Wednesday, February 12, 2014

Are the Earth’s Rainforests Carbon Sinks, or Have We Turned Them to Sources?

                The conventional wisdom has always been that the vast forests covering much of our planet must be offsetting a great amount of anthropogenic carbon emissions. CO­2 is an essential part of the photosynthetic process so an abundance of it would “logically” provide a surplus of resources for the forest and increase rates of photosynthesis. Generally, unless other limiting factors are present, this has proven to be true. Rates of COuptake generally increase in plants when exposed to higher atmospheric concentrations of the gas. However, in addition to the fact that CO2 release is greatly outpacing even this increased uptake, we may be damaging and using these forests to the point that they are no longer net carbon sinks. 

                A recent article in Nature magazine describes more recent measurements of atmospheric carbon over arguably the world’s most iconic forest, the Amazon. According to the research paper, the forest can vary its uptake from .02±22 Pg of carbon during one drought year to .30±.10 Pg of C during a relatively wet year. To put this in perspective, a petagram is one quadrillion grams of carbon. The variation alone in carbon uptake is equal to the weight of nearly 43 million average sized African Elephants. During the dry year, carbon released from the forest actually greatly exceeded the uptake. Forest fires (both natural and those started to clear forest for agriculture) released .48±.18 Pg more than the forest absorbed. Even during the wetter year uptake and release were only approximately even, with the average actually favoring carbon release. Given that the Amazon absorbed an average of .4 Pg of carbon every year for the two decades before 2005, this data is showing a potentially huge problem with our reliance on the world’s forests as carbon sinks.
                Every year, humans release approximately 9 Pg of carbon. This is about 3 and a half times the amount of carbon sequestered by all of the Earth’s forests (2.6±.7 Pg). Development of the third world is going to conflict with the conservation of the existing great forests, and the widespread use of slash and burn agriculture has the potential to throw the already damaged balance of atmospheric CO2 out the window. Conservation of the Amazon is now not only a matter of saving the incredible amount of biological diversity, but an important part of controlling greenhouse gas levels in the atmosphere.

Published by Stephen O'Brien


  1. The amount of carbon is absolutely astonishing; the scale of quadrillion grams and the variation of carbon uptake being equivalent to 43 million African elephants is insane. And it is shocking how much people release with slash and burning of forests, there's no wonder about how CO2 levels are so high; and there's definitely no way that we can hope for plants to reuptake it all to undo the mess. Really astonishing numbers here.

    Nicole Peterkin

    1. The scale of things is almost impossible to wrap your head around. I can't even imagine how much area those 43 million elephants would cover. Increases levels of CO2 and other greenhouse gases are poised to become a serious problem in coming decades. Proposed solutions range from fixation factories to purifying and storing CO2 deep underground, but any real effort toward a solution is very slow in coming.

      Stephen O'Brien

  2. This was a great read! It is really interesting to me that the tropical rainforests could actually be adding to the amount of carbon in the air. I wonder if this net output is accelerated by deforestation, or if the rainforests have always fluctuated between being a carbon sink and a carbon source. I would imagine that the reason the forests aren't always acting as carbon sinks has something to do with human intervention, but such a measurement would be very difficult to obtain, as I am not sure when carbon levels started being observed. I think this should really make us aware of where our paper and wood products come from.

    Tim Daly

  3. Left to its own devices the forest is always a net carbon sink for any given year. Intact forest plots measured for almost 30 years put estimates at net .4 Pg of carbon per year fixed (retained as physical plant growth) by the forest. Carbon uptake fluctuates as plants can't undergo photosynthesis as well under drought conditions, but it is almost impossible in nature for a forest to be a carbon source. As far as human effect goes, in addition to the CO2 being measured Carbon Monoxide was also measured and found to be at very high levels. Carbon Monoxide in that area is almost exclusively released by the burning of forest, so it can provide an accurate measure of burning rates when supplemented by satellite data.

    Stephen O'Brien