The manifold requirements for a world to sustain habitability on long timescales (continuous habitability) are delineated in this chapter. The first part offers a brief introduction to climate physics (e.g., greenhouse effect), and thereupon formulates the notion of the habitable zone, that is, the region where liquid water could exist on rocky planets orbiting stars; the boundaries of the habitable zone as a function of the stellar temperature are also presented. In the second part, the various stellar factors potentially involved in regulating planetary habitability are sketched: winds, flares and space weather, and electromagnetic radiation. The third part chronicles some planetary variables that may affect habitability: mass, plate tectonics, magnetic field, tidal locking, and atmospheric composition. The last part is devoted to examining the high-energy astrophysical processes that might impact habitability on galactic scales: candidates in this regard include supernovae, gamma-ray bursts, and active supermassive black holes.

We turn now to the third of the three branches of science that underpin our changing climate: radiant energy – the energy carried by light and other electromagnetic waves – and the ways it interacts with the gases and particles in our atmosphere. In this chapter we will outline the necessary science, and especially the laws of thermal emission. It turns out that we are bathed in electromagnetic radiation that we can’t see. We will then show how these laws lead to the counter-intuitive outcome that the ‘temperature’ of our planet is actually well below freezing! Fortunately, however, our Earth has a ‘good’ atmosphere, containing some very important gases – greenhouse gases – which act as a blanket, keeping the surface at a suitable temperature for liquid water, and hence for life. A simple model will help you gain a basic understanding of the greenhouse effect.

There is a distinct poleward zonation of climate defined by gradations of progressively colder annual mean temperature in tropical, subtropical, temperate, boreal, arctic, and polar latitudes. Additional climate zones are defined based on annual precipitation and the seasonality of temperature and rain. The climate at large spatial scales extending over thousands of kilometers is known as the macroclimate. It is determined by geographic variation in solar heating of the planet, which sets in motion large-scale atmospheric circulations that transport heat poleward from the tropics, and also by proximity to oceans, which similarly transport heat in ocean currents. Mountains and large lakes create a regional climate that can deviate from the macroclimate. Climate at this scale, generally up to a several hundred kilometers , is referred to as mesoclimate. Variation in topography, soils, and vegetation creates local climates at a spatial scale ranging from a few to tens of kilometers, known as microclimates. A south-facing slope has a different microclimate than a north-facing slope. Forests have a different microclimate compared with open land.

Planting trees to increase rain was the grand climate controversy of the nineteenth century. Some European scientists with diverse backgrounds in physics, meteorology, forests, and soils developed a new science of forest meteorology that blended meteorology, forest ecology, and forest hydrology. They sought answers in direct measurement of forest influences on climate and installed meteorological observatories in forested and open lands to obtain the necessary data. They explained forest influences in the laws of physics, fused with interdisciplinary knowledge of meteorology, forest hydrology, and forest ecology, and gathered the data to further their theories. It was an understanding based on observations of microclimates, but upon which was layered a dynamical framework applicable to macroclimates. Many of the findings have withstood the test of time, and the questions posed are still relevant to today’s scientists.

Chapter 1 offers a historical examination of the causes and discovery of global heating and the development of the scientific consensus that it is human-caused and can be curtailed only by cutting greenhouse gas emissions and describes how those developments nonetheless failed to lead to extensive action.

The Earth surface carbon cycle consists of regular flows between the major sinks: atmosphere, ocean, soils and biota. It is in balance, except for flows to and from long-term storage (methane formations, deep ocean, lithosphere), which comprise the deep Earth carbon cycle. At the beginning of life in earth history, about 500 million years ago, the atmosphere was carbon-rich, and Earth temperatures were vastly hotter than today. Over time most of this carbon was withdrawn from the surface carbon cycle through ocean sedimentation, reactions with weathering rock and migration of organic matter to the lithosphere, where they were transformed into fossil fuels. This eventually produced the temperate Earth, which facilitated human evolution, but now, by extracting and burning fossil energy, humans are recreating conditions for the hotter Earth of earlier times. These direct carbon emissions can be amplified by feedback mechanisms, whereby initial warming releases carbon stored in formerly stable methane deposits and alters the carbon cycle so as to increase the size of the atmospheric sink relative to other sinks or changes in the Earth’s albedo. The politics of controlling emissions is further complicated by lags in warming and subsequent climate impacts.

The greenhouse effect is a fundamental property of the Earth’s climate that is responsible for keeping the planet warm and habitable for life. The concept of the Goldilocks zone describing the habitability of planets is briefly introduced. Basic concepts in atmospheric physics, such as solar and infrared radiation, are introduced using concepts from everyday life, including an analogy with blankets to explain the greenhouse effect. The two main gases responsible for the greenhouse effect – water vapor and carbon dioxide – are described. The historical background of the discovery of the greenhouse effect, including the very first climate model that predicted global warming (formulated by Svante Arrhenius in the 1890s), as well as the recently uncovered role of Eunice Foote, is presented. Climate feedbacks, which can amplify the global warming, are explained.

The Geophysical Fluid Dynamics Laboratory (GFDL) is a pioneering institution in the field of climate modeling. Its founding director, Joseph Smagorinsky, was a member of the Princeton Meteorology Group. He hired a Japanese scientist, Syukuro Manabe, who formulated a one-dimensional model of climate, known as the radiative–convective model, that was able to calculate the amplifying climate feedback due to water vapor. This model provided one of the first reliable estimates of global warming. Manabe worked with other scientists to build three-dimensional climate models, including the first model that coupled an atmospheric model to an ocean model. The concepts of reductionism and emergentism, which provide the philosophical context for these scientific developments, are introduced.