Afforestation, which includes afforestation (planting trees in areas without recent forest cover) and reforestation (replanting trees in areas where they have been recently cleared), has been proposed as natural method of reducing CO2 in the atmosphereNew The study, published in collaboration with CECCR at King Abdulaziz University, uses twelve models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to quantify the biogeochemical and biogeophysical effects of net afforestation on climate by the end of the 21st century. The study examines a future net afforestation scenario, defined as the difference between the ssp370-ssp126Lu scenarios (global afforestation and no deforestation) and ssp370 (significant increase in greenhouse gases and global deforestation).

Biogeochemical effects: The main goal of afforestation is carbon sequestration in the terrestrial biosphere, thereby reducing atmospheric CO2 concentrations. This study estimates that clean afforestation leads to global multi-model average cooling of -0.08 ± 0.02 K through biogeochemical effects. The increase in soil carbon (cLand) represents a global average of 0.27 ± 0.13 kg m-2, corresponding to an annual CO2 reduction of approximately 0.99 GtCO2 per year. Most of this carbon (89 %) is stored in vegetation. This biogeochemical cooling is significantly greater as a global biogeophysical effect.

Biogeophysical effects: In addition to its effects on CO2, afforestation can also affect temperature through biogeophysical effects, which are defined as temperature changes due to changes in soil physical properties affecting the surface energy balance. Previous studies have suggested that these effects could compensate for biogeochemical effects by up to 50 %. However, this study found that biogeophysical effects lead to an insignificant global average cooling of -0.002 ± 0.041 K.

Regional differences in biogeophysical effects: Despite the globally insignificant effect, there are significant regional differences:

• Tropical regions they report cooling by -0.058 ± 0.058 KThis cooling is primarily caused by increased evapotranspiration (ET) a reduced incoming shortwave solar radiation (SWd) due to clouds and aerosols. Trees in the tropics cool the surface through high ET and increased latent heat flux. Increased humidity can lead to greater cloud formation and direct absorption of shortwave radiation by water vapor.
• On the contrary, in extratropical regions of the northern hemisphere (above 15° latitude) there is an insignificant warming of 0.035 ± 0.079 K. This warming is primarily associated with by reducing surface albedo (α), which is caused by surface darkening due to forest cover. The decrease in albedo is a significant and robust signal across models.

Impact on fires (fFire): Study also examined the impact of clean afforestation on carbon emissions from fires. It was found that clean afforestation leads to insignificant global increase in fFire. A decrease in fFire was observed in the tropics, while an increase was observed in areas such as the Sahel and southern Africa. This increase in emissions from fires was primarily driven by land cover change, specifically by increasing the proportion of grass due to agricultural land abandonment, rather than climate change caused by the biogeophysical effects of afforestation. Overall, the insignificant global increase in fFire only poorly offsets the benefits of carbon sequestration afforestation.

Remote effects: There are indications that pure afforestation can also cause long-range climate effects, manifested in significant changes in temperature over the oceanThese changes are consistent with changes in temperature over land and suggest an impact on atmospheric or ocean circulation. However, given the limited number of model runs, these changes may be partly due to internal climate variability and further research is needed.

Conclusion: The results of the study indicate that clean afforestation can help mitigate anthropogenically caused global warming, primarily through biogeochemical cooling. Although there are regional differences in biogeophysical effects (cooling in the tropics, warming in the northern extratropics), the overall global biogeophysical impact has been insignificant. The study highlights that Afforestation, tree protection, and reforestation of the tropics (15°S-15°S latitude) would reduce global CO2 concentrations, have latent cooling effects, and could reduce carbon emissions from fires in some tropical regionsHowever, given the relatively weak cooling effects, emissions reduction remains the dominant lever in addressing human-caused climate change. Further studies are also important to understand model differences and impacts on aerosols and the atmosphere. JRi

Study was published in the journal Nature