Forty years ago, Ahlmann considered the thermo-physical character of ice masses as a basis for differentiating glaciers into two broad geophysical groups: (1) polar and (2) temperate. About the same time, Lagally sub-divided glaciers into corresponding thermodynamic categories: (1) kalt and (2) warmen. By this it was understood that the temperature of a polar, or “cold”, glacier was perennially sub-freezing throughout, except for a shallow surface zone which might be warmed for a few centimeters each year by seasonal atmospheric variations. Conversely, in a temperate, or “warm”, glacier, the temperature below a recurring winter chill layer was consistently at the pressure melting point. As these terms are thermodynamic in connotation, glaciers of the polar type may exist at relatively low altitudes if their elevations are sufficiently great. Temperate glaciers may be found even above the Arctic Circle at elevations low enough that chilling conditions are not induced by the lapse rate.

In these distinctions, it is implied that regardless of geographical location a glacier’s mean internal temperature represents an identifiable characteristic which can be shown critically to affect the mass and liquid balance of ice masses and significantly to relate climatic influences to glacier regimes. The importance of these implications, and the fact that they are based on a gross, sometimes changing, and always difficult to measure, thermo-physical characteristic, makes some explicit terminology desirable.

To some extent Ahlmann addressed this problem by introducing a subordinate classification, sub-polar glaciers. In these, the penetration of seasonal warmth involved only a shallow surface layer at 0°C, but still to a depth substantially greater than the superficial warming experienced in summer on polar glaciers. Lagally also recognized an intermediate type which he called “transitional”, characterized by a relatively deep penetration of 0°C englacial conditions during the summer. These pioneering efforts reflect Ahlmann’s experience with glaciers in the high Arctic and Lagally’s with the Alpine glaciers of southern Europe. Although some confusion has resulted from alternate application of these different terms, both definitions can be useful. Further to refine the classification, a modified terminology is suggested by the writer. This involves introducing a fourth category, substituting the term sub-temperate for Lagally's “transitional” type on the basis that it is etymologically more consistent with the Ahlmann terminology which has remained most commonly in use. Thus, two distinct transitional categories are identified. These categories, sub-polar and sub-temperate, typify ice sheets during changes from fully polar to fully temperate englacial conditions—a situation pertaining during the waning and waxing stages of deglaciation and reglaciation,

A review of the literature reveals further problems. Flint and others have considered geophysically temperate glaciers as most typical of the inland glaciation which covered much of Europe, northern North America and Siberia during the expanded phases of the Pleistocene, whereas others including Ahlmann have suggested that the massive continental glaciers of the Pleistocene were geophysically polar. Thus, the latter advocates consider that present-day Antarctic and Greenland ice sheets represent conditions comparable to those which pertained in the Laurentide and Cordilleran ice sheets. New insights have developed, however, through deep drilling and englacial temperature measurements carried out in a number of different geographical locations in recent years. Such research has shown that each of the geophysical categories can pertain in a glacier system if there is sufficient range of latitude, area and elevation for the requisite climatological factors to pertain.

Because of the foregoing considerations, it is probable that polar, sub-polar, sub-temperate and temperate thermal conditions coexisted in different parts of continental glaciers during the Pleistocene maxima. At times of greatest extension, the ice sheet’s peripheries could have been thermo-physically polar and sub-polar, as on the margins of today’s Greenland icc sheet. But in their most regressive phases, the lower latitude margins were more than likely temperate, with only high interior sectors remaining “cold”. Such combined conditions characterize a fifth thermo-physical category, which in the geophysical sense may be termed polythermal. To some extent all glaciers are poly thermal, except in the final wasting temperate phase when they are fully isothermal.

To elucidate the characteristics of each of these five categories and to identify prototypes with suggested thermal parameters, selected held studies on existing glaciers are discussed and thermal measurements and characteristics illustrated. From the sampled data, arbitrary englacial temperature limits are suggested: for the main body of polar glaciers (—10 to — 70°C); for sub-polar glaciers (—2 to —10°C); for sub-temperate glaciers (-0.1 to —2°C); for temperate glaciers (in summer, 0°C throughout); and for polythermal glaciers (a range across at least two of the foregoing temperature zones). The significance of thermal anomalies, temperature sandwich structures, diagenetic ice zones, and measured shifts in thermodynamic characteristics over a number of years are considered as they aid in the interpretation of ice morphology, glacier regimes, and climatic change.

Type thermo-physical examples are briefly compared from the following areas: the Antarctic and Greenland ice sheets (polar and polythermal), the Nepal Himalaya, Svalbard (polar to sub-polar), Lapland (sub-polar), sub-Arctic Norway (sub-temperate), the Alps (polythermal to temperate), the Canadian Rockies (sub-temperate), the Juneau Icefield, Alaska (sub-temperate to temperate), the Alaskan-British Columbian coast (temperate), glacier systems on Mount Rainier, Washington State (polythermal), and icefields in the St Elias Mountains, Yukon Territory (temperate to polythermal).

The relationship of thermal anomalies is clarified and illustrated within (he defined framework of each category. It is noted how these are manifest by deformation irregularities, differing salinities, and varying heat capacities within the ice. Also discussed is the relationship of changes in thermo-physical characteristics to the sensitivity of ice flow, revealed by changes in entropy and negentropy of glacier systems and by observable shifts from parabolic to rectilinear to surging flow. Finally considered is the long-term implication of secular changes in climate and their influences on englacial thermal regimes which affect the hydrological capacity and fluvial discharge of glaciers as well as their terminal fluctuations. The strong interdependence of all these factors and the total systems analysis which they represent underscore the mandate for a rational thermo-physical classification of glaciers.