Carbon Cycle-Climate Feedbacks
Climate warming may stimulate microbial respiration of soil organic carbon, causing a carbon cycle-climate feedback whereby carbon is redistributed from soil to atmospheric CO2. The magnitude of this feedback is uncertain, in part because warming-induced shifts in microbial physiology could retard or accelerate soil carbon losses. It is vital to reduce the uncertainty in this feedback given that soils are a vast repository for organic carbon, meaning that even small proportional losses could dramatically affect atmospheric CO2 concentrations and hence demand much greater reductions in fossil fuel emissions to meet climate targets (as we make clear in the following Policy Brief). The high uncertainty is reflected in the large spread among Earth System Model results for how the global stock of soil carbon will respond to climate change.
In the last decade, physical climate scientists have dramatically improved our confidence in climate change projections. They have primarily made these gains by comparing climate models with different structures to represent and then refine contrasting viewpoints on the mechanisms that regulate the climate system. In contrast, many biological assumptions in climate models are represented through a common structure that reflects an outdated conception of the mechanisms thought to regulate soil carbon turnover.
Recognition of such limitations is spurring efforts to explicitly represent microbes in models to capture emerging understanding of how soil carbon is formed and stabilized. We are working on providing the process-level knowledge for how microbial physiology might be represented in the models, through field and lab experiments that seek to understand the ecological and evolutionary mechanisms whereby climate affects microbial activity. This work dovetails seamlessly with our research theme on Soil Carbon, Litter Decomposition and Scaling Theory, in that it demands a local lens to understand regional and global responses.
Overall, we seek to build confidence in Earth System Model projections so that soil organic carbon changes - driven by the climate system - are factored into setting allowable greenhouse gas emission targets and hence actually achieve the climate mitigation they are intended to do so (see paper below by Yona et al. 2020 Ambio).