Disentangling direct and indirect soil moisture effects onecosystem carbon uptake with Causal Modeling

Soil moisture affects gross primary production through two pathways. First, directly through
drought stress and second, indirectly through temperature via evaporative cooling of the near-
surface atmospheric layer. Because it is not possible to disentangle these effects experimentally
at a biome level, Humphrey et al. (2021) has used Earth system model experiments in which soil
moisture is fixed to its seasonal cycle and evaluated the effects on gross primary production. In
contrast, we aim to use causal modeling to infer impacts directly from observation. To predict the
effects of soil moisture anomalies on gross primary production, we extend existing causal mod-
eling frameworks to cover situations where two variables influence one other. A major challenge
in applying causal modeling here lies in the bidirectional relationship between soil moisture and
temperature via evapotranspiration. On one hand, higher temperature leads to higher evapotran-
spiration and thus lower soil moisture. On the other hand, lower soil moisture leads to lower evap-
otranspiration and thus higher temperatures. Therefore, neither soil moisture nor temperature can
be adequately modeled as a function of the other. To address this challenge, we extend existing
causal modeling frameworks to account for these situations where the variables are not functions
of each other but are determined by equilibrium. We show that our method identifies the correct
links between variables in synthetic data. We further evaluate whether our new approach is con-
sistent with the results of Humphrey et al. (2021) based on idealized counterfactual experiments
using Earth system models. To this end, we use the control runs of the models to directly predict
the results of the idealized counterfactual experiment as proof-of-concept. Finally, we apply our
method to observations and determine the direct and indirect effect of soil moisture anomalies on
gross primary production.

References:
Vincent Humphrey, Alexis Berg, Philippe Ciais, Pierre Gentine, Martin Jung, Markus Reichstein,
Sonia I Seneviratne, and Christian Frankenberg. Soil moisture–atmosphere feedback dominates
land carbon uptake variability. Nature, 592(7852):65–69, 2021.