The term "xerothermic" was suggested in 1891 by the Swiss, John Isaak Briquet, to designate a climatic period occurring after retreat of the last great continental glaciers that had apparently been drier and warmer than that of the present.
Louis Agassiz, in 1837, had presented evidence for the former existence of these glaciers over much of North America and Europe. Once this idea was accepted in the scientific world, the simplest explanation of climatic change was that there had been a gradual warming from glacial times down to the present. Doubt was cast upon this assumption, however, by studies of animal fossils and the layers of plant remains that had accumulated in old lake beds as peat, and as a result of increasing knowledge of the present distribution of plant species.
Peat deposits are especially abundant in regions that once were covered by glacial ice. Beginning in 1842 much attention was given to the remains of vegetation found in successive layers of peat, especially in Scandinavia. As a result it became clear that above the lower layers of cold climate species, such as spruce, there were remains of other kinds, such as oak, mistletoe, and ivy, that thrive today in warmer regions than those in which they were laid down. Above them was a marked increase in the species that are found living under the present, evidently somewhat cooler climate. There was, however, a considerable amount of controversy as to the details of this record and their interpretation.
The second reason for believing in a postglacial xerothermic period (or periods) came from the presence of steppe plants surviving in parts of Europe whose characteristic climate favors forest rather than grassland. In North America there are similar areas of grassland, despite a forest climate such as that of Ohio. Henry Allan Gleason, in 1915, discussing the presence in Illinois of communities of arid-climate species, advanced the explanation that they were relicts- surviving remnants of a type of vegetation much more abundant during a former drier and warmer phase of climate.
Impressive as these two lines of evidence were, debate continued. This is why the term "theory" has been employed, pending more positive proof.
Supporting evidence became possible with the development, by the Swedish scientist Ernst J. Lennart von Post in 1915, of a technique known as pollen analysis. Since windblown pollen is deposited in large amounts in lake sediments, one can, by coring these sediments and identifying and counting the different kinds of pollen therein, get a useful notion of the trends in adjacent vegetation, and from it infer climatic changes.
Thus a typical deposit in the Great Lakes region of North America reveals the following sequence, from bottom to top, based upon the changing proportions of pollen: spruce, pine, hemlock, oak and hickory, beech and maple. Knowing the moisture and temperature requirements of these forest types, we can be reasonably certain that the intervals of pine and the later oak-hickory were relatively drier than the other three. They were also clearly warmer than the initial spruce interval and very probably warmer than the climate of today, as well as drier. Acceptance of this idea by geologists, at least so far as temperature is concerned, is reflected in their use of the term "altithermal" and the later coined term "hypsithermal".
Willard Frank Libby's discovery of the method of dating organic materials by the use of radioactive carbon has made it possible to approximate the time scale of these changes, confining them to the last 10,000 years. Further refinements almost certainly will enable us to factor out the effects of moisture and temperature more closely than at present. Meanwhile, we know that the gradient from cool-moist (oceanic) to warm-dry (continental) climate in North America runs from northeast to southwest, and that climates in the past have shifted back and forth along this diagonal. Plants and animals h ave changed their distribution accordingly, and so doubtless have the activities of prehistoric man.