An area of the Amazon rainforest three times the size of the United kingdom was strongly affected by a drought that began in 2005, says a NASA-led team that includes researchers from Oxford.
The results, together with observed increases in rainfall variability and associated forest damage in southern and western Amazonia during the past decade, suggest these rainforests may be witnessing the first signs of potential large-scale degradation due to climate change.
The researchers analysed more than a decade of satellite microwave radar data collected between 2000 and 2009 over Amazonia. The observations included measurements of rainfall from NASA's Tropical Rainfall Measuring Mission as well as the moisture content and volume, or biomass, of the forests.
The scientists found that, beginning in 2005, more than 270,000 square miles (70 million hectares or three times the area of the UK) of pristine, old-growth forest in southwestern Amazonia experienced an extensive and severe drought. This drought caused widespread changes to the forest canopy that were detectable by satellite. These changes to the canopy suggest dieback of branches and tree falls, especially among the older, larger, more vulnerable 'canopy' trees that blanket the forest, say the researchers in a paper published in the journal Proceedings of the National Academy of Sciences.
'The biggest surprise for us was that the effects appeared to persist for years after the drought,' said co-author Professor Yadvinder Malhi from 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.'
While total rainfall levels gradually recovered in subsequent years, the forest canopy damage persisted all the way to the next major drought, which began in 2010. In fact, about half of the forest affected by the 2005 drought – an area one and a half times the size of the UK – did not recover by the start of the next drought.
Recent Amazonian droughts have drawn attention to the vulnerability of tropical forests to climate change. Satellite and ground data have shown an increase in wildfires during drought years and tree die-offs following severe droughts, but there has been no assessment of the long-term effects of these droughts across Amazonia. Large-scale droughts can lead to sustained releases of carbon dioxide from decaying wood, affecting ecosystems and Earth’s carbon cycle.
The team found that the impact of the 2005 drought was on a much larger scale than scientists had previously predicted. About 30 percent (1.7 million square kilometres) of the Amazon basin's total current forest area was affected in some way, with more than five percent of the forest experiencing severe drought conditions. The 2010 drought affected nearly half of the entire Amazon forest, with nearly a fifth of it experiencing severe drought conditions.
More than 231,660 square miles (600,000 square kilometres) of the areas affected by the 2005 drought were also damaged by the 2010 drought. This 'double whammy' punch by successive droughts suggests they may have a potentially widespread effect on forests in southern and western Amazonia, say the researchers.
The paper points out that the drought rate in Amazonia during the past decade is unprecedented over the past century. In addition to the three major droughts in 1997/8, 2005 and 2010, the area has also experienced several localised mini-droughts in recent years. Observations from ground stations show that rainfall over southern Amazonia declined by almost 3.2 per cent per year in the period from 1970 to 1998. Climate analyses for the period from 1995 to 2005 show a steady decline in plant water availability over the region. Together, these data suggest a decade of moderate water stress that led up to the 2005 drought, helping trigger the large-scale forest damage seen after the 2005 drought event.