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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 the 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 landmasses: dry, bare areas like deserts suffer greater daily and seasonal extremes of temperature than do wetter areas like forests. Thus, global maps of temperature zones hide a great deal of local variation.

microclimatic variation

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. 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.

ENSO and NAO

Long‐term temporal variations in temperature, such as those associated with the ice ages, were discussed in the previous chapter (Section 1.4.3). 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). The ENSO is an alternation between a warm (El Niño) and a cold (La Niña) state of the waters of the tropical Pacific Ocean off the coast of South America (Figure 2.16a), although it affects temperature and the climate generally in terrestrial and marine environments throughout the whole Pacific basin and beyond (Figure 2.16b). The NAO refers to a north–south alternation in atmospheric mass between the subtropical Atlantic and the Arctic (Figure 2.16c) and again affects climate in general rather than just temperature (Figure 2.16d). Positive index values (Figure 2.16c) 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 2.17.

Figure 2.16 Features of the El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). (a) ENSO from 1900 to 2017 as measured by sea surface temperature (SST) anomalies (differences from the mean) in the equatorial mid‐Pacific. El Niño events are defined as occurring when the SST is more than 0.4°C above the mean (red dashed line) and La Niña events when the SST is more than 0.4°C below the mean (blue dashed line). (b) Examples of El Niño (December 2009) and La Niña events (October 2007) as well as a neutral state (La Nada; January 2013) in terms of sea height above average levels. Warmer seas are higher; for example, a sea height 150–200 mm below average equates to a temperature anomaly of approximately 2–3°C. (c) NAO from 1864 to 2017 as measured by the normalised sea‐level pressure difference between Lisbon in Portugal and Reykjavik in Iceland. (d) Typical winter conditions when the NAO index is positive or negative. Conditions that are more than usually warm, cold, dry or wet are indicated. The positions of the Icelandic low pressure (L) and the Azores high pressure (H) zones are shown.

Source: (a) Compiled from the US National Ocean and Atmospheric Administration (NOAA), https://www.ncdc.noaa.gov/teleconnections/enso/indicators/sst.php). (b) From the US National Aeronautics and Space Administration (NASA), https://sealevel.jpl.nasa.gov/science/elninopdo/elnino/). (c) From https://climatedataguide.ucar.edu/climate‐data/hurrell‐north‐atlantic‐oscillation‐nao‐index‐station‐based). (d) From http://www.ldeo.columbia.edu/NAO/.

Figure 2.17 The abundance of three‐year‐old cod, Gadus morhua, in the Barents Sea is positively correlated with the value of the North Atlantic Oscillation (NAO) index. The mechanism underlying the correlation (a) is suggested in (b–d). (b) Annual mean temperature increases with the NAO index. (c) The length of five‐month‐old cod increases with annual mean temperature. (d) The abundance of cod at age three years increases with their length at five months.

Source: After Ottersen et al. (2001).