17 AUG 2026 (MON) 15:05 - 15:35

Soil Moisture Regulates the Cooling Benefit of Global Greening under Atmospheric Drying

Mr. XIAO Zhen 

( Supervisor: Prof Shunlin Liang )

Abstract:

Vegetation greening can cool the land surface by altering surface energy partitioning, yet whether this cooling benefit persists under climate change remains unclear. Here we quantify the land surface temperature response to a unit increase in leaf area index (𝑆𝐿𝐴𝐼) using global satellite observations during 2003–2020. We show a widespread but spatially heterogeneous cooling response, driven primarily by enhanced latent heat flux and partly offset by albedo-induced warming. Combined with the observed greening trend, vegetation greening resulted in a global mean cooling of −0.037±0.012 K from 2003 to 2020. However, 𝑆𝐿𝐴𝐼 shifted significantly towards warming in tropical and temperate regions, by 0.032±0.019 and 0.039±0.015 K LAI⁻¹ per decade, respectively. We further show that root-zone soil moisture regulates the influence of vapour pressure deficit (VPD) on 𝑆𝐿𝐴𝐼. Under relatively wet conditions, increasing VPD is associated with more negative 𝑆𝐿𝐴𝐼, whereas under dry conditions the relationship becomes nonlinear, with 𝑆𝐿𝐴𝐼 increasing above a critical VPD level. Future increases in VPD combined with declining root-zone soil moisture are projected to further shift 𝑆𝐿𝐴𝐼 towards warming. These results reveal a strong hydrological constraint on the biophysical cooling effect of global greening, suggesting that continued greening may become less effective at buffering land surface warming under increasingly warm and dry conditions.