INTRODUCTION

The Miocene (~ 23 to 5 Ma) belongs to the late phase of Cenozoic cooling, but climate is known to be still a general hothouse situation. Various proxy data suggest a warmer and more humid climate than today (e.g., Wolfe et al. 1994a,b; Zachos et al. 2001; Bruch et al. 2004, 2006, 2007; Mosbrugger et al. 2005). In particular, the Miocene equator-to-pole temperature was weak implying that primarily higher latitudes were warmer than they are today (e.g., Wolfe 1994a,b; Bruch et al. 2004, 2006, 2007). Corresponding to warm polar regions during the Miocene, it was commonly assumed that the large-scale glaciation of the Northern Hemisphere started in the latest Miocene or in the Pliocene (e.g., Kleiven et al. 2002; Winkler et al. 2002; Bartoli et al. 2005; St. John and Krissek 2007 [AUTHOR: ref. says 2002 not 2007]). However, some studies have raised doubts whether significant Arctic sea ice [AUTHOR: incomplete sentence / need singular verb of some sort? "had formed?"] (e.g., Helland and Holmes 1997; Moran et al. 2006). It remains an open debate of when sea ice appeared first on the Northern Hemisphere, although most recent evidences seem to support that the Arctic Ocean was already ice-covered in the Miocene (e.g., Moran et al. 2006). If so, this [EDITOR: this what?] could mean that polar regions in the Miocene may have been cooler than previously assumed.

The increase of carbon dioxide in the atmosphere is responsible for the future climate change (e.g., Cubasch et al. 2001; Meehl et al. 2007). Thus, somehow related to the question of how warm the Miocene was is the question of how high the amount of atmospheric CO₂ had been. Some studies suggest that the Miocene CO₂ level was close to the pre-industrial concentration (280 ppm) or a little higher (e.g., Pearson and Palmer 2000; Pagani et al. 2005). However, other evidences support a pCO2 being as high as 500 ppm (e.g., MacFadden 2005) and less than 700 ppm (e.g., Cerling 1991). Retallack (2001) even proposed that atmospheric carbon dioxide was higher than 1000 ppm until the Late Miocene. It appears to be enigmatic to understand warm high latitudes, if CO₂ was low in the Miocene, but it also seems to be strange that polar regions were ice-covered, if CO₂ was rather high.

So far, climate model experiments for the late Tertiary primarily concentrated on the role of geography and orography (e.g., Ramstein et al. 1997; Ruddiman et al. 1997; Kutzbach and Behling 2004) or on the role of the ocean (e.g., Bice et al. 2000; Steppuhn et al. 2006). In order to adapt their models to Miocene conditions, modellers are used to specify atmospheric CO₂. Owing to the fact that the Miocene CO₂ level is still debated (cf. above), model experiments for Miocene time intervals, hence, cover quite a large range (e.g., Kutzbach and Behling 2004; Steppuhn et al. 2006, 2007). Recently, Steppuhn et al. (2007) presented a sensitivity experiment for the Late Miocene, which analysed the effects of 2×CO₂ (700 ppm) as compared to 1×CO₂ (353 ppm). Even with 700 ppm, the Late Miocene model experiments demonstrated that the Arctic Ocean is still ice-covered (Steppuhn et al. 2007). These results might conflict with the fossil record (cf. above). Moreover, the heating of high latitudes occurred at the expense of warming lower latitudes, which tends to be in conflict with proxy data (Steppuhn et al. 2007). Steppuhn et al. (2007) conclude that high CO₂ cannot help to explain warm high latitudes, but maybe palaeovegetation can. In fact, the Miocene vegetation contributed to warm high latitudes (Dutton and Barron 1997; Micheels et al. 2007), but climate modellers still run into trouble when trying to understand warm high latitudes in the Miocene (Steppuhn et al. 2006, 2007; Micheels et al. 2007). A low atmospheric pCO₂ is not sufficient to explain the warm Miocene climate and a high concentration does not seem to help (Steppuhn et al. 2007; Micheels et al. 2007). But what if CO₂ was 'intermediate'?

Geological processes such as the mountain uplift from the Miocene until today and their influence on climate has repeatedly attracted the interest for model experiments (e.g., Ramstein et al. 1997). In contrast, sensitivity experiments with respect to CO₂ are rare (Steppuhn et al. 2007; Tong et al. 2009). Information about carbon dioxide in the Miocene is not known for sure (e.g. Retallack 2001; Pagani et al. 2005), but realistic model experiments strongly require a properly specified CO₂ level. Climate models are tools to test hypotheses and to understand specific processes. Carbon dioxide is a general key factor for climate (e.g., Meehl et al. 2007), but in terms of Tertiary climate modelling the relevance of carbon dioxide is poorly understood. This situation is not satisfying especially because the Miocene can serve as a possible analogue for the future climate change (e.g., Kutzbach and Behling 2004). Three basic points still remain open for discussions:

How high must atmospheric CO₂ be so that the Arctic Ocean is ice-free in the Miocene?
What is generally the climate sensitivity with respect to different enhanced pCO₂ in the Miocene? Is the Late Miocene comparable to future climate change scenarios?
How consistent are different Late Miocene CO₂ scenarios as compared to proxy data? Hence: Can we estimate a Late Miocene pCO₂ from comparing model and proxy data?

Addressing these questions, we perform some sensitivity experiments for the Miocene using the earth system model of intermediate complexity Planet Simulator. The reference of our sensitivity experiments is the Late Miocene. Based on the reference run, we present some sensitivity experiments, which consider a different pCO₂ from low (200 ppm) to high values (700 ppm). In addition, we perform a transitional experiment with a steady increase of CO₂ by +1 ppm starting with 200 ppm and ending up with 2000 ppm. Finally, we use quantitative terrestrial proxy data to validate the model results. The definition of our sensitivity experiments contributes to a better understanding of the role of CO₂ for the Cenozoic climate history.