In the race against climate change, global forestation has long been championed as a critical natural solution, touted for its ability to sequester carbon and cool the planet. However, groundbreaking research recently published in Nature Communications by Kan, Xu, Tang, and colleagues challenges this optimistic narrative by demonstrating that the biophysical cooling effects of forestation may wane significantly under increasing atmospheric CO₂ concentrations. This revelation compels the scientific community and policymakers alike to reassess the role of reforestation strategies in future climate mitigation portfolios.
The study delves deep into the complex interplay between atmospheric composition, vegetation physiology, and land-atmosphere energy exchanges that govern Earth’s climate system. Traditionally, large-scale afforestation has been viewed not only as a carbon sink but also as a natural cooling agent due to its ability to modify surface albedo, enhance evapotranspiration, and alter local energy budgets. Yet, this new research reveals that rising CO₂ levels impact these processes in nuanced ways, often dampening the cooling contributions attributed to expanding forest cover.
Central to the team’s findings is the effect of elevated CO₂ on plant stomatal behavior. Stomata—tiny pores on leaf surfaces—regulate gas exchange and water loss. As atmospheric CO₂ rises, stomata tend to close partially, reducing transpiration rates. While this can improve water-use efficiency for plants, it simultaneously diminishes latent heat fluxes—one of the key mechanisms through which forests cool the surface by transferring heat into the atmosphere via evaporated water. With less energy expended dissipating heat, the net biophysical cooling effect is therefore weakened.
Moreover, the altered transpiration patterns affect atmospheric humidity and cloud formation processes. The researchers used advanced climate-vegetation models incorporating dynamic stomatal responses, coupled to land surface and atmospheric modules, to simulate the behavioral shifts under varying CO₂ scenarios. Their results reveal that although photo-synthetic carbon uptake increases as CO₂ rises, this gain does not fully translate into proportional enhancement of forest cooling benefits. The complex nonlinear feedbacks in the Earth system mediate the overall impact, sometimes resulting in net warming in regions where forestation was once assumed to be unequivocally beneficial.
Beyond leaf-level physiological changes, the study also examines how shifts in canopy structure and albedo modulate radiation balance. Forested land typically has a lower albedo compared to grasslands or croplands, absorbing more solar energy. When combined with suppressed evaporative cooling due to CO₂-induced stomatal closure, the positive radiative forcing at the surface may counteract, or even override, the cooling gained through carbon sequestration. This effect is especially pronounced in boreal and temperate zones, where snow cover reduction under forest canopies further lowers albedo.
The authors emphasize that these results do not invalidate the carbon storage potential of global forestation but caution against overreliance on biophysical cooling in climate mitigation planning. They argue that afforestation policies must incorporate regional climatic contexts and dynamic plant-atmosphere feedbacks to avoid unintended consequences such as localized warming or shifts in hydrological cycles. Optimization of land management to maximize both carbon and biophysical cooling effects thus becomes imperative.
Furthermore, the study highlights the necessity of integrating biogeochemical and biophysical factors into Earth system models to accurately project future climate outcomes. Many current models underestimate or neglect the nuanced physiological responses of vegetation to rising CO₂, leading to potential overestimations of forestation’s cooling capacity. The incorporation of mechanistic stomatal conductance models represents a critical step forward in simulating these processes with greater fidelity.
The methodology employed by Kan and colleagues involved coupling state-of-the-art dynamic global vegetation models with radiative transfer and energy balance frameworks, validated against observational datasets across multiple biomes. By conducting sensitivity analyses under a range of CO₂ concentration trajectories aligned with climate scenarios, the researchers dissected how incremental increases in atmospheric carbon dioxide influence both carbon and energy fluxes at the land surface.
Attention to spatial heterogeneity featured prominently in their analysis. Different forest types—tropical, temperate, and boreal—exhibited distinct patterns in the dampening of cooling benefits. Tropical forests, for instance, maintained stronger evaporative cooling owing to consistently high moisture availability, whereas boreal forests showed elevated risks of biophysical warming owing to snow albedo dynamics and water limitations.
Importantly, the research confronts the misconception that more trees unequivocally translate to cooler climates. It underscores that the true climate impact of forestation hinges on a balance between carbon uptake and shifts in surface energy budgets. This balanced understanding is critical as many nations commit to ambitious tree-planting campaigns in pursuit of climate goals, potentially overlooking intricate climatic feedbacks.
The broader implications extend to global climate policy frameworks such as the Paris Agreement, which relies heavily on nature-based solutions. These findings suggest that forests, while indispensable in carbon sequestration, cannot be regarded as panaceas for atmospheric warming when considered in isolation from biophysical effects and atmospheric chemistry changes. Climate strategies must therefore be holistic and adaptive.
The study also invites renewed scrutiny of land-use decisions, since the net climate benefit of converting grasslands or croplands into forests depends heavily on local conditions and future CO₂ trajectories. Restoration ecology, afforestation, and reforestation projects must integrate these scientific insights to ensure they deliver intended climate benefits without adverse side effects.
Kan et al. advocate for increased observational campaigns utilizing remote sensing and in situ measurements to monitor real-world stomatal behavior under changing CO₂ regimes. Such empirical efforts are paramount to validate and refine model projections, ultimately enhancing predictive capacities concerning land surface-atmosphere interactions under climate change.
In summary, this transformative research reshapes our understanding of how rising atmospheric CO₂ complicates the biophysical climate benefits of forests. It calls for an urgent reevaluation of nature-based climate interventions through the lens of evolving biophysical and physiological archetypes, emphasizing the complexity and dynamism inherent in Earth’s climate system. Policy makers and scientists alike must heed these insights to craft nuanced, effective responses to the planetary climate crisis.
Article References:
Kan, F., Xu, H., Tang, S. et al. Diminished biophysical cooling benefits of global forestation under rising atmospheric CO₂. Nat Commun 16, 4410 (2025). https://doi.org/10.1038/s41467-025-59547-y
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