The role of atmospheric humidity in controlling land-atmosphere feedbacks over forest: regional and global-scale analyses

Abstract:

The interaction of land cover and atmosphere can affect the climate patterns via biogeochemical and biogeophysical process. The afforestation contributes to increase the biogeochemical cycles like carbon sequestration. Meanwhile, the landcover change modify the biogeophysical parameters perturbs the energy and water fluxes. The latter will be the most direct process to affect the atmosphere and its effects from landcover change outweigh radiative forcing triggered off by CO₂ emissions.

After the “Grain to Green Program”, the Loess Plateau (LP) has experienced a widespread forest expansion. Up to 2012, the extension of forest area in the central LP (Ningxia, Shanxi, and Shaanxi) accounted for 11.2 % of the area of the three provinces. The greening trend has changed the energy and water cycle, hence to a climate variability. The moist heat stress (a combined climate metric) has been recently investigated because it is directly related to human health. However, the affection of afforestation to moist heat stress is still unclear in LP.

In a recent study, we used the Weather Research and Forecasting (WRF) model to simulate the modulation of moist heat in LP caused by the afforestation. The result demonstrates that the intensive revegetation in LP shows a cooling effect on regional average near surface air temperature, especially in central LP. In addition, an increase of relative humidity caused by afforestation is detected. Driving by the near-surface temperature, sensible heat flux, and the subsidence of the planetary boundary layer the moist heat stress has obvious change after afforestation. The average moist heat stress decreases in central LP. While the decrease rate of moist heat stress is slower than near-surface temperature. It is worth noting that, an increased signal occurs in the maximum moist heat stress which might expose humans to the risk of moist heat stress. Our sensitivity results imply that the moist heat stress should be accounted for in climate change adaptation.

In ongoing work, we study the role of atmospheric VPD on mitigating land-atmosphere feedbacks over forest and non-forest land cover based on a global analysis of FLUXNET data. Preliminary results show a strong climate control on the effect of VPD on land-atmosphere exchange, in particular during heatwaves.

Reference: Zhang, S., Wang, W., Teuling, A. J., Liu, G., Ayantobo, O. O., Fu, J., & Dong, Q. (2022). The effect of afforestation on moist heat stress in Loess Plateau, China. Journal of Hydrology: Regional Studies, 44, 101209