Tree restoration, including reforestation and afforestation on previously unforested lands, is widely recognized as a key natural climate strategyThese measures can significantly contribute to limiting global warming through their ability to capture carbon dioxide (CO2) from the atmosphere – a process known as carbon sequestrationThis biogeochemical effect leads to a cooling of the climate.

However, the impact of tree restoration on climate is not simple and also includes other, so-called biogeophysical factorsThese include changes in surface albedo (surface reflectivity) and evapotranspiration (evaporation of water by plants). Increased forest cover can cause the surface to darken, especially in higher latitudes and snow-covered areas, leading to the absorption of more solar radiation and thus warmingOn the other hand, trees, especially tropical ones, increase evapotranspiration, which has cooling effectWithout considering atmospheric chemistry, these biogeophysical effects mitigate the biogeochemical cooling caused by carbon storage by up to 45 %.

However, recent research points to another, often overlooked factor: atmospheric chemistryStudies using climate models that also include interactive atmospheric chemistry show that this aspect significantly alters the overall climate response for tree restoration.

Simulations with and without interactive chemistry (CHEM) led to surprising findings. While the simulations without chemistry (NOCHEM) showed an overall global warming of surface air of 0.19 K, the simulations with chemistry (CHEM) recorded a significantly smaller global warming of only 0.07 K. This means that the effects of interactive chemistry led to net global cooling by approximately −0.11 K.

This additional cooling effect is largely attributed to the influence of atmospheric chemistry on organic aerosols and clouds. Tree restoration, especially tropical ones, leads to increased emissions biogenic volatile organic compounds (BVOCs)These compounds contribute to the formation of secondary organic aerosols (SOA) in the atmosphere. Increased amounts of aerosols, including SOA, can directly scatter solar radiation (aerosol direct effects) and also influence cloud formation and properties (aerosol-cloud indirect effects).

These aerosol and cloud effects led in the CHEM simulations to a more significant decrease in downward shortwave (solar) radiation reaching the surface compared to simulations without chemistry. This is the main driver of reduced warming, and especially cooling in the Southern Hemisphere. The CHEM simulations showed an increase in low cloud cover and changes in cloud microphysical properties (e.g., increased number of cloud condensation nuclei and smaller cloud droplet diameters) in the Southern Hemisphere, consistent with increased SOA levels.

The results also showed pronounced hemispheric asymmetriesWhile in the Northern Hemisphere moderate warming persists even with the inclusion of chemistry, in the Southern Hemisphere warming in the scenario without chemistry changes to pure cooling in the CHEM scenario. This stronger response in the Southern Hemisphere is related to the dominance of tropical trees in the restoration in this area, which are the main producers of BVOC and contribute to stronger evapotranspiration.

In addition to temperature, atmospheric chemistry and tree regeneration also affect other aspects of climate and the environment. Simulations suggest changes in fire activity (decrease in the tropics, increase in the mimotropics, but moderate increase with chemistry). Changes in atmospheric chemistry also lead to impacts on air quality. In the Northern Hemisphere, some locations are expected to experience increase in surface ozone (pollutant), while in the Southern Hemisphere there could be increase in fine particles (PM2.5), especially in areas with extensive tropical regeneration.

Although biogeochemical cooling due to carbon sequestration is the main benefit of tree restoration, biogeophysical effects themselves significantly mitigate this cooling. However including interactive atmospheric chemistry reduces the extent of this mitigation (from 45 % to 24 %, including the influence of methane). This confirms that atmospheric chemistry increases the potential of tree restoration to mitigate climate change, which is an aspect that is not commonly taken into account.

These findings highlight the complexity of interactions between land use change, climate and atmosphere and indicate that The climate benefits of afforestation and reforestation may be greater than previously thought, due to the effects of atmospheric chemistry, particularly through aerosol and cloud mechanisms. This research highlights the need for more comprehensive models when assessing the full climate potential of natural climate solutions. Spring

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Model study published in Communications Earth & Environment magazine

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Glossary of key terms

Afforestation: Afforestation of areas that were historically unforested.

Albedo: The degree of reflectivity of a surface or object. Surfaces with high albedo (e.g. snow) reflect more sunlight, while surfaces with low albedo (e.g. forests) absorb more.

Aerosol Direct Radiative Effect (ADRE): Direct impact of aerosols on the radiation balance by scattering and absorption of solar and terrestrial radiation.

