Because of its direct connection to sea level, glacier mass balance is a subject of much interest now, not only to glaciologists, but also to a wide variety of earth scientists and even the general public. Consequently, this comprehensive glossary of terms is timely and welcome to a wide range of readers and users.
This is not a glossary of glaciology—the only term related to sea ice, for example, is “sea ice”—but the authors have taken a pleasingly broad view of “Related Terms,” including terms such as brightness temperature, permittivity, and ice-core stratigraphy (but not ice core), and even organizations, like World Data Centres and the World Glacier Monitoring Service.
The authors, in their Introduction, give a lucid and apposite explanation of their scope and purpose: “The aim of this Glossary of Glacier Mass Balance and Related Terms is to update and revise what has long been the effective standard of mass-balance terminology (Anonymous 1969).” It “ … reflects changes in practice with conventional measurement tools, and also what is possible with the wide range of new tools ….” The GLOSSARY includes commentary on usage, particularly problematic usage, with recommendations where appropriate.
“The scope of the Glossary extends beyond the measurement of mass balance. There are articles covering such subjects as glacier zonation; the definition of glacier features and morphological types of glaciers; the administrative structures within which mass-balance data are archived once collected; and the modelling of mass balance …. Also included [are] some terms that are mainly of historical interest, and some technical terms from other disciplines that appear in reports of mass-balance measurements by newer methods.
“The purpose of the Glossary is … to promote clarity and reduce ambiguity in the communication of information about glacier mass balance, as well as to provide a range of ancillary material.” The authors “have tried to steer a middle course between being prescriptive, that is, laying down the law about how terms are to be used, and being descriptive, that is, simply recording the facts of current usage.” That purpose is well met by this Glossary.
Following the Introduction are 15 pages of explanatory material, beginning with a brief history of mass balance measurements going back to 1874. The next chapter is on terminology and is straightforward, although some users may not like the adoption of positive and negative signs for mass balance components according to changes in mass, so accumulation (c) is inherently positive and ablation (a) is inherently negative (i.e. an ablating surface has a negative ablation rate) and mass change is the sum of the two (c + a). The authors also describe and accept the alternative usage of positive signs indicating movement in a positive direction with respect to a coordinate axis, useful particularly for glacier flow. The main point is that reports of mass balance should state clearly which convention is used.
A crucial chapter comes next: Formulations of Mass Balance. Here the authors start with the basic equation for the mass balance rate of a column including all components of mass addition and loss, internal as well as external. To help prevent inaccurate usage of the term “surface mass balance” in the presence of non-zero basal and internal contributions, they introduce two new, closely related terms, “climatic mass balance,” for the sum of surface mass balance and internal mass balance, and “climatic-basal mass balance,” for the sum of climatic and basal balances. To get the total mass balance of the column the divergence of the flow vector must be added. The distinctions are important in light of the growing evidence for active internal and basal activity in and under glaciers and ice sheets; it will be interesting to see whether these new terms catch on with glaciologists.
Next there is an excellent discussion of the components of both the point mass balance and the glacier-wide mass balance, including a table of recommended symbols for them and figures depicting them in relation to a complete glacier and, separately, to an ice shelf. The chapter ends with short sections on alternative formulations and seasonal mass balance.
There follows a brief chapter that states what data should be included in mass balance reports and urges that those data be submitted to the World Glacier Monitoring Service for all glacier measurement projects.
Updating and revising the mass balance terminology of Anonymous (1969) was a central aim of this report; in Chapter 6 departures from the old standard are discussed. The authors find that the terminological distinction between equivalent quantities in the stratigraphic vis à vis fixed-date time systems has been so muddied over the years that it should be abandoned (although not the terms or the time systems themselves). The fact that geodetic measurements (repeated measurements of surface heights) do not fit into any established time system exacerbates the problem of intercomparison of data between systems. The authors do not offer any solutions in detail for this problem, but do emphasize the importance of reporting dates and methods of measurement accurately. They also diverge from Anonymous (1969) in giving primacy to mass units over volumetric or “water-equivalent” units, again not suggesting that the latter be discarded.
The units recommended for use are laid out in the next chapter starting with a look at the basic SI units (Système Internationale d'Unites) relevant to glaciology, and then taking up the case of the “year,” a non-SI unit, which they recommend be considered “a practical extension of the SI” for obvious historical and practical reasons.
The main body of the Glossary comprises 85 A4-sized pages of definitions, fuller explanations, comments, discussions, and cross-references. The entries range in length from one line to over a page—the Glossary is much more like a small encyclopedia than a dictionary, containing, as it does, a wealth of information. The entry for Zone, for example, covers two pages and includes easily legible diagrams of two temperate glaciers, one each with positive and negative mass balance, and one of a cold glacier, and a score of cross-references.
Perhaps that is to be expected for “zone.” More surprising is the entry for stake: a detailed description of the layers that develop (or disappear) in a year along a stake, again with clear diagrams, and comments about correcting for density changes, stakes that are not vertical, and measuring on a rough surface.
Actually, those two entries are the only ones over a page long, but most others are more than bare-bones definitions, many of them a lot more. All in all, it makes for an extremely interesting and useful glossary. The expanded nature of the entries leads to some redundancy among entries, but that is a positive aspect, not a negative one, because it means that many entries are complete in themselves without excessive cross-referencing—few people will read the Glossary through like a book (probably only reviewers).
Referring back to the cited “Purpose”—the authors have succeeded admirably in providing clarity and reducing ambiguity and in steering an appropriate course between proscription and description.
With regard to scope: the coverage is comprehensive within the boundary between what is included and what is not. The placement of that boundary, while not precisely defined, seems reasonable and appropriate, although in some places it is rather ragged. Examples of the latter: “Altimetry” includes radar and laser techniques but not the also important (particularly historically) aneroid altimetry; “sonic ranging” is here, but not microwave or laser ranging; “grounding line” is discussed but not the more realistic term, grounding zone; “radar sounding (‘ground-penetrating radar’)” is discussed, but there is no mention of seismic sounding; and, as already mentioned, “ice-core stratigraphy” is here, but not ice core. These slight irregularities in the scope boundary are trivial when stacked against the huge value of the Glossary.
This superb volume belongs on the bookshelf of all practicing glaciologists and, perhaps just as important, on the shelves of non-glaciologists who interact with glacier people.
It is unfortunate that the Working Group included nobody from outside western Europe and North America (one member was based in Chile for a year and a half and two other have done glaciological research in South America). That came about because the Working Group comprised glaciologists who volunteered at a mass balance workshop in Norway in 2008 that included few attendees, and no volunteers, from the rest of the world. The Working Group did have useful correspondence with several Russian experts, but since this Glossary and its recommendations will undoubtedly become a new world standard, a broader, worldwide representation in its construction would have been gratifying.