23 May 2022
23 May 2022

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

Huanhuan Wang1, Chao Yue2,3, and Sebastiaan Luyssaert4 Huanhuan Wang et al.
  • 1College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China
  • 2State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China
  • 3College of Forestry, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China
  • 4Department of Ecological Sciences, Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam 1081 HV, The Netherlands

Abstract. Satellite observations have been widely used to examine afforestation effects on local surface temperature at large spatial scales. Different approaches, which lead potentially to differed definitions of the afforestation effect, have been used in previous studies. The results were used in climate model validation and were cited in climate synthesis reports, but large differences existed in these results. Such differences were simply treated as observational uncertainty, which can be an order of magnitude bigger than the signal itself. However, it remains unclear whether these differences arise from methodological differences that can be reconciled or they represent intrinsic uncertainty of land surface temperature change following afforestation. Here, we provide a synthesis of three influential approaches (one estimates the actual effect and the other two the potential effect) used in the literature and use large-scale afforestation over China as a test case to examine whether the differences in the effects stem from methodological differences. We found that the actual effect (ΔTa) often relates to incomplete afforestation over a medium resolution satellite pixel (1 km) for which LST is observed and that it increases with the fraction of the pixel actually afforested (89 % variation in ΔTa being explained). One potential effect approach quantifies the impact of quasi-full afforestation (ΔTp1), whereas the other quantifies the potential impact of full afforestation (ΔTp2) by assuming a shift from 100 % openland to 100 % forest coverage. An initial paired-samples t-test shows that ΔTa < ΔTp1 < ΔTp2 for the cooling effect of afforestation ranging from 0.07 K to 1.16 K. But when all three methods are normalized for full afforestation, the observed range in surface cooling becomes much smaller (0.79 K to 1.16 K). While potential cooling effects could indeed be realized through full afforestation, they might not always be feasible, given other environmental constraints such as the high water consumption of forests and competition for land usage. Although potential cooling effects have a value in academic studies where they can be used to establish an envelope of effects, they are misleading in a policy-making context where the actual cooling effect better represents policy ambitions.

Huanhuan Wang et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2022-317', Chao Zhang, 13 Jun 2022
  • RC1: 'Comment on egusphere-2022-317', Anonymous Referee #1, 14 Jun 2022
  • RC2: 'Comment on egusphere-2022-317', Anonymous Referee #2, 22 Jun 2022

Huanhuan Wang et al.

Huanhuan Wang et al.


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Short summary
In this study we provided a synthesis of three influential methods to quantify afforestation impact on surface temperature. Results showed that actual effect following afforestation was highly dependent on intensity of afforestation. When full afforestation is assumed, actual effect approaches potential effect. Therefore, we provided evidence that these different methods could be reconciled. And actual effect has a direct policy relevance in evaluating the climate effects of afforestation.