Tropical forests now emit more carbon than they are able to absorb from the atmosphere as a result of the dual effects of deforestation and land degradation, a new study says.
The research challenges the long-held belief that forests act as “carbon sinks” by storing more carbon than they emit due to natural processes and human activity.
Instead, the world’s tropical forests could have experienced a net loss of around 425m tonnes of carbon from 2003 to 2014, the lead author tells Carbon Brief. This figure is considerably higher than previous estimates of carbon loss from tropical forests.
The findings suggest that curbing deforestation and protecting existing forests could be instrumental in removing greenhouse gases from the atmosphere and fighting future climate change, he adds.
Canopy carbon
Tropical forests are capable of storing large amounts of carbon. This is because trees absorb carbon from the atmosphere during photosynthesis and then use it to build new leaves, shoots and roots.
However, forests can also release carbon into the atmosphere. Some of this release is through natural processes such as plant respiration, droughts and wildfires. But emissions can be heightened further by human activities, such as deforestation and illegal logging.
Calculating the balance between the uptake and release of carbon allows scientists to determine whether tropical forests are a “carbon sink”, meaning they take in more carbon than they release, or a “carbon source”, meaning their carbon emissions exceed their intake.
For decades, the consensus has been that tropical forests are a moderate carbon sink.
However, a recent rise in human activity in the world’s forested regions could have disrupted the balance between uptake and emissions, says Dr Alessandro Baccini, a researcher at the Woods Hole Research Centre in Massachusetts and lead author of the new study published in Science. He tells Carbon Brief:
“The main discovery is that forests in tropical regions are not a carbon sink, but a carbon source. That means that the amount of carbon emissions from tropical regions are actually bigger than the carbon removal that this region is able to achieve.”
Calculating carbon loss
Traditionally, studies of carbon loss in tropical forests have relied on data taken from satellite images of tree cover.
This approach allows scientists to see the extent of deforestation in tropical regions, but it can overlook more subtle types of human activity, such as illegal logging, forest disturbance and land degradation. Baccini says:
“Degradation is a process where only a small portion of trees are removed from a forest. From a satellite image, the area will still look like an intact forest. But, when you lose even a small proportion trees, you lose a significant amount of carbon.”
Because of this caveat, the researchers opted instead to look for changes in “carbon density” from tropical forests spanning America, Africa and Asia.
“Carbon density is a measure of the weight of carbon that is held by forests. Even in the field we don’t really collect direct measurements of weight but we can collect direct measurements of the trees, such as their diameter and height, and then you use an equation to convert that into biomass.”