How much CO₂ can forests really store?

In order to achieve the climate targets of the Paris Agreement and keep global warming well below 2 °C, it is not enough to simply reduce CO₂ emissions. It is becoming increasingly necessary to actively remove carbon dioxide from the atmosphere. In almost all future scenarios that are compatible with the Paris climate targets, afforestation/reforestation plays a central role. It is therefore important to be able to estimate the future carbon storage potential of forests as accurately as possible.

In a recent study, a research team led by Sabine Egerer and Julia Pongratz investigated how afforestation/reforestation is depicted in climate models in a sustainable future scenario compatible with the 2-degree target. The analysis is based on simulations of eight different models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), which also form the basis of the IPCC climate report.

The research work was carried out during a two-month stay by Sabine Egerer at the National Center for Atmospheric Research (NCAR) in the USA, which was supported by the LMU Postdoc Support Fund. There, Egerer was able to make use of the extensive expertise in dealing with the American climate model CESM. In addition, scientists from all eight participating international modeling centers collaborated, which further promoted scientific exchange.

The results of the study show considerable differences in the representation of afforestation/reforestation and the associated carbon uptake between the climate models. The main reasons for the differences are varying interpretations of land use based on data from the Land Use Harmonization Project (LUH2). In addition, the climate models differ in the representation of historical afforestation, use different satellite data for calibration and work with different spatial resolutions. The modeling of the carbon processes themselves also contributes to significant deviations in the predicted carbon storage potentials.

At the end of the study, the researchers make clear recommendations on how the modeling of forest carbon uptake can be improved and standardized. These include a more consistent representation of tree cover and the carbon cycle. Implementing these proposals could significantly increase the informative value of climate models – especially with regard to the role of land use in long-term CO₂ reduction.

The Paper is published in the Environmental Research Letters – Special Issue on Carbon Dioxide Removal.

Egerer, S., Lawrence, D.M., Lawrence P.J., Argles, A., Arora, V., Barbu, A., Harman, I.N., Miller, P.A., Raddatz, T., Vuichard, N., Warland, D., Ziehn, T., Pongratz, J. (2025): Forestation in CMIP6: Wide model spread in tree cover and land carbon uptake, Environ. Res. Lett. https://doi.org/10.1088/1748-9326/adc93e

Sabine Egerer and Julia Pongratz are part of the CDRterra consortium STEPSEC.