The Impact of Forest Expansion on Earth's Energy Balance


In recent years, planting trees has gained significant attention as a nature-based solution to combat climate change by removing carbon dioxide (CO2) from the atmosphere. This approach, known as afforestation or reforestation, is seen as a key strategy to help achieve the warming targets of the 2015 Paris Agreement. However, a recent study by Weber et al. published in Science sheds light on the complex interactions between forest expansion and Earth's energy balance, raising questions about the effectiveness of this strategy.

The Impact of Forest Expansion on Earth's Energy Balance
Planting trees has diverse effects on composition and climate mitigation

Weber et al. used Earth system models (ESMs) to investigate the effects of extensive forest expansion on Earth's energy balance. They found that while planting trees can sequester CO2, it can also lead to changes in albedo (surface reflectivity) and atmospheric composition, which could offset up to one-third of the climate mitigation effects of the CO2 sequestered. This suggests that the benefits of forestation may have been overestimated in previous studies.

Forests cover 31% of the global land area, but their distribution is not uniform. The Bonn Challenge, the New York Declaration on Forests, and the UN Decade on Ecosystem Restoration aim to restore 350 Mha of degraded and deforested lands by 2030, which could cover about 2.7% of the global land area. This restoration effort includes afforestation, reforestation, and avoided deforestation.

Another land-based carbon mitigation option is bioenergy with carbon capture and storage (BECCS), where crops or short-rotation coppice species are grown for energy production, with the capture and storage of the CO2 produced during combustion. The carbon mitigation potential for afforestation and reforestation is up to 10 billion tonnes (Gt) of CO2 per year, whereas BECCS is estimated to be 0.4 to 11.3 Gt of CO2 per year.

The effectiveness of BECCS is strongly dependent on factors such as the choice of bioenergy crop, biomass and use efficiency, and the offset of fossil fuel emissions. Forest-based mitigation was found to be more effective than BECCS for atmospheric CO2 removal, especially if bioenergy crops replace ecosystems with high carbon content.

However, the study by Weber et al. highlights the need to consider the broader impacts of forest expansion on Earth's energy balance. Forests and vegetation, in general, are sources of biogenic volatile organic compounds (BVOCs), which can affect Earth's energy balance through the production of tropospheric ozone and aerosol particles.

The study found that extensive forest expansion can decrease albedo and increase aerosol formation and concentrations of greenhouse gases like methane and ozone, which can offset the climate benefits of CO2 uptake. This offset is smaller when forestation occurs alongside the reduction of emissions of CO2 and other pollutants.

In conclusion, while planting trees is a valuable strategy for mitigating climate change, it is essential to consider the broader impacts on Earth's energy balance. Future research should focus on quantifying these impacts and developing guidelines and tools for effective forestation. Engaging local communities and ensuring institutional alignment will be crucial for achieving forestation targets and maximizing the benefits for climate mitigation, biodiversity, and ecosystem services.

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