This paper gives a detailed analysis of the relative performance between the Earth Simulator and systems built using Alpha processors. The achieved performance results from an interplay of system characteristics, application requirements and scalability behavior. Detailed performance models are used here to predict the performance of two codes representative of ASCI computations, namely SAGE and Sweep3D. The performance models do not require access to a full sized system but rather rely on characteristics of the system as well as knowledge of the achieved single-processor performance. One result of this analysis is in the determination of an equivalent-sized Alpha-based machine that would be required to obtain the same performance as the Earth Simulator.