Where plant matters: How forests adjust aerosol cooling effect in surprising ways

The research team, led by Professors Pingqing Fu from Tianjin University has uncovered previously unrecognized complexities in how vegetation changes influence climate through interaction with aerosol formation utilizing an advanced Earth system model. The findings demonstrate that forestation initiatives must carefully consider regional variations in atmospheric feedbacks to maximize their intended climate benefits. While tree planting has been widely promoted as a climate solution, this research reveals the mechanisms are far more nuanced than previously understood.

Reforestation and afforestation have been recognized as a crucial strategy for mitigating global warming. However, beyond assessing its "carbon sink effect," it is equally important to holistically evaluate the dual climatic effects induced by vegetation changes through alterations in both surface and atmospheric conditions: Vegetation modifications regulate radiative balance by changing surface albedo, while also disturbing near-surface aerodynamic processes through structural changes in plant cover. These biogeophysical effects can trigger cascading climate responses. Simultaneously, biogenic volatile organic compounds (BVOCs) released by vegetation undergo atmospheric oxidation to form secondary organic aerosols (BSOA), which exert cooling effects by scattering solar radiation and modulating cloud microphysical processes.

The study reveals a dual modulation mechanism governed by distinct biogeophysical processes: When vegetation changes are primarily driven by reduced surface albedo, increased forest cover enhances solar absorption, leading to localized warming that significantly stimulates BVOC emissions from trees. This in turn amplifies the radiative cooling effect of BSOA. Conversely, when vegetation changes are dominated by enhanced updraft disturbances, the moisture uplift intensified by forests promotes the formation of dense cloud layers, which reduce surface solar radiation and consequently suppress BVOC emissions. This suppression diminishes BSOA formation and weakens its radiative cooling capacity.

This bidirectional modulation stems from fundamental regional differences in dominant biogeophysical mechanisms - albedo reduction acts as a "warming engine" that activates BSOA's cooling potential, while aerodynamic disturbance functions as a "sunshade" that inhibits BSOA's cooling capability. The contrasting effects highlight the complex interplay between surface and atmospheric processes in determining the net climate impact of afforestation.

Earth system modeling reveals pronounced spatial heterogeneity in how biogeophysical processes modulate BSOA radiative effects across vegetated regions. The simulations demonstrate that biogeophysical feedback acts as an "effect amplifier" in half of global vegetated areas, intensifying BSOA radiative effect variations by up to twofold. Conversely, in the remaining half of vegetated zones, these processes function as "dampening regulators," offsetting over 50% of BSOA radiative effect changes.

The study further reveals that biogeophysical feedback can induce large-scale climatic changes which subsequently cause disproportionately large variations in BVOC emissions even in areas with relatively minor vegetation changes. Such regional feedback effects are especially prominent in densely vegetated ecosystems like the Amazon rainforest. Importantly, failure to account for this spatial heterogeneity in biogeophysical modulation may lead to significant uncertainties in evaluating and predicting BSOA radiative effects under global vegetation change scenarios.

This landmark study has for the first time systematically revealed the dual regulatory mechanism through which vegetation changes modulate aerosol radiative effects via biogeophysical processes. "Our work provides the missing piece in understanding the complex 'afforestation-climate feedback' chain," said Prof. Pingqing Fu. "We now know tree planting isn't as simple as 'plant and cool' - it requires precision design based on regional dominant biogeophysical processes."