Last Friday I closed with a reference to the paper “Amazonia as a Carbon Source Linked to Deforestation and Climate Change”, by Luciana V. Gatti (from the Brazilian National Institute for Space Research) and co-authors.
The research team divided the 7.25 million km² Amazon basin (containing between 100 billion and 146 billion metric tons of carbon) into four regions: TAB_TEF (northwest), SAN (northeast), ALF (southeast), and RBA (southwest).
Using aircraft to collect air samples at different altitudes over 5 sampling sites (two of them, TAB and TEF, on the northwest region; of the remaining 3 sampling sites, one for each region), it was found that:
A quick comment. The table shows that, averaged over the nine years the study took, the Amazon basin has not acted as a carbon sink (see Net Carbon Flux column): no net flux is negative. [*]
Given the margin of error to be expected in empirical studies, one can take those results as showing that towards the northwest and southwest input and output of CO2 balance.
Not so on the east side of the Amazon basin, where deforestation has been more extensive and precipitation has fallen more sharply and where temperatures haven risen more: there (and I repeat averaged over the nine years the study took) the CO2 output (from respiration, decomposition, and fire) exceeds the input (CO2 absorption through photosynthesis). Therefore, over its eastern side the basin may be actually a net source of CO2.
Scott Denning’s “Southeast Amazonia is no Longer a Carbon Sink” explains and further elaborates and distinguishes within the eastern regions. He also discusses the relative contributions of forest clearing by fire and carbon sequestration (through the growth of biomass).
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Let’s try an intuitive explanation.
Plant tissue contains carbon, which comes from the air, more precisely from atmospheric CO2. Living plants absorb CO2 and release oxygen during daytime, while they absorb oxygen and release CO2 at night (that’s why it’s often recommended -- rightly or wrongly it’s a separate question -- not to the keep plants in your room at night).
Provided that the soil contains sufficient moisture, plants grow by absorbing more CO2 than they release through respiration. So, growing plants act as net carbon sinks. In other words: they sequester (or sequestrate, if you prefer) carbon by incorporating it in their mass (biomass).
Makes sense?
Of course that assumes two things: (1) plants are alive and (2) there is enough water.
Dead vegetal matter decomposes and part of the carbon it contains is released back into the atmosphere. Well, there is plenty of dead vegetal matter even in thriving, healthy forests. That release only increases when forests are cleared, particularly by fire.
And without enough water, even living plants cannot absorb CO2: dry seasons over the Amazon basin are growing longer and more intense.
In aggregate, those two things reduce the ability of forests to sequester CO2. That is happening more intensely over the eastern and southern parts of the Amazon basin, but I fear it may be only a matter of time until that extends even to the western and northern regions.
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So what?
As you read this, imagine that governments worldwide decided to do what scientists have been begging them to do. Human CO2 emissions instantly drop to 0.
Would that by itself stop stocks of atmospheric CO2 from increasing? And what happens when stocks of atmospheric CO2 increase?
A careful reading of Denning’s piece suggests that a ban on forest clearing by fire would go a long way into reducing CO2 emissions on the northeast region of the basin (see Figure 1). But not even that seems enough to restore the southeast region as a carbon sink.
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So, a big question now is whether there is a way to reverse those changes the Amazon basin is experiencing.
Note:
[*] In this context “flux” is synonymous with “flow”.
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