In recent years there has been some contradictory information about the potential of forests as CO2 sinks. Now a comprehensive study in the journal “Nature” confirms that biologically diverse forests can be large carbon sinks. Accordingly, restored natural forests with many different tree species could bind around 226 gigatons of carbon - in addition to the forests' current storage potential. However, this is only possible if we humans significantly reduce our greenhouse gas emissions and better protect biodiversity, the scientists emphasize.
Trees can absorb carbon dioxide (CO2) from the atmosphere through photosynthesis and store it in the form of biomass. Forests therefore have enormous potential to offset human CO2 emissions and stabilize the climate. Through reforestation they could absorb even larger quantities of this climate-damaging greenhouse gas. But scientists have not yet agreed on how much CO2 forests can actually bind and how great their climate protection effect is. A 2019 Science study found that reforestation could sequester 205 gigatons of carbon - equivalent to about 30 percent of the CO2 released into the atmosphere by humans. Further studies confirmed the results, other researchers considered the number to be exaggerated and determined that forest storage potential was four to five times lower.
New analysis of forests as CO2 storage
In order to find out how effective reforestation actually is, an international team led by Lidong Mo from the Swiss Federal Institute of Technology (ETH) Zurich has now re-evaluated the storage potential of forests. Hundreds of researchers from all over the world were involved in the study, some of them the same as in the 2019 study. The scientists used various methods and calculation models to evaluate an extensive data set on global biomass, which came from satellites and measuring stations on the ground and contained more comprehensive information than previous studies. From this they determined the carbon bound in trees, dead wood and soils of forests. Unlike 2019, the new study also takes into account how much additional carbon existing but damaged forests could sequester with better management.
The result: The natural storage potential of forests worldwide is currently not being fully exploited. Because we humans are clearing or damaging more and more forest areas for settlements and agriculture, their carbon storage capacity is around 328 gigatons of CO2 below the potential that would be possible without our influence. In some cases, the deforested or damaged areas are not used at all and could be renatured, i.e. converted into near-natural forests. On this area, forests worldwide could bind around 226 gigatons of carbon, as the researchers calculated.
But to achieve this, further measures are necessary. On the one hand, existing forests would have to be protected so that they can recover. As a result, around 61 percent of the previously unused potential (139 gigatons) could be achieved, report Mo and his colleagues. On the other hand, the forests would have to be actively renatured, for example by merging smaller forest areas into larger ones, planting new trees and managing the new areas sustainably. This would restore natural ecosystems and achieve the remaining 39 percent (87 gigatons) of unused CO2 storage potential.
Reforestation alone is not enough
The new data also shows that around half of the global storage potential of forests depends on their biodiversity. “Most of the world's forests are severely damaged,” explains Mo. “In order to restore biodiversity worldwide, deforestation must above all be stopped.” If forests are also reforested, many different tree species would have to be used and they would have to be planted sustainably in order to protect the natural environment To promote biodiversity and achieve its full storage capacity, the scientists report. This is a community task and is the responsibility of us humans to protect the forests and thus the climate and ultimately our own livelihoods.
“Restoration does not mean planting masses of trees to offset carbon emissions,” emphasizes senior author Thomas Crowther from ETH Zurich. Rather, restoration means supporting local communities, indigenous peoples and farmers who, through their actions, promote biological diversity around the world. “Only when healthy biodiversity becomes the preferred choice for local communities will we achieve the full CO2 storage potential as a positive side effect in the long term,” explains Crowther.
Meadows and moors also store CO2
But it's not just trees that are important as CO2 sinks and for an intact environment, but also other plants. The research team warns that environmentally friendly restoration of forests should not come at the expense of other ecosystems such as tundras or grasslands. “We must protect the natural biodiversity of all ecosystems that are important for life on earth – this also includes meadows, moors and wetlands,” says co-author Constantin Zohner from ETH Zurich. Overall, the study shows that natural, species-rich forests could actually bind up to 30 percent of the carbon dioxide emissions caused by us humans - as the 2019 study also showed.
However, Mo and his colleagues have now made the conclusion that the researchers drew from their findings more precise: We humans should not only focus on restoring forests and nature in order to protect the climate. It is just as important to reduce our fossil emissions. The researchers warn that these are already threatening the forests through their effects such as persistent droughts, forest fires and global warming, which in turn means that they are able to store less and less CO2. “We cannot choose between reducing emissions or protecting nature – both are urgently needed. We need nature for the climate and we need climate protection for nature,” emphasizes Crowther. “The more greenhouse gases we emit, the greater the danger to people and nature.”
In addition, even with immediate reforestation, the effect as an effective carbon sink would take a long time, because trees need time to grow. “The maximum possible reserves calculated here, regardless of the area size, would probably only be expected in 100 to 200 years if we started immediately everywhere at the same time,” says Christian Körner from the University of Basel, who was not involved in the study . “Preventing the deforestation of old forests, on the other hand, has an immediate effect.” Future influences on forests due to human actions and climate change are not taken into account in the study because they are difficult to predict.
Source: Lidong Mo (ETH Zurich) et al., Nature, doi: 10.1038/s41586-023-06723-z