The Amazon rainforest really can’t catch a break. First they had industrial deforestation and widespread human encroachment, and now it’s looking like their seasons are getting all jacked up. A new study published in this week’s Proceedings of the National Academy of Sciences has found that the southern Amazon’s dry season is beginning to last longer, extending further into the Southern Hemisphere’s early Spring months, from October on into November. This seasonal shift, which seems to have begun around 1979, is beginning to affect the rainforest’s carbon cycle, keeping the forest from fixing more carbon by hampering tree growth, and thus impairing its ability to reduce atmospheric carbon levels. The rainforest’s natural ability to fix atmospheric carbon has been very important in helping to combat global climate change and weakening that ability will only help to accelerate the destabilization of the world’s climate regimes and further loss of biodiversity worldwide.
The group combined data from two different sources of rainfall records, the National Oceanic and Atmospheric Administration’s (NOAA) Climate Prediction Center (whose records run from January 1978 to December 2007) and the NOAA Climate Diagnostics Center (whose records run from January 1940 to December 2011). They found that the length of the dry season had increased by about six days per decade since 1980 and that the end of the dry season came about 5 days later each decade. These increases have also led to more fires towards the end of the dry season, when conditions usually begin changing to prevent extensive burning.
The big question here is why? Why has the dry season continued to grow since the 1980s began? Well, to be honest, researchers don’t really know why. They examined the possibility that cyclical weather patterns could be affecting the Amazon’s rainfall by checking out the Pacific Decadal Oscillation, the Atlantic Multidecadal Oscillation, and the El Niño Southern Oscillation. None of these large-scale climate cycles could be linked conclusively to the changes in the dry season. Of course, many would immediately point to the possibility of a human answer and look to connect this newfound trend to global climate change. The authors are tentative to do so though, as much of the data that the Intergovernmental Panel on Climate Change uses underestimates the variability of the Amazon’s dry season. As a result, their models aren’t all that accurate when it comes to accounting for these apparent changes.
Regardless of the cause, the progressive growth of the Amazon’s dry season is a serious issue. A very large number of climate models rely on the continued carbon fixation of the world’s rainforests and the Amazon is one of those we rely on most to help combat our own carbon emissions. The more the wet season shrinks, the more the forest will shrink, and the more our prospects will shrink along with it.