The Earth’s Climate

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The Earth's Climate




The climate of our planet is the result of three main factors: solar energy, the greenhouse effect, and atmospheric and oceanic circulation. In addition, the geographic and seasonal variations in solar energy are determined by the curvature of the Earth, the inclination of its axis and its orbit around the Sun. These factors produce different climatic zones, which in turn affect the distribution of plant, animal and human populations.

1 - The sun and the greenhouse effect

The Sun is the central star of our solar system, which consists of 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.The "surface" of the Sun is very hot, with a temperature of about 6,000 degrees Celsius. Due to its position, neither too near nor too far from the Sun, the Earth is the only planet in the solar system that can host abundant life, in particular because its average surface temperature of 15 degrees Celsius enables the presence of liquid water. Solar energy and the presence of an atmosphere are the two main elements that condition the Earth's temperature. Like the glass panels of a greenhouse, certain gases that are naturally present in the atmosphere, notably water vapor and carbon dioxide, trap heat from the sun, maintaining this mild average temperature on the planet's surface. This natural phenomenon is called the greenhouse effect. Without it, the surface temperature of the Earth would be –18 degrees Celsius and life, if any life could be sustained, would be quite different. The Sun emits radiation consisting of a small amount of ultraviolet rays and large quantities of visible light and near infrared. 30% of this radiation is immediately reflected by clouds, the atmosphere and the Earth's surface. The remaining 70% is absorbed: 20% by gases naturally present in the atmosphere and 50% by the oceans and land. Consequently, only half of the initial solar radiationreaches the Earth's surface. The absorption of solar energy warms the atmosphere and, most importantly, the surface of our planet. Converted into heat, this energy is then released back into space by the atmosphere and the Earth's surface in the form of middle infrared radiation. 10% of this terrestrial infrared radiation escapes directly into space, while the remaining 90% is trapped by greenhouse gases that are naturally present in the atmosphere. After trapping the heat from the sun as well as the heat rising from the Earth's surface, these gases, mainly water vapor and carbon dioxide, re-release this energy in the form of infrared rays. Two-thirds of these rays return to the surface, warming it a second time with the same solar energy, and the remaining one-third escapes into space.This is a simplified explanation of the greenhouse effect disregarding certain factors, for example the phenomenon of water evaporation on the Earth's surface, which leads to the formation of clouds. This evaporation contributes to the warming of the atmosphere, and also plays a key role in the water cycle. This constant exchange of energy between the Earth's surface and the greenhouse gases is what enables the planet to maintain its average temperature of 15 degrees Celsius. Our atmosphere's natural greenhouse effect is thus a beneficial phenomenon, without which life as we know it would not be possible. The greenhouse effect is often mentioned in the news as a dangerous phenomenon, but what really is dangerous is the intensification of the natural greenhouse effect. This is the result of an increase in greenhouse gases due to human activity, in particular the combustion of fossil fuels. It is thought that this intensification of the greenhouse effect is a major cause of the global warming trend that has been observed over the past 50 years. Today, scientists are trying to predict more accurately how this trend is likely to develop between now and the end of the 21st century.


Solar radiation

The Sun has been in existence for several billion years. The Earth is constantly exposed to its rays, which warm the surface of our planet. Without this energy input, the Earth would be a frozen wasteland. But what exactly is this solar radiation that makes our planet inhabitable? Solar energy reaches the Earth in the form of electromagnetic radiation, part of which is perceptible as visible light. Electromagnetic radiation can be broken down into millimeter and radio waves, infrared emissions, visible and ultraviolet light and, beyond these frequencies, X-rays and gamma rays. A cold mass like the Earth primarily emits radio, millimeter and infrared radiation, whereas a hot mass like the Sun emits waves covering the entire electromagnetic spectrum. It sends us a steady flow of energy dominated by the visible part of the spectrum. Part of this radiation is absorbed by the atmosphere. Ultraviolet and X-rays are mainly absorbed at altitudes above 100 kilometers, while infrared rays and visible light are partly reflected by the atmosphere and clouds.
The Earth's surface, which with an average temperature of 15 degrees Celsius is not very warm, re-emits part of the sun's energy, which can be absorbed by the atmosphere (greenhouse gases) and clouds.
The amount of solar energy that reaches the Earth is not constant over time. The Sun goes through a "solar cycle" of about 11 years, a phenomenon that has been observed by astronomers for centuries through variations in the number of sunspots (of dark patches on the Sun's surface). Still, the energy flow emitted by the Sun varies only by about one one-thousandth over the course of such a cycle.

