Reconciling different approaches to quantifying land surface temperature impacts of afforestation using satellite observations

[摘要] Satellite observations have been widely used to examineafforestation effects on local surface temperature at large spatial scales.Different approaches, which potentially lead to differing definitions of theafforestation effect, have been used in previous studies. Despite theirlarge differences, the results of these studies have been used in climatemodel validation and cited in climate synthesis reports. Such differenceshave been simply treated as observational uncertainty, which can be an orderof magnitude bigger than the signal itself. Although the fraction of thesatellite pixel actually afforested has been noted to influence themagnitude of the afforestation effect, it remains unknown whether it is a keyfactor which can reconcile the different approaches. Here, we provide asynthesis of three influential approaches (one estimates the actual effectand the other two the potential effect) and use large-scale afforestationover China as a test case to examine whether the different approaches can bereconciled. We found that the actual effect ( Δ T a ) often relatesto incomplete afforestation over a medium-resolution satellite pixel (1 km). Δ T a increased with the afforestation fraction, which explained89 % of its variation. One potential effect approach quantifies the impactof quasi-full afforestation ( Δ T p 1 ), whereas the otherquantifies the potential impact of full afforestation ( Δ T p 2 ) byassuming a shift from 100 % openland to 100 % forest coverage. Aninitial paired-sample t test shows that Δ T a Δ T p 1 Δ T p 2 for the cooling effect ofafforestation ranging from 0.07 to 1.16 K. But when all three methods arenormalized for full afforestation, the observed range in surface coolingbecomes much smaller (0.79 to 1.16 K). Potential cooling effects have avalue in academic studies where they can be used to establish an envelope ofeffects, but their realization at large scales is challenging given itsnature of scale dependency. The reconciliation of the different approachesdemonstrated in this study highlights the fact that the afforestationfraction should be accounted for in order to bridge different estimates ofsurface cooling effects in policy evaluation.

[学科分类] 大气科学