest in southwestern Amazonia experienced an extensive, severe drought in summer 2005 that caused widespread changes to the forest canopy detectable by satellite, the researchers said. The changes suggest dieback of branches and tree falls, especially among the older, larger, more vulnerable canopy trees that blanket the forest.

Rainfall levels recovered in subsequent years but the damage to the forest canopy persisted all the way to the next major drought beginning in 2010, the scientists said.

"The biggest surprise for us was that the effects appeared to persist for years after the 2005 drought," study co-author Yadvinder Malhi of the University of Oxford in Britain said. "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."

Megadroughts can have long-lasting effects on rainforest ecosystem, JPL scientist Sassan Saatchi said.

"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."