Lower boundary conditions in Land Surface Models. Effects on the permafrost and the carbon pools

Abstract

Earth System Models (ESMs) use bottom boundaries for their land surface model components which are shallower than the depth reached by surface temperature changes in the centennial time scale associated with recent climate change. Shallow bottom boundaries reflect energy to the surface, which along with the lack of geothermal heat flux in current land surface models, alter the surface energy balance and therefore affect some feedback processes between the ground surface and the 5 atmosphere, such as permafrost and soil carbon stability. To evaluate these impacts, we modified the subsurface model in the Community Land Model version 4.5 (CLM4.5) by setting a non-zero crustal heat flux bottom boundary condition and by increasing the depth of the lower boundary by 300 m. The modified and original land models were run during the period 1901-2005 under the historical forcing and between 2005-2300 under two future scenarios of moderate (RCP 4.5) and high (RCP 8.5) emissions. Increasing the thickness of the subsurface by 300 m increases the heat stored in the subsurface by 72 ZJ 10(1 ZJ = 10 21 J) by year 2300 for the RCP 4.5 scenario and 201 ZJ for the RCP 8.5 scenario (respective increases of 260% and 217% relative to the shallow model), reduces the loss of near-surface permafrost between 1901 and 2300 by 1.6%-1.9%, and reduces the loss of soil carbon by 1.6%-3.6%. Each increase of 0.02 W m −2 of the crustal heat flux increases the temperature at the soil-bedrock frontier by 0.4 ± 0.01 K, which decreases near-surface permafrost area slightly (0.3-0.8%), but reduces the loss of soil carbon by as much as 1.1%-5.6% for the two scenarios. 15Geosci. Model Dev. Discuss., https://doi.and snow cover (Hansen and Nazarenko, 2004). In these Land Surface Models (LSMs) the bedrock layer present below soil is impermeable, and when explicitly modeled, the only process taking place in bedrock is thermal diffusion.Thermal diffusion in the subsurface allows the land system to act like a heat reservoir, contributing to the thermal inertia of Earth's climate. However, this contribution is relatively small as the capacity of the oceans to absorb energy is orders of magnitude above that of the continents (Stocker et al., 2013). Estimates of the energy accumulation during the second half of 5 the 20th century in the land system show that the heat stored in continents (9 ± 1 ZJ, where 1 ZJ = 10 21 J) is less than the uncertainty on the heat stored in oceans during the same period (240±19 ZJ) (Beltrami et al., 2002;Levitus et al., 2012;Rhein et al., 2013). This justifies many ESMs to only consider the land subsurface to the shallow depth (3 − 4 m) needed for soil modeling (Schmidt et al., 2014;Wu et al., 2014) and neglect the bedrock entirely. Still, the thermal regime of the subsurface affects the energy balance at the surface, which in turn influences the surface and soil processes with a feedback on the climate 10 system. Energy variations at the land surface propagate underground, and the use of a too shallow subsurface in land models impl...

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