The high latitudes form an important component of
the Earth’s carbon cycle. It is therefore important to
capture this in Earth System Models (ESM’s). However,
most carbon-cycle development and evaluation
in ESM’s focuses on lower latitudes, and therefore
there is an urgent need to address Arctic carbon-cycle
Here, we run land-surface schemes from ESM’s at
the site level at various Arctic sites, performing a detailed
evaluation of the carbon dynamics in the models.
They are process-based models, and therefore
point-scale evaluations contribute directly towards
improving the large-scale results.
The sites chosen for the simulations are the five
principal field sites from the recently-concluded EU
project PAGE21. This gives the distinct advantage
that detailed data are available. In particular, data
on the physical state of the climate and permafrost at
these sites, and large datasets of soil carbon stocks and
fluxes. The sites cover a range from low Arctic discontinuous
permafrost to high Arctic desert, and a range
of soil types from thick peat to mineral soils with little
organic matter. The models involved are land surface
schemes from three European ESM’s: UKESM (UK),
IPSL (France) and MPI-ESM (Germany). The models
all have improved process representation as part
The simulations are first compared with physical observations
from the sites: specifically snow depth, soil
temperature, soil moisture and maximum thaw depth.
All models capture the physics with a reasonable accuracy,
and certainly capture the major differences
between sites, with a few exceptions. In particular,
we see the importance of simulating the physical properties
of the soil organic layer.
Comparing simulated soil organic carbon with observations
shows the importance of including vertical
soil carbon profiles. In one model this is not represented,
which results in a failure to capture the differences
in soil carbon in different physical conditions.
Including cryoturbation mixing is key to simulating
the vertical soil organic carbon profile. When vertical
mixing is included, the profile of soil organic carbon at
mineral soil sites matches very well with observations.
However, none of the models are able to simulate the
correct profile at sites with organic soils, highlighting
the need for further process representation of peat
Finally, the land-atmosphere carbon fluxes are assessed
using different observations, and we discuss
the meaning of these measurements in terms of the
land surface model output and how they can be most
usefully compared. The simulation of carbon fluxes
depends on every aspect of the models: The physical
state, the soil carbon stocks and most importantly,
the vegetation. Large errors result from the models
growing the wrong type of vegetation or no vegetation
at all, as tundra vegetation types are not represented
in two of the three models. Focussed work is required
to better represent Arctic vegetation in such models,
and our results highlight the next steps to take.
The observational datasets are more detailed than
those used in past studies and this work will be used
both to facilitate and to justify the development of
Arctic carbon cycle processes in Earth system models.
EPIC Alfred Wegener Institut