Long-term deglacial permafrost carbon dynamics in MPI-ESM

Abstract. We have developed a new module to calculate
soil organic carbon (SOC) accumulation in perennially
frozen ground in the land surface model JSBACH. Running
this offline version of MPI-ESM we have modelled
long-term permafrost carbon accumulation and release from
the Last Glacial Maximum (LGM) to the pre-industrial (PI)
age. Our simulated near-surface PI permafrost extent of
16.9106 km2 is close to observational estimates. Glacial
boundary conditions, especially ice sheet coverage, result in
profoundly different spatial patterns of glacial permafrost
extent. Deglacial warming leads to large-scale changes in
soil temperatures, manifested in permafrost disappearance in
southerly regions, and permafrost aggregation in formerly
glaciated grid cells. In contrast to the large spatial shift in
simulated permafrost occurrence, we infer an only moderate
increase in total LGM permafrost area (18.3106 km2)
– together with pronounced changes in the depth of seasonal
thaw. Earlier empirical reconstructions suggest a larger
spread of permafrost towards more southerly regions under
glacial conditions, but with a highly uncertain extent of noncontinuous
permafrost.
Compared to a control simulation without describing the
transport of SOC into perennially frozen ground, the implementation
of our newly developed module for simulating
permafrost SOC accumulation leads to a doubling of
simulated LGM permafrost SOC storage (amounting to a
total of 150 PgC). Despite LGM temperatures favouring
a larger permafrost extent, simulated cold glacial temperatures
– together with low precipitation and low CO2 levels –
limit vegetation productivity and therefore prevent a larger
glacial SOC build-up in our model. Changes in physical
and biogeochemical boundary conditions during deglacial
warming lead to an increase in mineral SOC storage towards
the Holocene (168 PgC at PI), which is below observational
estimates (575 PgC in continuous and discontinuous
permafrost). Additional model experiments clarified the
sensitivity of simulated SOC storage to model parameters,
affecting long-term soil carbon respiration rates and simulated
ALDs. Rather than a steady increase in carbon release
from the LGM to PI as a consequence of deglacial permafrost
degradation, our results suggest alternating phases of soil carbon
accumulation and loss as an effect of dynamic changes in
permafrost extent, ALDs, soil litter input, and heterotrophic
respiration.