DISCUSSION

We defined Late Miocene climate models 1) to test how much CO₂ is necessary to produce ice-free conditions on the Northern Hemisphere in the Miocene, 2) to analyse the Miocene climate sensitivity with respect to variations of CO₂, and 3) to validate the consistency of the model results with proxy data in order to estimate how high CO₂ might have been in the Late Miocene.

How much CO₂ is necessary to produce ice-free conditions on the Northern Hemisphere in the Miocene? The first question can now be easily answered: based on our model results, a pCO₂ of at least 1500 ppm is necessary to produce an ice-free Arctic Ocean. Even the most optimistic independent estimations of Miocene carbon dioxide concentrations give values of about 1000 ppm or less (e.g., Cerling 1991; Retallack 2001). Thus, our model results suggest that other processes than CO₂ are necessary to explain an ice-free Arctic Ocean in the Miocene. Recent evidence from the Miocene fossil record suggests some significant cooling events and supports the hypothesis that sea ice covered part of the Northern Hemisphere (e.g., Moran et al. 2006; Kamikuri et al. 2007; Jakobsson et al. 2007).

What is the Late Miocene climate sensitivity with respect to different concentrations of CO₂? Our present-day simulations, the global temperature increased by 2.5°C between TORT-360 and TORT-700. McGuffie et al. (1999) demonstrated that a doubling of CO₂ under present-day conditions leads to a warming of +2.5°C to +4.5°C. The most recent future climate change projections of the IPCC (Meehl et al. 2007), which consider a doubling of CO₂, give a similar temperature increase between +2°C to +4.5°C. Our present-day sensitivity experiments are at the lower end of that prediction, but still within the range of these climate change scenarios. Our Late Miocene simulations demonstrate a global temperature increase of +1.9°C when atmospheric CO₂ doubles (TORT-280 vs. TORT-560, TORT-360 vs. TORT-700). The Late Miocene climate sensitivity to changes in pCO₂ was weaker than today. Steppuhn et al. (2007) observed that a doubling of CO₂ under Miocene boundary conditions led to a global warming of +3°C. This is well within future climate change predictions, but stronger than we found in our simulations. The sea ice cover in Steppuhn et al.'s (2007) Tortonian reference run is close to the modern situation. In our simulations, even TORT-200 had a lower ice volume than CTRL-360 (Figure 4). Thus, the lower sea ice-albedo feedback dampens the Late Miocene climate sensitivity to CO₂ increase, although the Late Miocene is still comparable to the modern situation. The amount of sea ice is lower in our Miocene simulations than in previous Tortonian experiments (Micheels et al. 2007; Steppuhn et al. 2006, 2007) because we specified the modern ocean flux correction (Figure 3), whereas the previous studies considered a weaker-than-present ocean heat transport. The reduced sea ice cover in our Tortonian runs as compared to the present-day simulations is explained by the warming effect of Greenland (no glaciers and lower elevation, Figure 1) greater forest cover (Figure 2). These results are consistent to the previous studies.

Future climate change projections also demonstrated that continents are more affected by global warming than are oceans (e.g., Meehl et al. 2007). This is consistent to our simulations. However, other climate change predictions indicate a pronounced warming of higher latitudes (e.g., Meehl et al. 2007), which is consistent with the changes between TORT-360 and TORT-700. In contrast, our Late Miocene simulations do not show as much high-latitude warming (Figure 4). The reduced ice-albedo feedback in the Tortonian runs as compared to the present-day situation reduces the high-latitude response to CO₂. In the lower latitudes, future climate change projections and our Miocene runs are quite comparable. In contrast to Steppuhn et al. (2007), the different boundary conditions between Miocene and modern make a difference in the climate response to enhanced CO₂-scenarios. In general, our Miocene experiments demonstrate a weaker sensitivity to higher CO₂ than future climate change scenarios because sea ice is already reduced in the Miocene reference run.

