Life-giving rains have returned over the past two months to Earth’s greatest rainforest–the mighty Amazon–after it experienced its second 100-year drought in five years this year. The record drought began in April, during the usual start to the region’s dry season, when rainfall less than 75% of average fell over much of the southern Amazon (Figure 2.) The drought continued through September, and by October, when the rainy season finally arrived, the largest northern tributary of the Amazon River–the Rio Negro–had dropped to thirteen feet (four meters) below its usual dry season level. This was its lowest level since record keeping began in 1902. The low water mark is all the more remarkable since the Rio Negro caused devastating flooding in 2009, when it hit an all-time record high, 53 ft (16 m) higher than the 2010 record low. The 2010 drought is similar in intensity and scope to the region’s previous 100-year drought, which hit the Amazon in 2005, according to Brazil’s National Institute of Space Research. Severe fires burned throughout the Amazon in both 2005 and 2010, leading to declarations of states of emergencies.
The reason this is worrisome is that the amazon is a carbon dioxide sink during normal conditions – about 2 billion tons of carbon dioxide. But in severe drought periods like this year, it emits closet to 3 billion tons, a 5 billion ton swing.
And these last two droughts were also like none other in the last 100 years or so. Droughts then happened about every 12 years during an El Niño year, when the Pacific is warmer. The drought in 2005 and 2010 did not happen in El Niño years. In fact, 2010 was a La Niña.
The last two droughts appear to have been caused by unusually warm Atlantic Ocean waters. And modeling from before this year suggested that major droughts would happen once every 20 years. Yet we have had two in 5 years. This indicates that there is a positive feedback loop in place here that is already moving faster than expected.
One researcher said “I’m genuinely quite alarmed by this. In some ways it kind of reminds me of when they figured out than the Greenland ice sheet was melting much faster than the climate models predicted it would.” If the Amazon is releasing more carbon dioxide than expected due to droughts, then that would be alarming.
Here’s hoping that this is just an example of a stochastic series of events occurring rather than a deterministic link due to increasing carbon dioxide. The nonscientific words would be that I hope these two events are due to random events and not to increasing carbon dioxide.