3.2 Cold environments
The Arctic regions are the exact opposite of deserts as far as severe climate is concerned. Organisms in the Arctic regions have adapted to habitats influenced by extreme cold, resulting in short growing seasons for plants, land that produces little food, and lack of shelter. Some animals migrate and thus avoid the extreme cold for large parts of the year.
At high latitudes, the Sun’s rays always strike the Earth at a large angle from the vertical so they travel through a thicker layer of atmosphere and are attenuated by the time they reach the ground. Because the Earth’s axis of rotation is inclined to its path around the Sun, there are large seasonal changes in day length and the Sun is continuously below the horizon for a period in winter and continuously above the horizon for an equivalent period in summer. The range of annual temperature change is much greater at higher latitudes, and in mid-winter (January and February), the range about the mean is more than 12 °C. In polar climates, the temperature can change abruptly and often unpredictably. In coastal areas, the sea keeps the climate much more equable. Further inland, fluctuations in temperature are even greater. Polar organisms are thus adapted both to the extreme cold and to abrupt fluctuations in temperature.
The Arctic Circle (66° 30′N), and the equivalent latitude in the Southern Hemisphere, are defined as the latitude above which the Sun is continuously below the horizon for at least one day each year. Warm, moist air from the temperate zone rarely reaches high latitudes, so in most polar areas precipitation is low. Much of the water is locked away as ice, which has a low vapour pressure, and the air is very dry (often as dry as a tropical desert) and ground water is inaccessible to plants as well as to animals.
Terrestrial environments in the Arctic are, by geological standards, relatively new, most of the land having been completely covered with a thick layer of ice as recently as 10 000 years ago. Consequently, the soil is thin and fragile, and poor in organic nutrients. The optimum temperatures for plant growth do not coincide exactly with peak sunshine. At Longyearbyen, continuous daylight begins in late April, but the mean temperature does not rise above 0 °C (and so the snow and ice do not melt) for another two months.
These circumstances, combined with the severe climate, mean that the growing season for plants is short but intensive, and total productivity on land is low, producing little food and still less shelter for animals.