Better representation of the stomatal conductance scheme and its impact on pan-Arctic warming and greening

Opening and closing of stomatal pores in terrestrial plants are a key control of both photosynthetic CO₂ uptake and transpirational loss of water by land ecosystems. Their accurate modelling in response to variation in atmospheric conditions is important because it controls global water and carbon cycle even energy balance on the land surface. However, it is reported that simulations of transpiration by the Earth System Models (ESMs) are significantly diverse and it seems that leading uncertainty factor in future climate projections especially in terms of climate- carbon feedback. The current ESMs make use of the coupled photosynthesis-stomatal conductance models to determine the transpiration flux and gross primary productivity based on an empirical model, but the most ESMs are composed of parameterization for constants of empirical stomatal conductance model to only distinguish C₃ and C₄ plants. In recent studies, these constants are observed based on in-situ flux towers with various values depending on Plant Function Type (PFT) such as needleleaf, broadleaf tree, arctic (cold) grass, and shrub. Here, we modified the stomatal conductance scheme based on observed constant in the Community Land Model (CLM) 4.5 as a part of the Community Earth System Model (CESM). Especially, transpiration in boreal needleleaf forest is significantly reduced by modification of stomatal conductance scheme and it directly leads temperature increase by reduced latent heat and increased sensible heat flux between land and atmosphere in the AMIP run. As a result, leaf area index (LAI) has significant increasing trend during recent 30 years in modified simulation, while the control simulation does not show increasing trend of surface temperature and LAI which is observed in pan-Arctic.