Over a period of time, Earth’s outer shell (atmosphere, hydrosphere, cryosphere, biosphere) winds along a unique trajectory toward an ever-changing, and hence elusive, radiative equilibrium. It is elusive partly because Earth’s overwhelming external boundary condition, solar irradiance, is never constant thanks to continuous variations in both orbit about the Sun and solar output. As such, the best Earth, and any other planet, can do is achieve a sequence of states that are in quasi- (radiative) equilibrium over a period of time that spans at least several annual cycles. Even if boundary conditions were static, it is now recognized that Earth’s climate would not settle down to a single state or even a fixed cycle. Instead, it would execute a non-repeating sequence of, potentially very diverse, states that approximate radiative equilibrium. This chaotic character is supported by the inexorable intertwining of internal processes that operate at radically different time-scales. Indeed, the life giving/supporting character of Earth’s climate system, that begins with absorption of solar radiation and ends with infrared emission to space, owes much of its richness, and worthiness of study, to the four-dimensional interaction between radiation and the three phases of water.