Variations in temperature on and within the surface of the earth have a variety of causes: latitudinal, altitudinal, continental, seasonal, diurnal and microclimatic effects and, in soil and water, the effects of depth.
Latitudinal and seasonal variations cannot really be separated. The angle at which the earth is tilted relative to the sun changes with the seasons, and this drives some of the main temperature differentials on the earth's surface. Superimposed on these broad geographic trends are the influences of altitude and 'continentality'. There is a drop of 1°C for every 100 m increase in altitude in dry air, and a drop of 0.6°C in moist air. This is the result of the 'adiabatic' expansion of air as atmospheric pressure falls with increasing altitude. The effects of continentality are largely attributable to different rates of heating and cooling of the land and the sea. The land surface reflects less heat than the water, so the surface warms more quickly, but it also loses heat more quickly. The sea therefore has a moderating, 'maritime' effect on the temperatures of coastal regions and especially islands; both daily and seasonal variations in temperature are far less marked than at more inland, continental locations at the same latitude. Moreover, there are comparable effects within land masses: dry, bare areas like deserts suffer greater daily and seasonal extremes of temperature microclimatic variation than do wetter areas like forests. Thus, global maps of temperature zones hide a great deal of local variation.
It is much less widely appreciated that on a smaller scale still there can be a great deal of microclimatic variation. For example, the sinking of dense, cold air into the bottom of a valley at night can make it as much as 30°C colder than the side of the valley only 100 m higher; the winter sun, shining on a cold day, can heat the south-facing side of a tree (and the habitable cracks and crevices within it) to as high as 30°C; and the air temperature in a patch of vegetation can vary by 10°C over a vertical distance of 2.6 m from the soil surface to the top of the canopy (Geiger, 1955). Hence, we need not confine our attention to global or geographic patterns when seeking evidence for the influence of temperature on the distribution and abundance of organisms.
Long-term temporal variations in ENSO and NAO temperature, such as those associated with the ice ages, were discussed in the previous chapter. Between these, however, and the very obvious daily and seasonal changes that we are all aware of, a number of medium-term patterns have become increasingly apparent. Notable amongst these are the El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) (Figure 2.11) (see Stenseth et al., 2003). The ENSO originates in the tropical Pacific Ocean off the coast of South America and is an alternation (Figure 2.11a) between a warm (El Niño) and a cold (La Niña) state of the water there, though it affects temperature, and the climate generally, in terrestrial and marine environments throughout the whole Pacific basin (Figure 2.11b; for color, see Plate 2.1, between pp. 000 and 000) and beyond. The NAO refers to a north-south alternation in atmospheric mass between the subtropical Atlantic and the Arctic (Figure 2.11c) and again affects climate in general rather than just temperature (Figure 2.11d; for color, see Plate 2.2, between pp. 000 and 000). Positive index values (Figure 2.11c) are associated, for example, with relatively warm conditions in North America and Europe and relatively cool conditions in North Africa and the Middle East. An example of the effect of NAO variation on species abundance, that of cod, Gadus morhua, in the Barents Sea, is shown in Figure 2.12.
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