Scientists Predict Huge Pulse of Carbon into Atmosphere from 2010 Amazon Drought

In November 2010, we reported on extreme drought conditions in the Amazon in Another Extreme Drought Hits the Amazon and Raises Climate Change Concerns.  The posting compared the 2005 and 2010 "once in a century" droughts in the region, noted connections to rising global greenhouse gas concentrations, and raised several concerns including the impacts on global carbon emissions.

In The 2010 Amazon Drought, published in the 4 February 2011 issue of Science, scientists predict that the drought of 2010 ultimately may result in the release of about 2.2 × 1015 grams of carbon (1015 grams = 1 Petagram (Pg) = 1 billion metric tonnes) into the atmosphere. That is:

  • Well over a third more than the emissions associated with the historic 2005 drought in the Amazon (estimated at 1.6 billion metric tonnes of carbon ). 
  • Roughly equivalent to a quarter of annual global carbon emissions from fossil fuel use (in 2009, emissions from fossil fuel use were 8.4±0.5 billion metric tonnes C; source: Global Carbon Project).

The authors note that one reason emissions from the 2010 event are predicted to be higher than in 2005 is that last year 57% of Amazonia had low rainfall  -- compared to 37% of the region in 2005. 

The authors emphasize that "[c]onsiderable uncertainty remains."  They bracket their estimates with "95% confidence intervals" (95% CI):

  • 2005: 1.6 billion metric tonnes of C, with 95% CI between 0.8 and  2.6 billion metric tonnes of C
  • 2010: 2.2 billion metric tonnes of C, with 95% CI between 1.2 and 3.4 billion metric tonnes of C

They are 95% confident that the actual emissions from these droughts fall within those ranges.  

They further explain:

"These values are relative to the predrought carbon uptake and represent the sum of (1) the temporary cessation of biomass increases over the 2-year drought measurement interval (~0.8 Pg C) and (2) biomass lost via tree mortality, a committed carbon flux from decomposition over several years (~1.4 Pg C after the 2010 drought). In most years, these forests are a carbon sink; drought reverses this sink."

They conclude (footnotes omitted; see original article for references):

"The two recent Amazon droughts demonstrate a mechanism by which remaining intact tropical forests of South America can shift from buffering the increase in atmospheric carbon dioxide to accelerating it. Indeed, two major droughts in a decade may largely offset the net gains of ~0.4 Pg C year−1 in intact Amazon forest aboveground biomass in nondrought years. Thus, repeated droughts may have important decadal-scale impacts on the global carbon cycle.

Droughts co-occur with peaks of fire activity. Such interactions among climatic changes, human actions, and forest responses represent potential positive feedbacks that could lead to widespread Amazon forest degradation or loss. The significance of these processes will depend on the growth response of tropical trees to increases in atmospheric carbon dioxide concentration, fire management, and deforestation trends. Nevertheless, any shift to drier conditions would favor drought-adapted species, and drier forests store less carbon. If drought events continue, the era of intact Amazon forests buffering the increase in atmospheric carbon dioxide may have passed. "

Online Resources:

The 2010 Amazon Drought.  By Simon L. Lewis, Paulo M. Brando, Oliver L. Phillips, Geertje M. F. van der Heijden, and Daniel Nepstad.  Science, Vol. 331, no. 6017, p554 (4 Feb 2011).

Another Extreme Drought Hits the Amazon and Raises Climate Change Concerns By Nick Sundt, WWF Climate Change Blog, 23 November 2011.

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