Ž . Global and Planetary Change 20 1999 125–156 Postglacial sea level: energy method Ming Fang ) , Bradford H. Hager Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 USA Received 20 December 1996; received in revised form 15 March 1998 Abstract The equation governing changes in sea level caused by the redistribution of ice and water masses on the earth’s surface is rederived based on the least potential energy principle. This energy method deepens our understanding of the coupled ice–sea–Earth system, and lays the foundation of a global solution used in this study. A phenomenological parameterization of ‘realistic’ viscosity models is proposed based on microphysical considerations. Continuously varying viscosity structures Ž . are determined using a few characteristic viscosity values parameters at seismically identified boundaries. By using a set of admissible ‘realistic’ viscosity models and two models of deglaciation histories, we find that satisfactory convergence can be Ž . reached for a global solution for postglacial relative sea level RSL at about harmonic degree 50, and the convergence appears independent of ice model. This relatively lower tolerable truncation level is a consequence of global nature of the ice–sea–Earth system. We further examine the sensitivity of postglacial sea level to ‘realistic’ viscosity structure and the lithospheric thickness combined. We find that variation of lithospheric thickness does not alter our previous conclusion ŽFang, M., Hager, B.H., 1996. The sensitivity of post-glacial sea level to viscosity structure and ice-load history for . realistically parameterized viscosity profiles. Geophys. Res. Lett. 23, 3787–3790 that there is a correlation of RSL sensitivities between ice history and viscosity structures, i.e., at sites less sensitive to the ice model, the resolving power for viscosity structure is also less. Furthermore, models having a thicker lithosphere tend to permit better resolution of lower mantle viscosity. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Sea level; Redistribution; Ice–sea–Earth system 1. Introduction The sea level change associated with glacial isostatic adjustment is one of the classical problems in geophysics. A unique feature of the problem is that this is a coupled system—ice sheets, ocean, and the solid earth. The melted ice mass spills into the sea, and rearranges the mass distribution on the surface of the solid earth. The solid earth in turn, deforms in response to the redistribution of its surface loads, and causes the sea level to adjust itself to reach an equilibrium with the remaining ice sheets and the deformed solid earth. The melting of ice sheets has its astronomical causes and strong climatic consequences, while the deformation of the solid earth depends crucially upon creep processes in the interior. Thus, the dynamics of the coupled system ) Corresponding author. Tel.: q1-617-253-3077; Fax: q1-617-253-1699; E-mail: fang@chandler.mit.edu 0921-8181r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0921-8181 98 00067-8