A quantitative calculation of f$_{b}$ is not possible within this qualitative model, but spectral data 8 suggest values of several tenths. Then deviations from equilibrium grow exponentially for 0 . 3 < f$_{b}$ ≲ 0 . 7 . (12) ν 0 = 1 . 04 × 10 − $^{2}$/ y . (13) The mean ocean depth, averaged over the entire Earth (including zero depth for land areas) is about 2.65 km. However, the flow of heat to the deep ocean, while apparently reflecting the warming of the last century $^{11}$, is expected to be confined to downwelling regions and to be comparatively slow. For the purpose of an order of magnitude estimate we suppose a nominal mean depth of 1 km contributes to C$_{p}$ and find VII. CONCLUSION If 0 . 3 < f$_{b}$ ≲ 0 . 7, as suggested by the empirical water vapor spectrum $^{8}$, then instabil- ity would be expected for any state between an ice age (when p$_{v}$ is low enough that the assumption τ 0 ≫ 1 is not satisfied) and the warmest interglacials (when the assumption 1 − f$_{b}$ ( T 0 ) > τ − 1 / 2 0 is not satisfied as nearly the entire thermal infrared spectrum is blan- keted with water lines and f$_{b}$ → 1, or energy is carried by convection, ignored in this na¨ıve model). Hence the climate would be stable only in these two limiting states. The paleoclimatic data, as shown in Figure 1, are qualitatively consistent with this model. The historic record of the Medieval Climatic Maximum, the Little Ice Age, and fluctuations on similar time scales of O (100) years at other epochs, is also qualitatively consistent with the estimated ν 0 . VIII. DISCUSSION Climatologists have struggled with the question of what upsets the stable ice age and interglacial states since the discovery of the ice ages, and this na¨ıve model cannot contribute to its answer. Even the most complete and sophisticated general circulation models struggle with this problem, and its resolution remains controversial. However, even the conclusion that intermediate states may be intrinsically unstable has significant implications. One implication is that there is no “normal” climate, nor can the Earth be expected to return 6