Geoscience Reference
In-Depth Information
In his 1901 paper, Harmer had suggested that some well-known case studies of
anomalous past climates, such as the existence during the Pleistocene epoch of
humid conditions in the Sahara (a region now arid) might be explained meteo-
rologically by supposing that the distribution of high and low pressure areas, and
the prevalent tracks of Atlantic depressions were different to those which now
prevail, and that such a situation was not only contemporaneous with but also due
to the existence of the great ice sheets of North America and Europe.
Harmer referred to an article by the meteorologist, Henry N. Dickson
(1866-1922) concerning the mean temperature of the atmosphere and causes of
glacial periods also published in 1901. While accepting the view that the case
study of the Sahara and other similar geological anomalies might be explained
meteorologically, Dickson argued that the shifting of the Atlantic storm tracks
towards the south during the glacial period was due to the increased difference in
temperature then existing between the polar and equatorial regions. Refrigeration
having been, in his opinion, considerably more in the former than in the latter, the
steepness of the thermal gradient would be greater than at present and this would
have increased the wind force of the North-East Trades and the mid-latitude
Westerlies, and displaced the sub-tropical high pressure belts to latitudes nearer
the equator. This increased vigour of the circulation would tend the development
of cyclonic pressure systems, the stronger the flow the more intense and frequent
these depressions would have become; also the 'normal' storm tracks would have
been displaced to lower latitudes, running more from west to east rather than from
southwest to northeast—as at present.
Dickson had studied at Edinburgh University where, among other projects, he
acted as a volunteer assistant at the Ben Nevis Observatory. Harmer well
acknowledged Dickson 'from whom I have received some friendly and valuable
criticism'. Dickson's paper, 'Mean temperature of the atmosphere and the causes
of glacial periods' was read at a Meeting of the British Association and published
in The Geographical Journal for November 1901.
Starting with the assumption that secular variations of climate in the past have been due to
changes in the mean temperature of the atmosphere, the author [Dickson] pointed out that
such changes had probably been associated with large alterations in the temperature
gradient between the equator and the poles. But as this difference of temperature is the
primary cause of the overall planetary circulation of the atmosphere, the form and
intensity of the latter must have varied with it. Thus a lowering of the mean temperature
[with relatively greater cooling at the poles] would be accompanied by an increase in the
equator-poleward gradient, a rise by a diminution of it, and in the former case the plan-
etary circulation would become more active and the sub-tropical high pressure belts would
be displaced to lower latitudes. It is suggested that the effect of these changes on the
distribution of precipitation and on the position and directions of the major storm tracks,
may explain some peculiar features of glacial phenomena (Dickson
1902
).
Harmer continued that conditions not only of greatly increased rainfall but also of
torrential downpours must have existed in southern Europe during the glacial
epoch (as had already suggested in his 1901 article) and these effects may have
been due to the greater strength of the general circulation together with the
