Not so long ago Brazil was home to not one, but two of the Earth´s largest tropical rainforest biomes, the Amazon and the Atlantic Forest.

Including true rainforest, dry tropical forest and mangroves the Atlantic Forest used to span an approximate 1.5 million square kilometers*. Human slash and burn practices have reduced that to a mere 4,000 km² – and as it turns out the wider region continues to experience the consequences.

[*) Or 580,000 square miles. For comparison, that equals some 27 percent of the current Amazonian forest cover.]

Unlike the Amazon rainforest the Atlantic forest is not fed by local rainfall recycling of fluvial waters from the Andes Mountains, but instead by direct precipitation from the South Atlantic Ocean. Because of typical air circulation patterns the region is not only characterised by summer monsoon rains, but also by mild and wet winters, allowing the rainforest biome to stretch much further south than its Amazonian counterpart.

The remaining Atlantic Forest still has high unique biodiversity. Its role as a terrestrial carbon store has however greatly diminished – in an ongoing process, an international group of scientists led by the German Max Planck Research Group for Marine Geochemistry of the University of Oldenburg states in a recent publication in Nature Geoscience.

When people start burning up entire forest biomes the primary concern for climatologists is of course CO2 emissions. As for instance seen in the 2010 Amazon drought these CO2 emissions can be significant.

As a secondary concern slash and burn practices lead to large black carbon emissions. And as a GHG-independent warming forcer this black carbon – or soot – too is significantly bad for the climate.

The reason soot is the lesser evil is that, much like any aerosol [and very much unlike CO2], the floating black carbon particles tend to fall down back to Earth, after which their albedo deteriorating effect usually [not in case of soot (or any aerosol) precipitation on continental glaciers!] quickly disappears.

Millions of tonnes of black carbon

With the incomplete burning of biomass in wildfires the soot production is by no means small compared to the CO2 emissions. According to the researchers by 1973, when the Brazilian Atlantic Forest slash and burn practices halted, some 200 to 500 megatonnes of black carbon had been released. Much of this black carbon settled somewhere within the original forested area.

That may sound like good news for ecology, as at least the black carbon could then be restored in the local organic soils – but unfortunately the forest was not allowed to recover after the fires as the land is now in use for agriculture – and without the dense tree vegetation the organic soils are prone to erosion, fluvial erosion, as indeed the high annual rainfall in the coastal areas is relatively unaffected by the deforestation.

Due to this soil erosion the relict Atlantic Forest continues to have a negative carbon flux. Almost 40 years after the last systematic forest fire the studied Paraíba do Sul river basin in the southeast of Brazil for instance still exports 50,000 to 70,000 tonnes of former forest carbon to the Atlantic Ocean, the researchers write.

© Rolf Schuttenhelm | www.bitsofscience.org