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SUMMARY AND CONCLUSIONS
With the earth system model of intermediate complexity Planet
Simulator, we perform CO2-sensitivity experiments for the Late
Miocene. Therein, CO2 ranges from 200 to 2000 ppm, whereas all
other boundary conditions remain unchanged. We summarise our modelling results
in the following:
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An atmospheric carbon dioxide of 1500 ppm is necessary to
produce an ice-free Northern Hemisphere in our model. This value is much too
high to be reasonable for the Miocene (e.g.,
Cerling 1991;
Retallack 2001). Our
sensitivity experiments support evidences for an onset of the Northern
Hemisphere's glaciation before the Miocene (e.g.,
Moran et al. 2006;
Kamikuri et al. 2007;
Jakobsson et al. 2007).
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The climate sensitivity on enhanced concentrations of
greenhouse gases is reduced in the Late Miocene as compared to the modern
situation. The Late Miocene represents a hothouse climate with a reduced ice
cover, which dampens the ice-albedo feedback. However, the climate response
on increases in CO2 is only slightly weaker than in future
climate change scenarios (e.g.,
Meehl et al. 2007). With some limitations, the Late Miocene can serve as
an analogue for the future situation.
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The Late Miocene simulations with a pCO2
from 280 to 630 ppm agree reasonably well with quantitative terrestrial
proxy data. If CO2 is higher, the consistency to proxy data is
successively getting worse. Based on our results, an 'intermediate'
concentration of CO2 between 360 and 460 ppm is realistic for the
Late Miocene.
We present some sensitivity experiments with an earth system
model of intermediate complexity. The results should not be over-estimated
because of uncertainties in the model (e.g., simplified physical
parameterisations) and its setup (e.g., the ocean), but they are quite
consistent to other studies. The comparison of model results and proxy data
emphasises the need for proxy data from crucial regions such as the high
latitudes and Africa. More quantitative climate information from the fossil
record from these poorly covered regions would be helpful to better estimate
the reliability of climate model experiments. |
