We would like to thank Prof. Dr. L. Bengtsson and his group, in particular Dr. E. Roeckner and Mr. U. Schlese for developing and providing the ECHAM-4 model, Dr. E. Maier-Reimer for helping in developing HOPE-2, and Dr. Klaus Arpe for many fruitful discussions. This work was supported by the Bundesminister für Forschung und Technologie under Grant 07VKO1/1 and the European Union under Grant EV5V-CT94-0507.
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