A study by Weber et al. (2024) recently published in Science diminishes hopes to simply offset excessive emissions by planting more trees.
While forests are usually considered carbon sinks and as having a cooling effect on the climate, things are not as straightforward as they may seem.
Weber et al. investigated reforestation and afforestation by taking into account the influence of changed surface albedo and aerosol scattering of sunlight and concluded that those two effects combined offset about one third of the cooling by carbon dioxide removal associated with forestation.
The authors compared different scenarios regarding global forest expansion (comprising afforestation, reforestation and forest enhancement (increased tree cover density)) under different warming scenarios. They showed that forestation, modelled in suitable regions, lead to increased aerosol scattering. Methane and ozone, both greenhouse gases, followed increased biogenic organic emissions (so-called biogenic volatile organic compounds (BVOCs) that are released by forests). The second effect was a change in the proportion of radiation reflected from the surface, a change in surface albedo. Simply speaking darker surfaces reflect less radiation but rather absorb it, leading to warming effects. Under a 4°C warming scenario, these two effects combined offset up to a third (23-31%) of the additional CO2 removal from re- or afforestation, under a 2°C warming scenario offsetting effects were smaller (14-18%).
These results do not mean that trees or forests are bad or do not have a role to play in climate change mitigation. They do. Moreover, they are also essential for biodiversity and many ecosystem services, and they do still sequester carbon. However, the findings confirm that reforestation and afforestation efforts need to be combined with substantial emission reduction (in order to not trigger too strongly the responses in the Earth System that diminish the positive effects on forestation on the climate). Second, calculations of using tree planting to offset emissions are almost inevitably flawed, due to the Earth System’s complexity, and most likely tend to overestimate the amount of emissions that can be offset. This means planting enough trees can not be a substitute to decarbonising. Reforestation and afforestation can still be beneficial, but do not diminish the importance of rapid resolute action to reduce emissions from all economic and societal activities!