I. INTRODUCTION Models of climate change, a subject that concerns time scales from decades to billions of years, range from the simple and qualitative to atmospheric and oceanic general circu- lation models running on the fastest computers. Despite several decades of effort by many scientists, the causes of such important phenomena as the alternation of glaciation and in- terglacials, and the ice ages themselves, remain controversial. In this pedagogical paper I describe an elementary and qualitative model of the consequences of one important effect, the temperature dependence of the opacity of atmospheric water vapor, the most important greenhouse gas $^{1}$. This is a consequence of the temperature dependence of the water vapor pressure and the physics of pressure broadening of saturated Lorentzian spectral line profiles, and lies on the border between physics and geophysics. The paleoclimatic data are summarized in Figure 1. Alternation of ice ages and inter- glacials is evident. Intermediate states, such as the present climate, are not stable or steady, but show continual variation. This is also evident in the historical record of the Late Me- dieval Climatic Maximum ( c. 800–1300), followed by the Little Ice Age ( c. 1300–1800), followed in turn by a warming trend. II. PALEOCLIMATIC DATA III. WATER VAPOR OPACITY Although water vapor is the greatest source of atmospheric opacity in the mid-infrared (8– 14 µ ) band in which the Earth’s thermal emission peaks, it is not even mentioned in a classic monograph 6 on climatological dynamics. The HITRAN database 7 is the comprehensive source of spectral information. Measurements of atmospheric opacity, such as those shown in Fig. 3 of Hiriart and Salas $^{8}$, illustrate its sensitivity to even small amounts of water vapor. Tropospheric water vapor is not usually considered in the context of anthropogenic green- house gas (GHG) emissions because it equilibrates with surface water very rapidly, with a characteristic time of a few days or less, and any anthropogenic contribution has no long term effect. Tropospheric water vapor opacity is controlled by evaporation and precipitation to a level determined by the surface temperature. 2