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

<p>Soil moisture affects gross primary production through two pathways. First, directly through<br>drought stress and second, indirectly through temperature via evaporative cooling of the near-<br>surface atmospheric layer. Because it is not possible to disentangle these effects experimentally<br>at a biome level, Humphrey et al. (2021) has used Earth system model experiments in which soil<br>moisture is fixed to its seasonal cycle and evaluated the effects on gross primary production. In<br>contrast, we aim to use causal modeling to infer impacts directly from observation. To predict the<br>effects of soil moisture anomalies on gross primary production, we extend existing causal mod-<br>eling frameworks to cover situations where two variables influence one other. A major challenge<br>in applying causal modeling here lies in the bidirectional relationship between soil moisture and<br>temperature via evapotranspiration. On one hand, higher temperature leads to higher evapotran-<br>spiration and thus lower soil moisture. On the other hand, lower soil moisture leads to lower evap-<br>otranspiration and thus higher temperatures. Therefore, neither soil moisture nor temperature can<br>be adequately modeled as a function of the other. To address this challenge, we extend existing<br>causal modeling frameworks to account for these situations where the variables are not functions<br>of each other but are determined by equilibrium. We show that our method identifies the correct<br>links between variables in synthetic data. We further evaluate whether our new approach is con-<br>sistent with the results of Humphrey et al. (2021) based on idealized counterfactual experiments<br>using Earth system models. To this end, we use the control runs of the models to directly predict<br>the results of the idealized counterfactual experiment as proof-of-concept. Finally, we apply our<br>method to observations and determine the direct and indirect effect of soil moisture anomalies on<br>gross primary production.</p><p>References:<br>Vincent Humphrey, Alexis Berg, Philippe Ciais, Pierre Gentine, Martin Jung, Markus Reichstein,<br>Sonia I Seneviratne, and Christian Frankenberg. Soil moisture&#8211;atmosphere feedback dominates<br>land carbon uptake variability. Nature, 592(7852):65&#8211;69, 2021.</p>