Aerosol optical density (AOD): The degree to which aerosols scatter and absorb light, which affects the passage of light through the atmosphere.

Biogenic volatile organic compounds (BVOCs): Organic compounds emitted by plants that can react in the atmosphere to form secondary aerosols and ozone.

Biogeochemical factors: Processes related to the carbon cycle and other biogeochemical cycles, such as carbon storage by vegetation and soil.

Biogeophysical factors: Physical processes that affect the Earth's energy balance and climate, such as changes in albedo and evapotranspiration.

Cloud radiative effect (CRE): The effect of clouds on the radiative balance, which includes cooling by reflecting solar radiation and warming by trapping long-wave radiation.

Cloud Condensate Nuclei (CCN): Small particles in the atmosphere on which water vapor can condense to form cloud droplets.

Cloud water number concentration (CDNC): The number of cloud droplets per unit volume of air.

Community Atmosphere Model (CAM6): The atmospheric component of the Community Earth System Model (CESM2), which simulates atmospheric processes including dynamics, physics, and chemistry.

Community Land Model (CLM5): The terrestrial component of the Community Earth System Model (CESM2), which simulates terrestrial processes including vegetation, the carbon cycle, and fires.

Evapotranspiration: The combined process of evaporation of water from the earth's surface and transpiration of water from plants into the atmosphere.

Effective Radiated Power (ERF): The immediate change in the net radiative flux at the top of the atmosphere after a new atmospheric equilibrium is reached (including rapid adjustments).

Instantaneous Radiative Force (IRF): An instantaneous change in the net radiative flux at the top of the atmosphere due to a perturbation, without any system reactions.

Short-term climate factors (SLCF): Substances in the atmosphere that have a relatively short lifespan compared to long-lived greenhouse gases but can significantly affect the climate (e.g. aerosols, ozone, methane).

Hydroxyl radical (OH): A highly reactive molecule in the atmosphere that plays a key role in removing many pollutants and greenhouse gases, including methane.

Indirect effects of aerosol clouds: The influence of aerosols on cloud properties (such as the number of cloud droplets and droplet size), which in turn affect the radiative properties of clouds.

Soil carbon: Carbon stored in soil in the form of organic matter.

Latent heat flux (LH): The transfer of energy from the surface to the atmosphere through evapotranspiration.

Net primary productivity (NPP): The net uptake of carbon by vegetation through photosynthesis after subtracting plant respiration.

NOCHEM simulation: Simulation performed without interactive atmospheric chemistry.

Nitrogen oxides (NOx): A group of reactive gases containing nitrogen and oxygen that play a role in ozone formation and nitrogen deposition.

Nitrogen deposition: The deposition of nitrogen compounds from the atmosphere to the Earth's surface, which can affect ecosystem productivity and the carbon cycle.

Ozone (O3): A gas in the atmosphere that is a pollutant at ground level and protects against UV radiation at stratospheric level (the ozone layer).

Radial cores: Mathematical tools used to estimate the radiative impact of small changes in climate variables.

Quick adjustments (RAP_ADJ): Changes in atmospheric or surface processes that occur rapidly in response to climatic forcing that are not caused by changes in surface temperature.

Reforestation: Reforestation in areas that were previously forested but have been deforested.

Effective radius of cloud droplets (Re): The average size of droplets in a cloud.

Secondary organic aerosol (SOA): Aerosol particles that form in the atmosphere from the oxidation of gas-phase organic compounds, such as BVOCs.

Surface Energy Balance (SEB): The balance of incoming and outgoing energy at the Earth's surface, which determines the surface temperature.

Sensible heat flux (SH): The transfer of energy from the surface to the atmosphere through conduction and convection.

Shortwave radiation (SW): Radiation from the Sun.

Longwave radiation (LW): Radiation emitted from the Earth and the atmosphere.

Simulation with a weak ocean model: A type of climate model where the ocean is represented by a mixed layer whose temperature is calculated based on an energy balance, but there is no ocean current dynamics.

Transient Climate Response to Cumulative CO2 Emissions (TCRE): A warming rate based on the amount of cumulative CO2 emitted into the atmosphere, which provides an estimate of warming from changes in carbon storage.

Tropospheric ozone (TO3): Ozone found in the troposphere, the lowest layer of the atmosphere, acts as a greenhouse gas and pollutant.