Unit 5: The Earth's major bioclimatic zones
It is rather ironic that the very place where life, and then complex life, evolved will be its ultimate destructor. Even the most extreme microbes will only last another 3 billion years and then it will be too hot even for them.
Mark Maslin, Climate. A very short introduction.
1. The torrid zone
The torrid zone is located between the Tropics of Cancer and of Capricorn.
The temperature is high (above 18ºC).
There are three types of bioclimates in this area: equatorial, tropical and desert. The latter is azonal, so we will study it separately.
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1.1) Equatorial Bioclimate
It occurs around the equator.
The temperature is high (above 25ºC)
Precipitations are high throughout the year (and, therefore, the trade winds are constant and atmospheric pressure is low).
The rivers carry a lot of water (Congo River, Amazon River ...).
The typical vegetation is the rain forest.
Congo River
Amazon River
Rain forest in the world
Climate graph of Iquitos, Peru. Constant precipitations all through the year and an average temperature of around 26º C. Under these conditions, the type of vegetation would be rain forest.
1.2) Tropical Bioclimate
The tropical bioclimate is located between the equatorial zone and the Tropic of Cancer to the north and between the equatorial zone and the Tropic of Capricorn to the south.
The temperatures are quite high: they never go below 18ºC.
The precipitations are high, but irregular, because it does not rain the same all the year: it rains much more in summer than in winter.
The tropical bioclimate is divided into two:
1.2.1) Humid tropical bioclimate: The dry season lasts less than three months. The vegetation is rain forest, but less dense than in the equatorial zone.
1.2.2) Dry tropical bioclimate: Also called savanna bioclimate. The dry season lasts more than three months.
The rivers carry more or less water depending on the season.
The vegetation consists of few trees and very tall grasslands, where animals such as zebras, lions and gazelles graze.
African Savannah
Venezuelan Savannah
Savannah in the world
Climate graph of Ancalho, Guinea-Bissau. There is a very dry winter followed by a summer of heavy rainfall. This produces a vegetation with few trees and an abundance of grasslands.
Climate graph of El Casar de Talamanca, Guadalajara, Spain. Dry summer and moderate precipitations the rest of the year, and a remarkable temperature contrast between winter and summer.
Mediterranean forest in the world
2. The temperate zone
The temperate zone is located between the tropics and the poles, both in the northern hemisphere and in the southern hemisphere. The most notable feature is the thermal contrast between stations, with a marked thermal oscillation. There are, in addition to the desert, four other bioclimates: Mediterranean, oceanic, continental and Chinese
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2.1) Mediterranean: It occurs around the Mediterranean Sea and in some areas of western America, Africa and Australia near the sea.
Temperature: low thermal oscillation, due to the proximity of the sea.
Precipitation: moderate, with dry summers.
Vegetation: Mediterranean forest (specially holm oaks) and scrubland.
Oceanic climate in the world
2.2) Oceanic.
It is present specially in the western side of the continents in the temperate zone, since the westerly winds bring clouds to these areas. However, it is also present in some eastern areas, like in the East coast of North America.
Temperature: Mild, with no pronounced thermal oscillations, because the proximity of the sea. Winters are not very cold, and summers are not very hot either.
Precipitation: Regular and constant all thorough the year, due to the proximity of the sea.
Vegetation: Decidious woodland.
Picture and climae graph of Bidarray, French-Basque Country. Temperatures are never too high or too low, and the constant precipitations favor the presence of the decidious woodland.
2.3) Continental.
It is present in the interior of the great continental masses.
Temperature: There are strong contrasts. The winters are very cold and the summers are hot. This is due to the remoteness of the sea.
Precipitation: There is a lot, especially in summer. In winter it precipitates less, but in the form of snow.
The rivers freeze in winter and take a lot of water in spring due to the thaw.
Vegetation: It is very varied and depends on the latitude, because this bioclimate covers enormous extensions. The most common types are the taiga, the grass lands and, in dry places, the steppe.
Left: Taiga continental forest in Central Europe. Right: Climate graph of Cracovia (Poland). Winters are very cold (at least two months below zero) and summers are mild, with higher precipitations during the summer.
Taiga Grasslands Steppe
Continental Climate in the world
2.4) Chinese bioclimate:
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It is a subtropical humid climate that is dominated by the Southeast Asian monsoon, a type of wind that comes from the equator and moves in summer from south to north and which causes the summer rains to be more intense and prolonged than those in winter. The monsoon in China does not "explode" as suddenly, nor does it rain as heavily as in India. It is typical of the northern hemisphere, specifically East Asia and occurs in China, Taiwan, Korea and Japan. In winter the winds are reversed due to the intense pressure on Siberia.
