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corresponds to the 2°C criteria, referred to in discussions on ‘stabi-
lization of greenhouse gas concentrations in the atmosphere at a
level that would prevent dangerous anthropogenic interference with
the climate system’ (United Nations Framework Convention on
Climate Change). Anthropogenic CO
2
emission pathways required
to achieve the concentrations calculated by MIROC-ESM must also
be calculated.
What follows from this is that, based on MIROC-ESM results,
to stay below the 2°C criteria, it is necessary to reduce emission to
nearly zero by the 2040s and further continue the reduction so that
emission is negative by the 2070s, going into an artificial sequestra-
tion phase of atmospheric CO
2
. It has to be pointed out that fulfilling
such a requirement is an extremely difficult task, though it cannot
be ruled out that such an artificial sequestration of atmospheric CO
2
could be achieved in future thanks to technological advancements.
Carbon accumulation on land from 2000, simulated by MIROC-
ESM, demonstrates that RCP3PD and RCP8.5 have the lowest
carbon storage. This is rather a counter-intuitive result because the
scenario with the lowest carbon emission (RCP3PD) and the highest
(RCP8.5) fall into the same category of low terrestrial accumula-
tions, while the scenarios with medium emission are associated with
high accumulations. Analysis has shown that the key for under-
standing this apparent paradox is the LUC scenarios. That is, in
RCP3PD and RCP8.5, agricultural land use is projected to expand,
preventing forest growth and its large carbon sink, because biomass
energy is promoted in the former scenario and economic growth has
a higher priority in the latter. Due to this suppressed forest growth,
carbon accumulation on land is also suppressed.
This finding is ironic in that biomass energy produc-
tion for reducing CO
2
emission could act to shrink the
terrestrial carbon sink. It is at the same time intriguing
that human activities such as LUC, which can be control-
led to some extent by policies, could have a significant
impact on the global carbon cycle. Future decision-
making on energy and climate mitigation policies should
pay attention to such possibilities, although it has to be
borne in mind that the results here are obtained by a
single particular model and further examinations with
multi-model ensembles are desirable.
Vegetation shift
A visualization of changes in vegetation distribution in
the experiment under RCP4.5, where the model inte-
gration is extended to the year 2300, puts the focus on
boreal forest, where the changes are most drastic. In
this simulation, the boreal evergreen forest expands
northward into northern Siberia, which was origi-
nally covered by tundra, whereas boreal deciduous
forest almost disappears from eastern Siberia, perhaps
because the survival strategy of deciduous trees no
longer functions in the warmer climate. If boreal
evergreen forests expand as is projected, this might
affect the physical surface conditions of the forest. In
late autumn or early spring, when the surface land is
covered with snow, boreal evergreen forest will help
absorb more solar energy through the decrease of
surface albedo.
Again, care should be taken because what is shown
is a result from a single particular model and examina-
tion of the validity of the model processes critical for
the biome shift is by no means sufficient because of
the inherent difficulty of observing phenomena with
centennial or even longer timescales. It can be said,
however, that a possibility has been demonstrated
by a vegetation model that the Earth has a consider-
ably different distribution of forests after a couple of
hundreds years ahead. It should also be pointed out that
decisions made by our generation can be crucial factors
for the ecosystem centuries later. When that is the case,
results from 300-year projections could be a matter for
consideration in establishing mitigation policies.
We have seen that ecosystem dynamics in future,
ranging from plant distribution to carbon cycling, can
be significantly modified, not only though emission
of greenhouse gases but also by more direct means,
such as deforestation. It has at times been pointed out
that the Earth’s history has entered a new geological
period called ‘anthropocene’, and that the ‘anthrop-
osphere’, consisting of human societies, should be
regarded as a sub-system of the entire Earth system
as well as other sub-systems such as the atmosphere,
ocean (or hydrosphere) and biosphere. It appears that
the topics presented here demonstrate the validity of
the concept and the fact that interactions between the
biosphere (including forests) and anthroposphere are
one of the most important agents for the future of the
global environment.
The grid system of MIROC-ESM
Source: JAMSTEC/MEXT
In Earth system models, Earth’s atmosphere and oceans are divided into grids in
which variables such as temperature and humidity are calculated. The horizontal
grid size illustrated roughly corresponds to the actual one adopted in MIROC-ES