to such a state even without anthropogenic interventions such as the emission of GHG. A second implication is that if some climate state is determined to be optimal for humanity, maintaining it would require continual active intervention (geoengineering). I thank R. W. Cohen, M. Gregg, W. Happer, R. Muller, W. Munk and C. Wunsch for useful discussions. Acknowledgments ∗ katz@wuphys.wustl.edu 1 J. T. Kiehl and K. E. Trenberth, Bull. Am. Meteorol. Soc. 78 , 197 (1997). 2 http://en.wikipedia.org/wiki/File:Ice_Age_Temperature.png (Accessed Dec. 30, 2009). 3 EPICA, Nature 429 , 623 (2004). 4 J. R. Petit, J. Jouzel, D. Raynaud, N. I. Barkov, I. Basile, M. Bender, J. Chappellaz, M. Davis, G. Delaygue, M. Delmotte, et al., Nature 399 , 429 (1999). 5 L. E. Lisiecki and M. E. Raymo, Paleooceanography 20 , PA1003 (2005). 6 B. Saltzman, Dynamical Paleoclimatology (Academic Press, San Diego, 2002). 7 L. S. Rothman, I. E. Gordon, and A. Barbe, J. Quant. Spect. Rad. Trans. 110 , 533 (2009). 8 D. Hiriart and L. Salas, Rev. Mex. Astron. Ap. 43 , 225 (2007). 9 R. M. Goody, Atmospheric Radiation I. Theoretical Basis (Oxford U. Press, Oxford, 1964). 10 F. Reif, Fundamentals of Statistical and Thermal Physics (McGraw-Hill, New York, 1965). 11 B. D. Dushaw, P. F. Worcester, W. H. Munk, R. C. Spindel, J. A. Mercer, B. M. Howe, K. Metzger, T. G. Birdsall, R. K. Andrew, M. A. Dzieciuch, et al., J. Geophys. Res., Oceans 114 , C07021 (2009). 7