How consistent are the different Late Miocene CO₂-scenarios as compared to the fossil record and can we estimate how high CO₂ was in the Late Miocene? Our model fits with proxy fossil data quite well when pCO₂ is between 360 to 560 ppm (Figure 8). TORT-200 to TORT-460 produced a global temperature that was less than ±1°C different from the proxy data. Micheels et al. (2007) found a global discrepancy to proxy data of –2.4°C in a Tortonian simulation with the AGCM ECHAM4/ML using almost the same boundary conditions and proxy data base. If discrepancy of this magnitude is acceptable, then even TORT-700 is a realistic scenario for the Late Miocene (Table 2).

There are, however, some crucial points which limit our interpretations. Most Late Miocene fossil proxy data cover the European realm, whereas most other key regions such as the high latitudes, Africa and Asia are poorly covered. Our sensitivity runs demonstrate that lower to mid-latitudes are warmer than suggested by proxy data, but this statement is based on only a few localities. With regard to the quality of proxy data, there are some shortcomings that may influence detailed data comparisons. This includes the time span covered by the proxy data, which may hide temporal climatic variability, palaeogeographical changes, which may change the exact position of data points with respect to the grid cell (e.g. in the Pannonian realm, cf. Erdei et al. 2007), and possible taphonomic biases, which may influence the results of different reconstruction methods (e.g., Liang et al. 2003; Uhl et al. 2003, 2006). Despite some possible shortcomings, quantitative climate proxy data based on plant fossils are consistent on a larger scale (Bruch et al. 2004, 2006, 2007), and they provide reliable information to validate global climate model results (Micheels et al. 2007; Steppuhn et al. 2007).

For our purposes, we consider the proxy data to be more reliable than our model results. The model experiments can be unrealistic because of weak points in the model and its boundary conditions. The EMIC Planet Simulator with its simplified parameterisation schemes and the boundary conditions explain rather more inconsistencies in our experiments. For some localities (e.g., in Africa, North America), the orography in the model might be unrealistic because of the coarse model resolution, the spectral transformation method or simply because the reconstruction of the orography is not fully correct. The coarse model resolution also does not allow for representation of details of the land-sea distribution. For example, the model cannot resolve Iceland (Figure 8), which may explain why the changes between TORT-200 and TORT-630 indicate conditions in the North Atlantic region that are too cool. In addition, our settings for the ocean include some uncertainties. As mentioned in the model description, the sea ice model tends to overestimate the amount of ice and the performance of the ice model for annual ice is better than for perennial ice. However, with increasing CO₂ concentrations in our runs, the amount of perennial ice decreases, and the model performance should improve. The absence of ocean and sea-ice dynamics is a potential source of errors in our study, but it is computationally expensive to run fully-coupled atmosphere-ocean general circulation models in particular for a series of sensitivity experiments.

For this reason, AGCM experiments for the Miocene used either prescribed sea surface temperatures and sea ice (e.g., Ramstein et al. 1997; Lunt et al. 2008) or slab ocean models (e.g., Dutton and Barron 1997; Micheels et al. 2007). For past climates, ocean dynamics plays an important role (e.g., Bice et al. 2000). Due to an open Panama Isthmus, the northward heat transport in the Atlantic Ocean was weaker than today in the Miocene (e.g., Bice et al. 2000; Steppuhn et al. 2006). Because of using the present-day flux correction, the climate in Europe should be too warm in our sensitivity experiments. If we consider that temperatures in Europe might need to be corrected to slightly cooler conditions, the scenarios TORT-460 to TORT-630 would be more realistic. Taking all weak points into account, the scenarios TORT-360 and TORT460 fit best with the fossil record. Hence with some limitations, the simulations support a slightly higher-than-present (intermediate) pCO₂ between 360 ppm and 460 ppm in the Late Miocene. Consistent with this result, MacFadden (2005) proposed a pCO₂ of 500 ppm in the Miocene, and Haywood and Valdes (2004, 2006) proposed a pCO₂ of 400 ppm for the Middle Pliocene.