Ozone

Ozone is a gas whose molecule is formed by 3 oxygen atoms. It is highly reactive, and its abundance in the atmosphere is the result of a balance between the formation and breakdown processes.
Ozone is found mainly in two atmospheric zones, and its role in relation to living organisms is beneficial in one and toxic in the other. The first zone is the stratosphere, a layer of air located between about 10 to 50 kilometers above sea level. Under the atmospheric pressure of the Earth's surface, this layer would be only about 3 millimeters thick. In the springtime, the stratospheric ozone is partially broken down in the polar regions due primarily to the particular meteorological conditions in these zones during the winter, and to the presence of chlorine and bromine atoms in the stratosphere resulting from human emissions of chlorofluorocarbons (CFCs) and halons. The destruction of the ozone layer above Antarctica in September and October is quite extensive, causing "the ozone hole." The phenomenon is also observed regularly in the Northern Hemisphere, although to a lesser degree. The ozone breakdown, which also occurs to some extent in the middle latitudes, has led to a steady depletion of the ozone layer over the past few decades. The beneficial role of stratospheric ozone arises from its capacity to absorb solar radiation in a particular ultraviolet range, UVB, thus protecting living cells on the Earth's surface against UVB rays' destructive action. Ozone is the only atmospheric compound, when it is present in sufficient quantities, capable of absorbing UVB rays in this wavelength range.

This depletion phenomenon was first noticed in the late nineteen-seventies. Following the signature of the Montreal Protocol, which called for the phasing out of CFC and halon emissions, the further breakdown of stratospheric ozone came to halt in the late nineties, although it will be several decades before the ozone layer is completely "repaired."
The second atmospheric zone where ozone is more or less uniformly present is the troposphere, the layer between the Earth's surface and an altitude of 10 to 16 kilometers. The ozone concentration here is only one-tenth that of the stratosphere. Although it is indeed the same molecule, tropospheric ozone is a pollutant. Its powerful oxidative action makes it a toxic gas when it reaches excessive levels. Vehicle exhaust fumes, which contain pollutants that undergo chemical reactions resulting in the production of ozone, are partly responsible for the increased ozone levels in the troposphere. When a certain "ozone peak" threshold is reached, public health warnings are issued and vehicle speed limits are reduced in densely populated areas.



Greenhouse gases

The most abundant natural greenhouse gases are water vapor, which accounts for half of the natural greenhouse effect, and carbon dioxide, which is responsible for another quarter even though it represents only 0.03% of the molecules that make up the Earth's atmosphere. Other gases, including methane, ozone and nitrogen protoxide, also contribute to the natural greenhouse effect, but to a lesser extent.
Human activity results in the release of large quantities of greenhouse gases into the atmosphere. These gases of anthropogenic origin are in all likelihood responsible for the climatic trends observed since 1975. They include:
- Carbon dioxide (CO2), which mostly comes from the combustion of fossil fuels (petroleum, coal, natural gas) used for transportation and heating, and secondarily from deforestation;
- Methane (CH4), which is abundant in wet zones, either natural or man-made (like ricefields), and is also produced by the digestion of ruminants, landfills, and losses during the extraction, transport and use of natural gas;
- Nitrogen protoxide (N2O), which is used in fertilizers;
- Halocarbons, including the now-infamous CFCs, which in most cases do not exist in nature. These gases were used for many years in the industrial production of substances like cooling and propellant gases.
The amount of time a gas spends in the atmosphere is called its "retention time." Water vapor only stays for a few days, but most of the other greenhouse gases remain for much longer periods, ranging from a decade in the case of methane to thousands of years for certain halocarbons.


Albedo

Albedo is a physical value that indicates the quantity of incident sunlight reflected by a given surface. In relation to the climate, this is an important variable because it expresses the proportion of solar radiation reflected into space by the Earth's surface and atmosphere, which therefore does not heat the planet.

