Strengths and challenges for transient Mid- to Late Holocene simulations with dynamical vegetation
2019
Abstract. We present the first simulation of the last 6000 years
with a version of the IPSL Earth system model that includes interactive
dynamical vegetation and carbon cycle. It is discussed in the light of a set
of Mid-Holocene and preindustrial simulations performed to set up the model
version and to initialize the dynamical vegetation. These sensitivity
experiments remind us that model quality or realism is not only a function
of model parameterizations and tunings but also of experimental setup. The
transient simulations shows that the long-term trends in temperature and
precipitation have a similar shape to the insolation forcing, except at the
Equator, at high latitudes, and south of 40 ∘ S. In these regions
cloud cover, sea ice, snow, or ocean heat content feedbacks lead to smaller
or opposite temperature responses. The long-term trend in tree line in the Northern Hemisphere is reproduced and starts earlier than the southward
shift in vegetation over the Sahel. Despite little change in forest cover over
Eurasia, a long-term change in forest composition is simulated, including
large centennial variability. The rapid increase in atmospheric CO 2 in
the last centuries of the simulation enhances tree growth and
counteracts the long-term trends induced by Holocene insolation in the
Northern Hemisphere and amplifies it in the Southern Hemisphere. We also
highlight some limits in the evaluation of such a simulation resulting from
model climate–vegetation biases, the difficulty of fully assessing the result
for preindustrial or modern conditions that are affected by land use, and
the possibility of multi-vegetation states under modern conditions.
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