Climatic changes and variations in the atmospheric and ocean circulations

Until quite recent years meteorology was in no position to make an effective contribution to the understanding of paleoclimates. Lacking were both a theory of the general circulation of the atmosphere and the upper wind observations which have provided the essential foundation of such theoretical understanding of the circulation as we now possess. The most notable early contributions from meteorology wereSimpson's (1934) schematic construction of a presumed mean circulation at the time of the Quaternary ice age maximum andWillett's (1949) proposal of two alternative modes of the general circulation, summarized as expansions and contractions of the circumpolar vortex of upper westerly winds. Both theories lacked an observational basis, especially for their more sweeping generalizations (e.g. simultaneity of ice-ages and lowlatitude pluvials, limitation to only two main atmospheric circulation types etc.) as well as for their author's further proposals about variations of the solar radiation. Nevertheless, they marked a valuable step forward, because they stimulated workers in many fields to interpret their evidence of climatic changes as part of a world phenomenon in which the general wind circulation must be involved. A strictly empirical study of the broadest possible scope of the climatic changes observable within the last 100–1000 years was badly needed. Examination reveals that significant changes have taken place within this period. Study of them is instructive. Whatever changes (if any) of effective radiation available at the Earth's surface occur, the atmospheric and ocean circulations are the mechanism whereby the heat and moisture are distributed. As in synoptic meteorology, circulation maps have been found the most useful means of obtaining a unified view of the climatic changes of recent times. The first successful studies of this kind were byDefant (1924), who showed that the mean circulation over the North Atlantic had intensified materially from the 1880s to the present century, i.e. in part of the period of Arctic warming and increasing mildness of winters in Europe. The present paper shows how our knowledge of changes such asDefant found has been extended over more of the world and a longer period of time. This leads to some indications about the probable nature and possible origins of the phenomenon, which is revealed as rather simple and world-wide in scope. We learn some of the types of variation that actually occur. Examination of the charts indicates inter alia progressive changes of intensity and position of the mainstreams of the atmospheric circulation, and of the ocean currents and climatic zones that correspond to them, within the last 100–250 years. Consideration of the nature of the Little Ice Age epoch of 1550–1850, and comparisons with what is known of the great ice ages of the Pleistocene, appears to throw some new light on the ice age problem.