Albedo is a dimensionless quantity. Its value is expressed either by a percentage between 0 and 100 representing the percentage of light reflected in relation to the quantity received, or by a figure between 0 and 1 correspondind to the fraction of reflected light.

Consequently, a perfectly white surface reflects all light, and its albedo is 100%. Conversely, a perfectly black surface reflects no light and thus absorbs all of the solar radiation it receives. Its albedo is 0%.

To give a few examples, the oceans have an albedo of 5 to 10%, sand between 25 and 40%, ice about 60%, and fresh thick snow up to 90%. The continents have a higher albedo than the oceans, which is why in satellite photos the land is lighter than the bodies of water. All surfaces combined, the Earth's average albedo is 30%.

The melting of the icecaps and variations in the use of land, as in the case of massive deforestation, affect the planet's albedo, which in turn impacts its energy exchanges and thus its climate. Changes in the cloud cover affect the Earth's albedo and the transmission of infrared radiation, thus modifying the greenhouse effect and the terrestrial cycles of heat and water exchange.

The water cycle

The four reservoirs of the hydrosphere are: the seas and oceans, the continental surface and subterranean waters, the atmosphere and the biosphere. The constant exchange of water that takes place among between four compartments is called the external water cycle. Its driving force is the Sun, whose radiated thermal energy keeps the masses of water in constant movement.

The water cycle comprises all three of the possible forms of water: liquid water (liquid state) in rivers, oceans and clouds, water vapor (gaseous state) in the atmosphere, and ice (solid state) in the polar icecaps, glaciers and pack ice. The water present on Earth, whose quantity has remained unchanged for several billion years at about 1400 billion cubic kilometers, is constantly in motion, passing through the different physical states. The circulation of water takes place in several stages:
1. Evapotranspiration: water evaporates from the surface of the oceans, as well as from the continents, including evaporation from the ground and transpiration from plants, thus passing from the liquid to the gaseous state;
2. Condensation: evaporated water condenses in the atmosphere, passing from the gaseous to the liquid state, thus forming clouds;
3. Precipitation: water from the clouds falls back to the Earth's surface in the form of raindrops or snowflakes, depending on the air temperature;
4. Infiltration and run-off: the precipitated water infiltrates directly into the subsoil, eventually reaching the subterranean water table, or drains into rivers and streams, eventually reaching the oceans.

And water's long journey starts over again, with evapotranspiration, and so on.


On average, for all of the Earth's continents over an entire year, 65% of the precipitation that reaches the ground evaporates and the remaining 35% remains as run-off and infiltration water.

During this continuous cycle, water is stored in a number of "reservoirs." It remains for only one or two weeks in the atmosphere or the rivers and streams, a few thousand years in the oceans and glaciers, and can be stored for over a million years in the polar icecaps, for example in Antarctica. The water cycle contributes to the transfer of heat between the Earth's surface and the atmosphere. Evaporation is a transition from the liquid state, in which the water molecules are bonded together, to the gaseous state, in which the molecules are independent from each other. It takes energy to break the bonds that link the molecules together in the liquid phase. This energy is teken where the liquid water evaporates, which is why surface evaporation (from the oceans, ground and vegetation) results in the cooling of the surface in question. Conversely, when water vapor condenses in the atmosphere, the same quantity of energy is released into the ambient air. The evaporation–condensation cycle thus transfers heat from the planet's surface to the atmosphere. The warmed air is then transported by atmospheric circulation.


