SUMMARY AND CONCLUSIONS

We performed CO₂-sensitivity experiments for the Late Miocene using the earth system model of intermediate complexity Planet Simulator. We modelled CO₂ concentrations ranging from 200 to 2000 ppm, with all other boundary conditions unchanged. We found that:

• 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).

• The climate sensitivity to enhanced concentrations of greenhouse gases is reduced in the Late Miocene compared to the modern world. The Late Miocene represents a hothouse climate with a reduced ice cover, which dampens the ice-albedo feedback. However, the climate response to 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.

• The Late Miocene simulations with a pCO₂ from 280 to 630 ppm agree reasonably well with quantitative terrestrial fossil proxy data. At higher CO₂ levels, the consistency with proxy data becomes progressively worse. Based on our results, an intermediate concentration of CO₂ between 360 and 460 ppm is realistic for the Late Miocene.

The results of our modelling experiment should not be over-interpreted because of uncertainties in the model (e.g., simplified physical parameterisations) and certain features of the model configuration (e.g., the ocean setup). The agreement between our 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.