Earth Sciences / 81tions in solar output. Volcanic eruptions can depress surface temperatures and ENSO events
enhance them. It may be that emissions of greenhouse gases are now overwhelming these natural
forcing factors, but this does not remove them: predictions of future climate must take them into
account, inherently unpredictable though some of them may be. Those attempting predictions
must also bear in mind the possibility that once climate begins to change the rate of change may
accelerate dramatically and that we seem to be living in unusually stable times. Predictions are
concerned with the future, of course, but they must incorporate evidence gleaned from the past.
Palaeoclimatologists, who study ancient climates, supply information that is vitally important
to forecasters.
21. Climatic regions and floristic regions
Climates can be classified. At the simplest level, latitude, proximity to the ocean, and the convective
cells transporting warm air away from the equator and cool air away from the poles provide a basic
classification. Equatorial regions are warm and humid, subtropical regions, where dry air descends,
are warm and dry, polar regions are cold and dry, and the mid-latitudes are mild and humid or dry
with temperature extremes according to whether they are maritime or continental.
Unfortunately, it is not quite so simple as it sounds, because ‘warm’, ‘cool’, ‘dry’, and ‘humid’ are
relative terms that mean little by themselves. Aridity, for example, depends not on annual precipitation,
but on ‘effective precipitation’, which is precipitation minus evaporation, this being what determines
the amount of moisture reaching the ground water. This, in turn, is related to temperature and a
figure for the average annual temperature may conceal a very wide difference between summer and
winter. Many attempts have been made to base a classificatory system on the general circulation of
the atmosphere, the earliest dating from the 1930s.
The most successful scheme of this kind, proposed in 1950 by the German climatologist H.Flöhn, is
illustrated in Figure 2.29. Flöhn took account of the global wind belts and distribution of precipitation
(BARRY AND CHORLEY, 1982, pp. 358–373). In 1969, A.N.Strahler proposed an even simpler
system based on the air masses which produce climates, dividing all climates into three types: low-
latitude; mid-latitude; and high-latitude. These were subdivided according to variations in temperature
and precipitation to produce 14 regional types, with a separate category for upland climates.
The two most widely used classifications, however, were introduced between 1900 and 1936 by the
Russian-born German climatologist Wladimir Peter Köppen (1846–1940) and in 1931, with important
revisions in 1948, by the American climatologist C.Warren Thornthwaite (1899–1963). The Köppen
classification is widely used by geographers, that of Thornthwaite by climatologists.
Köppen took account of the distribution of vegetation, based originally on studies published in
the last century by Alphonse de Candolle (1806–93), whose Géographie botanique raisonée
(1855) considered the geographical distribution of plants in relation to their physiology. From
this it emerges that a summer temperature of 10°C marks the limit of tree growth, a winter
temperature above 18°C is necessary for some tropical plants, and if the average winter temperature
is below -3°C there will be at least some snow cover. Using these criteria and records of monthly
average temperatures, Köppen defined six climatic types. In tropical rainy climates temperatures
are above 18°C throughout the year; in warm, temperate, wet climates temperature in the coldest
month is -3–18°C; in cold boreal-forest climates temperature in the coldest month is below -
3°C and in the warmest month above 10°C; in tundra climates temperature in the warmest
month is 0–10°C; in polar climates the temperature never rises above 0°C; and a final category