2 - Movements and inclination

The average temperature at a given point on the Earth's surface is not constant all year due to the changing seasons. In the temperate zones the year is divided into four successive seasons.
There are three reasons for this phenomenon: the revolution of the Earth around the Sun, the planet's round shape, and the inclination of its axis of daily rotation, the axis of the poles, in relation to the plane of its orbit around the Sun.
The Earth revolves around the Sun in an elliptical orbit contained within a plane called the ecliptic plane. Because its orbit is only slightly off-center, our planet moves in a nearly circular path. The Earth makes one complete rotation around the Sun in one year.
Because the Earth is round, the Sun's rays are perpendicular to its surface at the Equator, and increasingly oblique as they got closer to the poles.
This means that for a given quantity of solar energy reaching the ground, the heated surface will be smaller at the Equator than at the poles. The Equator therefore receives more energy per surface unit than the poles.
In addition, the closer one gets to the poles, the greater distance the Sun's rays must travel through the atmosphere, losing energy along the way.
As a result, the Equator receives twice as much solar energy than a location at 60 degrees latitude.
However, the orbit around the Sun and the Earth's shape do not explain the phenomenon of the seasons, which is due solely to the fact that the axis of the poles is not perpendicular to the ecliptic plane. In other words, the Earth's equatorial plane is not congruent with the ecliptic plane. There is an angle, or obliquity, of 23.5 degrees between them.
If this angle were zero, at any given latitude, for example in Paris, the quantity of solar energy received would be the same in December as in June, and there would be no difference in temperature between winter and summer.
In fact, the Sun's rays are very oblique at this latitude in December. The quantity of solar energy received is lower resulting in cold winter weather.
On the other hand, in June the Sun's rays reach at the same latitude at much more perpendicular angle. The quantity of energy received is higher. It's summer.
This regular succession of four distinct seasons does not occur outside the temperate zones. For example, between the two Tropics, the Sun is always close enough to perpendicular, so there is no major difference in temperature between summer and winter. In these parts of the world, there are usually only two "seasons" in the climatic sense, a rainy season and a dry season.



Solar energy

Solar energy results from nuclear fusion in the Sun's core, where temperatures exceed 10 million degrees Celsius. This energy is radiated first to the Sun's "surface," where temperatures are still as high as 6,000 degrees Celsius, and then into the solar system in the form of electromagnetic radiation. Solar radiation is the main source of energy that heats the surface of the Earth. Every day the Sun sends us a considerable quantity of energy, of which less than 3% is consumed by man over an entire year. In comparison, geothermal energy, generated by the Earth's internal heat, represents less than a thousandth of the solar energy the planet receives.
The other sources of energy—cosmic radiation like he light from stars—amount to a mere one-millionth of solar energy.
On average (day and night, winter and summer, at the Tropics and at the poles), the Sun emits 342 watts per square meter at the levels of the upper atmosphere. About 30% of this energy is reflected back into space.
Since 1978, instruments on man-made satellites have been taking precise measurements of the variation in solar irradiance. It is very low—only 0.1%—and mostly concerns ultraviolet rays, which are largely absorbed at high altitudes, in the stratosphere.
The alternation between glacial and interglacial periods is apparently determined by variations in the amount of solar energy that reaches the polar zones in the summer, which in turn are due to slight variations in the Earth's rotation axis and orbit in cycles of 19,000 to 400,000 years.


The Earth's position

The position of the Earth in relation to the Sun changes constantly depending on three parameters:
- The eccentricity of the Earth's orbit varies between 0.005 and 0.05 over a period of 100,000 years. It is currently about 0.016;
- The inclination of the Earth in relation to the ecliptic plane varies between 22 and 25 degrees over a period of 41,000 years. The Earth's inclination is currently about 23.5 degrees;
- The precession of the equinoxes induces a movement of the Earth's axis of rotation around a cone of revolution over a period of 21,000 years.
Variations in these orbital parameters are constantly modifying the Earth's position and exposure to the Sun. However small, these variations, are sufficient to modify the amount of solar energy that reaches the Earth. They have given rise to a theory, called the Milankovitch Astronomical Theory, which explains the major climatic changes our planet has undergone over the past two million years.
Slight orbital variations result in major climatic cycles over periods of 100,000 years. The Earth has experienced a series of long glacial periods, followed by shorter and warmer interglacial periods lasting from 10,000 to 20,000 years. The average temperature difference between these periods is approximately 5 degrees Celsius on the planet's surface.
The interglacial period in which we now live began 11,000 years ago, and could continue for tens of thousands of years.


