Continental United States may lose 1.8 petagrams of soil organic carbon under climate change by 2100

Aims: High-resolution information on soils’ vulnerability to
climate-induced soil organic carbon (SOC) loss can enable environmental
scientists, land managers, and policy makers to develop targeted
mitigation strategies. This study aims to estimate baseline and decadal
changes in continental US surface SOC stocks under future emission
scenarios.   Location: Continental United States   Time
Period: 2014-2100   Results: Baseline SOC projections from ML
approaches captured more than 50% of variability in SOC observations,
whereas ESMs represented only 6-16% of observed SOC variability. ML
estimates showed a mean total loss of 1.8 Pg C from US surface soils under
the high-emission scenario by 2100, whereas ESMs showed no significant
change in SOC stocks with wide variation among ESMs. Both ML and ESM
predictions agree on the direction of SOC change (net emissions or
sequestration) across 46%–51% of continental US land area. These
differences are attributable to the high-resolution site-specific data
used in ML model compared to the relatively coarse grid represented in
CMIP6 ESMs.   Main conclusions: Our high-resolution estimates of
baseline SOC stocks, identification of key environmental controllers, and
projection of SOC changes from US land cover types under future climate
scenarios suggest the need for high-resolution simulations of SOC in ESMs
to represent the heterogeneity of SOC. We found that the SOC change is
sensitive to key soil related factors (e.g. soil drainage and soil order)
that have not been historically considered as input parameters in ESMs,
because currently more than 95% variability in the SOC of CMIP6 ESMs are
controlled by net primary productivity, temperature, and precipitation.
Using additional environmental factors to estimate the baseline SOC stocks
and predict the future trajectory of SOC change can provide more accurate
results.