January 18, 2013
Amazon Rainforest Under Threat From Climate Change
April Flowers for redOrbit.com - Your Universe Online
The results of this study, combined with the observed recurrences of droughts every few years and associated damage to forests in both the southern and western Amazon over the past decade, reveal that the rainforest might be showing the initial signs of potential large-scale degradation due to climate change.
An international team of scientists analyzed more than a decade of satellite microwave radar data collected beginning in 2000 over the Amazon rainforest. The observations, which included measurements of rainfall from NASA's Tropical Rainfall Measuring Mission (TRMM) and measurements of the moisture content and structure of the forest canopy (top layer) from the Seawinds scatterometer on NASA's QuikScat spacecraft, showed that during the summer of 2005, more than 270,000 square miles of pristine, old-growth forest in southwestern Amazonia underwent an extensive, severe drought. Satellite images were able to detect the widespread changes to the forest canopy caused by this megadrought, including dieback of branches and tree falls. This is especially true among the older, larger, more vulnerable canopy trees that blanket the forest.
NASA's QuikScat spacecraft launched in 1999, tasked with recording sea-surface wind speed and direction data under all weather conditions using Seawinds, an active radar scatterometer. QuikScat made approximately 14 full rotations of the Earth in a day, allowing it to capture over 400,000 measurements and cover over 90% of the Earth's surface each day.
The findings of this study were recently published in Proceedings of the National Academy of Sciences.
Even though rainfall levels gradually recovered in the following years, the damage to the canopy persisted until the next major drought started in 2010. Approximately half the forest affected by the 2005 drought had not recovered by the time QuikScat stopped gathering global data due to a failure in the antenna spin mechanism for the primary instrument in 2009 and before the start of the 2010 drought, which was more extensive.
"The biggest surprise for us was that the effects appeared to persist for years after the 2005 drought," said 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."
The vulnerability of tropical forests to climate change was highlighted by the recent Amazonian droughts, with satellite and ground data showing an increase in wildfires during these years and tree die-offs following severe droughts. Previous to this study, there had been no satellite-based multi-year impact assessment of these droughts across all of Amazonia. The implications of this assessment are important because large-scale droughts can lead to sustained releases of carbon dioxide from decaying wood. This can affect ecosystems and Earth's carbon cycle.
The team — which included scientists from JPL, Oxford, UCLA, University of Exeter in the UK, the National Institute for Space Research (INPE) in Brazil, Boston University, and NASA's Ames Research Center - ascribes the cause of the 2005 drought to long-term warming of the 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," Saatchi said in a statement. "An extreme climate event caused the drought, which subsequently damaged the Amazonian trees."
Such megadroughts as the 2005 and 2010 events can have long lasting effects on rainforest ecosystems, Saatchi pointed out.
"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," he said. "This may alter the structure and function of Amazonian rainforest ecosystems."
The researchers found that the area affected by the 2005 drought was much larger than previously predicted. Around 1.7 million square miles, or 30 percent, of the Amazon basin's total current forest was affected. More than five percent of the forest experienced severe drought conditions.
In 2010, nearly half the entire Amazon forest was affected with nearly a fifth of it — roughly the size of California - experiencing severe drought conditions. Over 231,660 square miles of the area affected by the previous drought of 2005 were also impacted by the 2010 drought. Potentially long-lasting and widespread effects on forests in southern and western Amazonia are suggested by this "double-whammy" of successive droughts.
In the past century of records, the drought rate in Amazonia during the past decade is unprecedented. Several localized mini-droughts in recent years have been experienced in addition to the 2005 and 2010 megadroughts. Ground station observations show that rainfall over the southern Amazon rainforest have declined by approximately 3.2 percent per year from 1970 — 1998.
A steady decline in water availability for plants in the region was revealed by climate analysis of the period from 1995 — 2005. Examined together, these datasets suggest a decade of moderate water stress contributed to the 2005 drought, helping to trigger the wide-scale forest damage that followed.
The new findings, according to Saatchi, sheds light on a major controversy surrounding how the Amazonian forest responded following the 2005 drought. Using conventional optical satellite data, previous studies produced contradictory results. This was likely due to the difficulty of correcting the optical data for cloud and other atmospheric condition interference.
QuikScat's scatterometer radar, in comparison, was able to see through the clouds and penetrate into the top several feet of vegetation. This provided daily measurements of the forest canopy structures and allowed the team to make estimates of how much water the forest contains. Forest areas that are drought-damaged produce a lower radar signal than those collected over healthy forest. This indicates that either the forest canopy is drier or it is "less rough" due to damage or early tree death.