Four years ago, the team led by Thomas Crowther from ETH Zurich published the first study on the underestimated potential of forests in climate protection: global reforestation could retain 205 gigatons of carbon. The estimate at the time was immediately criticized by many researchers as being too high; it was possibly up to four to five times higher than the real possibilities.
There was also criticism of the viability of the theoretical potential; mass planting was not a solution to the climate crisis. Since then, the debate surrounding the global CO2 storage capabilities of forests has continued. Recently, forestry projects, in particular, have been the target of criticism related to global emissions trading: their effectiveness in terms of CO2 is drastically overestimated, according to a study carried out in August this year.
Potential in stock maintenance
Crowther, together with authors from more than 200 institutions around the world, now presents a new article on the topic in the specialized journal “Nature Climate Science”, which takes these objections into account. This time, the focus is on targeted management of existing forests.
Due to deforestation, natural forest stocks store 328 gigatons less than in their original state, according to the study. However, some former forest areas are now home to settlements or are used for agriculture. It’s not easy to plant new trees there. According to the study, the storage potential lies mainly where the original forest still grows today or in wooded areas where human influence is low, Crowther and his team explain.
This is demonstrated, among other things, by measurements of biomass on the ground using satellite data. In addition to trees, root systems, dead wood and soils are also taken into account. According to the researchers’ modeling, the global storage potential in areas with little human influence is 226 gigatons of carbon. For comparison: global fossil CO2 emissions in 2022 corresponded to around ten gigatons of carbon.
What does restoration mean
The majority of this (61 percent) would be the restoration of degraded forests – which would require, among other things, the promotion of their natural diversity. The remaining 39 percent would be covered by the reforestation of deforested but unused areas. Therefore, there would be minimal conflicts over land use. As Crowther highlights in a press release, the term restoration needs to be redefined: “Restoration does not mean planting large quantities of trees to offset CO2 emissions.” Instead, it’s about supporting indigenous communities and farmers to promote biodiversity.
Natural forests with high diversity could therefore retain much more carbon. But this does not replace a significant reduction in greenhouse gases, emphasizes the research team. Because if emissions continue to increase, forests will also be increasingly threatened by warming, for example through droughts and forest fires.
Uncertainties in the model
Although Crowther researchers had some reservations about the 2019 study, the new work is once again receiving criticism, as shown in statements obtained from the German Science Media Center. The content is that the modeling still contains many uncertainties.
Some of the experts interviewed criticize that climate change has not been sufficiently taken into account, such as Florian Zabel, geographer at the Ludwig Maximilians University of Munich: “For example, an increase in droughts and forest fires due to higher temperatures, but also an possible stronger plant growth, CO2 fertilization. It is also not taken into account that pressure on land may increase in the future due to increased demand for food – in particular due to greater meat consumption – and therefore there is less area available for the proposed measures than expected.
Wrong basic assumptions
Christian Körner from the University of Basel also highlights that despite the sound methodology, the study is based on an incorrect basic understanding of forests. Because true forests are dynamic systems that, even in pristine nature, constantly alternate between slow construction – lasting centuries – and sudden collapse – within hours to weeks – due to fires, windbreaks or insects: “Forests that permanently provide maximum, optimal storage,’ as assumed here, does not exist.”
The study feeds an illusion: “The reforestation of fallow forest lands is ecologically absolutely desirable, but the effect as carbon storage is greatly delayed. The maximum possible reserves calculated here – regardless of the size of the area – would probably only be expected within 100 to 200 years if we immediately started everywhere at the same time,” says Körner. As he emphasizes, the authors themselves point out these limitations in their study.Overall, however, the work provides a very nuanced picture that is very valuable for global questions about the status of forests and possible developments.