The seasons

The Earth's trajectory around the Sun and the inclination of its axis of daily rotation in relation to its orbital plane (the ecliptic plane) generate seasonal weather and temperature variations. In the course of the year, depending on the Earth's position in its orbit, a given point on the Earth's surface does not receive the same amount of solar radiation. The closer the Sun's rays are to perpendicular, the more heat they release. The more oblique their angle when they reach the Earth, the less heat they generate.
One year has:

- Two equinoxes: one around the 20th or 21st of March and the other around the 22nd or 23rd of September. At the equinox, the Sun's rays are vertical in relation to a point on the Equator. The durations of day and night are equal, hence the name equi-nox.
- Two solstices: one around the 20th or 21st of June and the other around the 21st or 22nd of December. At the solstice, the angle between the Earth's equatorial plane and the direction of the Sun's rays reaches its maximum. In the Northern Hemisphere the June solstice is the summer solstice, with the longest period of daylight in the year. In the Southern Hemisphere, the June solstice is the winter solstice.
In the temperate zones, in the middle latitudes, the astronomical seasons correspond to four phases in the year's climatic progression: winter, spring, summer and autumn. In the tropical regions the term "season" is also used, but in reference to a dry season and a rainy season. The names of the seasons and the associated climatic phenomena are reversed in the two hemispheres—when it's summer in France it's winter in New Zealand.

3 - Atmosphere and oceans

The driving force behind the Earth's atmospheric movements is the Sun. Solar radiation heats the surface of our planet, which in turn heats the air that surrounds it. Masses of air are warmed contact with the Earth's surface, thus tending to rise as warm air is less dense than cold air. A cyclone, also called a depression or low-pressure zone, then forms at ground level. Conversely, cold air masses tend to drop lower in the atmosphere and form anticyclones, or high-pressure zones, at ground leve.l
The warm air cools as it rises, thus descending towards the ground, where it is warmed again. This air circulation cycle occurs on a planetary scale in patterns determined by the Earth's energy balance. Overall the planet's energy balance is zero, but there is an accumulation of energy at the lower latitudes and a deficit at the poles. Masses of air circulate at ground level from the polar high-pressure zones towards the equatorial low-pressure zones, and back again at high altitudes. In addition, each hemisphere has not just one, but three zonal air circulation cells.
The warm, humid air that rises from the ground in the equatorial low-pressure regions moves toward the North and South Poles on either side of the Equator, getting gradually cooler. At about 30 degrees latitude this tropical air meets the cold polar air, drops back towards the surface and returns to the Equator in the form of trade winds. This tropical cell transfers heat from the Equator to the tropics. Between 30 and 60 degrees latitude an inverse cell forms, characterized by winds that blow from south to north at ground level. Further north, the cold, high-density air flows towards the temperate latitudes, forming the third cell.
In addition, the Earth's rotation affects the movements of these air masses. The winds blowing from the high-pressure towards the low-pressure zones are deviated to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The warm, humid air that rises from the ground in the equatorial low-pressure regions is deflected to the east as it moves north, transforming at about 30 degrees North into a powerful jet stream that flows over the region where the tropical and polar air meet on the ground. This region is characterized by an unstable thermal front that causes atmospheric disturbances whose activity transfers heat very efficiently from south to north.
Thermal energy is also transferred from the Equator to the poles by the oceans, in which a system of currents offsets the uneven distribution of thermal energy received on the surface. Surface currents in the oceans are primarily caused by wind action and are sensitive, like the winds, to the rotational "Coriolis Force." They are also affected by variations in sea level and pressure fields.
Generally, the oceans convey heat from the Equator to the poles via the major western boundary currents, the Gulf Stream and the Kuroshio in the Northern Hemisphere and the Brazil and Agulhas currents in the Southern Hemisphere. These waters are cooled, causing them to sink in the temperate latitudes and then return towards the Equator as deep currents. However, this general principle is modified by the specific geographic characteristics of each region. In fact, only the Pacific Ocean follows the pattern exactly. The Indian Ocean, blocked to the north by the barrier of the Indian subcontinent, transfers heat southwards at all latitudes, and the Atlantic Ocean, which opens onto the Arctic Ocean, transfers heat northwards at all latitudes.
This characteristic of the Atlantic, which plays a key role in the endless "conveyor belt" of oceanic circulation, is linked to its capacity to form deep waters in the Subarctic region. One stream of the warm, saline water of the Atlantic rises towards the Arctic along the coastlines of Europe, gradually cooling and becoming denser. When the water reaches freezing point, part of it transforms into pack ice, releasing its salt into the surrounding water and thus further increasing its density. Gravity pulls this cold, high-salinity, high-density water down to depths of between 2,000 and 4,000 meters, where it forms a deep current that conveys the cold northern water southward. The result is a deep transport in the North Atlantic comparable to that provided by surface currents.


