Physics of Radiation and Climate

In Physics of Radiation and Climate, Michael and Gail Box state in their introductory chapter that the key questions related to climate change “… revolve around the potential changes to the radiation fields. Thus, radiation physics must take centre stage.” This is the premise for their book, and captured in the title: radiation first, then climate. The theme is repeated throughout the text, as the authors never stray far from their target: to understand climate change in the context of perturbations in radiative balance that are driven by changes in atmospheric composition. After several chapters on background material—the appetizer—in atmospheric physics and chemistry, Box and Box remind the reader at the start of Chapter 8, Radiative Interactions: here comes the main course. So it is on this basis that I reviewed the book and I'll provide my summary at the start: Box and Box deliver what they promise.

Initially, I had some misgivings. My first reaction, even before opening the book, was to ask, “yet another book on atmospheric radiation?” Then after scanning the contents, my second reaction: “probably a mile wide but an inch deep.” Happily, the authors alleviated the worst of my fears.

It is true that the last few decades have been witness to a flood of texts on atmospheric radiation. This arose after the field had largely relied on Chandrasekhar¹ for the theoretical foundation of radiation transfer and Goody² for many of the other elements of the broader field of atmospheric radiation, for example, molecular spectroscopy and particle scattering, among many others. After these two works it might be asked, what else is needed? Then, starting in the 1980s and continuing through the past decade, several books complete with problems and applications were developed for undergraduate and graduate students. The range of styles varies widely, from purely mathematical on one hand to a more physically intuitive approach on the other, each appealing to the diverse natures of scientists in the community (see Ref. 3 for a humorous take on the different languages used by Chandrasekhar and Minnaert to describe the principle of reciprocity to one another), but all drawing on the same fundamentals of physics and mathematics that govern the flow of electromagnetic radiation through the atmosphere and, perhaps, the ocean. Was there a need for another book? The Boxes convinced me, yes.

The Boxes have written a text focused on the physics of atmospheric radiation in the broader context of climate. For students interested in weather, a required course in atmospheric radiation may seem like an unnecessary burden. For climate specialists, it is unavoidable—Box and Box drive this point home. The temporal and spatial scales relevant to climate elevate radiation physics to “centre stage.” Their emphasis is justified. And although they do not ignore dynamical processes in their background treatment—Ch. 6, Circulation of the Atmosphere and Oceans, and Ch. 7, Fluid Dynamics—they argue, “… dynamics is a subject deserving a book of its own.”

The logic of their organization is apparent and the organization can be quite challenging for atmospheric radiation; no single ordering of topics will please everyone. A thorough treatment of the physics of emission, absorption, and scattering may delay the introduction of the equation of radiative transfer to the point where some may lose interest; for others it may be just the opposite. Box and Box manage to tie the microscopic to the macroscopic in a way that maintains a reader's interest. They are also helpful to tell readers when sections may be skipped without risk of missing the broader focus of a chapter or section.

Every chapter finishes with a synopsis that summarizes its contents, ties it to the central theme of climate, and portends how the material will be integrated into topics yet to come—Box and Box take the reader on a journey, a progression through interwoven topics that build on those that precede them. And regarding the range of styles exhibited in other texts, Physics of Radiation and Climate strikes a nice balance between physical insight and mathematical formalism (again, no single method will appeal to all).

The Boxes lay a solid foundation for the primary emphasis on radiation with chapters on atmospheric thermodynamics, chemistry, and general circulation—what they identify as background, roughly the first third of the book. These topics are at the level of a solid introductory text in atmospheric science, much in the manner of the widely read Wallace and Hobbs,⁴ and they serve the authors' purpose quite well.