Clmate graph of Canton, China. Regarding its precipitations, it is somehow similar to the continental climate, because summers are wetter than winters, but regarding the temperature they are very different: the Chinese climate does not have low temperatures, as it is located in places not far from the tropics.
Chinese climate in the world
3. Desert
The desert bioclimate can be found both in the torrid zone and in the temperate zone. Even the poles are deserts, although in the frigid zone.
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In the torrid zone, the deserts are located more or less at the height of the tropics. For example, the Sahara desert is located on the Tropic of Cancer, and the Kalahari desert, on the Tropic of Capricorn.
In the temperate zone, the deserts are located in the interior, far from the coast, or behind the mountains, where they prevent the clouds from reaching.
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Temperature:
In the torrid zone, the temperature is very high during the day and low at night, due to the lack of humidity, which causes a very rapid cooling.
In the continental deserts of the temperate zone, the summers are very hot and the winters are very cold.
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Precipitation:
Precipitations are very low and irregular in all deserts.
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Vegetation:
The vegetation is very scarce. Normally there are cacti or thorny shrubs. Spines and small leaves reduce evaporation, so they need very little water and are resistant to sunlight. In addition, these plants have deep roots that can find moisture in the soil.
In the steppe, there are low grasses and isolated bushes.
Deserts in the torrid zone
Sahara desert
Kalahari desert
Climate graph of Riad (Saudi Arabia). Precipitations are very scarce and practically there is no rainfall during the summer
An example of anthropic activity in the desert: During the Cold War, the Nevada desert, USA, served as a testing ground for the atomic bomb.
Deserts in the temperate zone
Gobi desert. Remains of the Great wall of China
Climate graph of Nukus (Uzbekistan). There is a big contrast in temperatures between winter and summer. Winters are very cold due to the long distance from the sea and the low humidity
Review: what is this phenomenon called?
Vegetation
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Thorny plants. They waste very little water
Oasis. They appear because of the aquifers
Steppe landscape. A little more humid than the desert, that's why we can find shrubs
Deserts of the world
Climate graph of McMurdo base, at the Antarctica. This is a glacial environment, so temperatures are always below 0ºC
Climate graph of Nuuk, Greenland. This is a periglacial environment, so temperatures go slightly above 0ºC during the summer
Temperature:
In the glacial environment it is always below 0º C. In the periglacial environment they rarely reach 10º C.
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Precipitation:
They are very scarce. Besides, the harsh cold favors the presence of anticyclons.
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Vegetation:
In the glacial environment there is no vegetation, as the subsoil is always frozen. This phenomenon is known as permafrost. In the periglacial zone we find a typical landscape: the tundra, which consists in low-lying plants, such as mosses and lichen, and short shrubs with wide and deep roots.
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There is no vegetation in the ice caps.
Tundra landscape in Scoresby Sund, Greenland
Tundra in the world
4. The frigid zone
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They are located in the inside of the Artic and Antarctic circles. Their main characteristics are:
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-Harsh cold
-Limited sunlight
-Low precipitation
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There are two zones:
-Glacial environment: The ice caps
-Pericglacial environment: It surrounds the ice caps or is located near them, but there is no permanent frost.
Mosses
Lichens
5. Alpine bioclimate (Mountains)
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There are two zones:
-Low mountain climate.
-Alpine or high mountain climate: it begins where the temperature is always below 10º C and trees cannot grow. We will focus on this one.
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5.1) Alpine climate:
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-Temperature: It never goes above 10º C. It drops 0.6 º C every 100 meters you climb.
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-Precipitation: It is more abundant as you climb, but
in the torrid zone, it starts decreasing at an altitude of around 3500 meters
in the temperate zone, it starts decreasing at an altitude of around 2500 meters
At the peak, there is perpetual snow due to the low temperatures.
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-Vegetation: There is different types of vegetation depending on the altitude. By the hillside we may find deciduous trees, such as oaks or beeches. As we ascend in altitude, we may find perennial trees, such as pines. Around 2500 meters, we will find pastures. When we get above 3000 meters, we may find tundra vegetation, and finally, above 3500 meters, there will be no vegetation, just permanent snow.