Once convinced about the uniqueness of their approach, the adherence to the single theme of climate, I was left to tackle the depth of their coverage. The breadth of topics is astounding. Indeed, that alone separates Physics of Radiation and Climate from the seemingly similar books referenced above. In addition to what might be considered standard fare for a text on atmospheric radiation—absorption, emission, scattering, spectroscopy, polarization, Rayleigh's law, Lorenz-Mie theory, multiple scattering, the equation of transfer and methods of solution, distinction between solar and terrestrial radiation transfer, optical properties of molecules, aerosol particles, and clouds—Box and Box include chapters on remote sensing and inverse theory, absolutely essential for linking radiometric observations from space and from surface networks to climatically relevant geophysical variables. There are more detailed works from which the fundamentals of these (and other) topics are drawn, and numerous references are identified in the text, but there is much to be said for having the essentials covered in a single text. This coverage contains sufficient detail to be far more than a survey text.

The third and final part of the book, the dessert, provides the links to climate and climate change. It includes the afore-mentioned material on remote sensing, a bridge, so to speak, to changes in atmospheric composition, perturbations to radiative balance, and climate modeling. Box and Box pay particular attention to the Intergovernmental Panel on Climate Change (IPCC) reports in this part but they introduce the role of the IPCC right at the start, in the Ch. 1 introduction. I find this quite helpful for several reasons. Snippets from IPCC reports are broadly disseminated in the popular media without providing the necessary background on what the IPCC really is and how it functions. Moreover, by providing context from the IPCC, Box and Box are assured of being as relevant and up to date (not entirely, as you will see below) as possible with the latest reviews of climate science—an immeasurably difficult task without the resource of the IPCC reports.

The book is not without flaws. The authors state that some of the material is from lectures given over the past quarter century. Perhaps some—not much—of that is dated, better suited for the historical section of a museum. One example is the discussion on band models in a chapter on thermal radiation transfer. In most of the widely used and broadly distributed radiative transfer codes in favor today these have been replaced by the k-distribution method (also discussed in this chapter), itself a type of band model but one that achieves exceedingly high accuracy, close to that of a line-by-line model because it orders absorption lines by their strengths rather than their frequencies in order to simplify numerical quadrature. Another example: the Optical Properties of Aerosols and Clouds database is outdated, at least for ice cloud particles, ignoring the more recent work of Yang et al.⁵ 

There are also some inconsistencies. Three different values for the Sun's irradiance at 1 AU are used at various places in the text. The authors identify the most accurate value—why not use that? The energy budget diagram in the introductory chapter uses a less accurate, outdated value for solar irradiance, but the entire diagram has been replaced by updated versions, ones that even include uncertainties (for example, see Ref. 3). While on this topic, I must add that the term solar constant should be stricken from any text on radiation and climate. Box and Box say that it varies only a little; relatively, yes, but in absolute flux density it is a substantial quantity. In a text that includes a figure showing three decades of measured solar irradiance from space, another figure on the sunspot record spanning two and a half centuries, and a section on solar forcing of climate, the term solar constant is best avoided. Thankfully, I could not find it in the index. One final comment on solar radiation: the spectrum of solar irradiance shown in Ch. 12 dates to 1954; there are far more accurate measurements from the last two decades and it has been measured continuously from space since 2003.

It is admirable that the authors have provided a text that is affordable, perhaps even for students. Presumably, avoiding color helped in that regard. Unfortunately, one consequence is that some of the figures are nearly indecipherable, especially those that are copied from the IPCC reports. This is exacerbated by figure captions that read more like citations than descriptions of the figures. I suppose that one could have a copy of the latest IPCC report handy to gain understanding of these figures but that undermines the appeal of the text as a general resource for a broad range of topics on radiation and climate. Finally, the number of exercises does not do justice to the breadth of material covered; an instructor using this book may need to develop many more but would not need to go far (this very text) for inspiration.

Despite these few shortcomings, the Boxes have achieved their stated goal: they have managed to make radiation the focus of a climate-centric text directed to the senior undergraduate or entry-level graduate student. I see far more than a single course developed from the material. For the generalist, a student or researcher who will not be specializing in radiation and remote sensing, it might be the only book they need. For the specialist, it is a nice reference. Indeed, this is a book I will use, despite having many of its references already in my collection.