The Coriolis Force

The Coriolis Force is a force that deflects the trajectory of an object in movement on the surface of an object in rotation. It affects all bodies in movement on Earth due to the rotation of the planet around its polar axis. The force is maximal at the poles and zero at the Equator.
The effect of the Coriolis Force is to deflect moving bodies to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It applies in particular to masses of air and water in motion, affecting the direction of the wind rotation in depressions and anticyclones (counterclockwise and clockwise respectively in the Northern Hemisphere).
The Coriolis Force also affects the direction of the Trade Winds, which blow from the tropics towards the Equator, northeast to southwest in the Northern Hemisphere and southeast to northwest in the Southern Hemisphere.

Thermohaline circulation

At tropical latitudes, the water at the surface of the oceans receives a considerable supply of heat that enables it to reach temperatures between 25 and 30 degrees Celsius. In the polar regions, surface water reaches the freezing point of seawater, which is about –2 degrees Celsius.
At depths comprised between a few dozen and several hundred meters, a layer of water called the "Mixed Layer" has a uniform temperature, close to that of the surface due to the action of the wind and waves. Beneath this layer, under the thermocline that designates a zone of wide temperature variation, exchanges of mass and energy are sharply reduced. Therefore, most of the ocean is made up of cold, high-density water. This water originates from the zones where surface water, having become cold and salty and thus very dense, is pulled by gravity under the warm, less saline water to a depth where it balances the density of the surrounding water (about 2,000 meters in the Subarctic region, 4,000 meters in the Antarctic, or 1,000 meters in the Western Mediterranean). The depths of all the oceans are filled with these waters, whose temperature, overaging 0 to 4 degrees Celsius, varies only slightly from the poles to the Equator. These so-called "deep convection zones" are few and far between. They are located in the higher latitudes, mainly in the Labrador Sea and off the coasts of Greenland and Norway, and to a lesser extent in the Weddell Sea. But they are also found in the Mediterranean, where the succession of Mistral winds in late winter can generate very high-density water.
The slow mixing of the ocean waters has been diagramed in the form of a global-scale "conveyor belt." The deep waters, which mainly form in the North Atlantic, flow in the direction of the South Atlantic. At about 60 degrees South, they join the Antarctic Circumpolar Current, in which they move west to east, gradually rising towards the thermocline and spreading throughout the South Atlantic, Pacific and Indian Oceans. The return of this massive flow to the North Atlantic takes place through warm currents near the surface, whose circulation is linked to the atmospheric currents. The entire process can take from several hundred to a thousand years.
The intensity of this circulation and the location of deep water formation sites vary between glacial and interglacial periods. During a glacial period, the presence of pack ice in the higher latitudes prevents the formation of deep water.


4 - Climates and biomes

The climate of a region is defined by the average values and variations of its meteorological data. The distribution of living organisms in a given zone is determined primarily by its temperature and precipitation (rain, hail or snow).
Temperatures on Earth vary according to the location, season, and time of day. They can range from -80 degrees Celsius in the depth of the Antarctic winter to +60 degrees Celsius in certain desert zones at midday in the summer. The distribution of average annual temperatures on the continents' surface divides the globe into five main zones: a warm zone located between the two Tropics, two frigid zones around the poles and two temperate zones in between.
Rainfall is essentially governed by atmospheric circulation. Desert regions are associated with zones of downward atmospheric motion, near the Tropics, and with the very cold zones near the poles. Rainy regions are associated with zones of upward atmospheric motion, located around the Equator and in the low-pressure zones of the middle latitudes. Rainfall averages 2 meters per year at the Equator, 70 centimeters in the semi-arid zones of the Tropical latitudes, 1 centimeter in the Subtropical deserts, and 1 meter in the middle latitudes.
The climate map shows the temperature and rainfall zones associated with the different latitudes: in red for the warm and humid Equatorial zone, yellow for the arid zones, green for the temperate zones and blue for the frigid zones.
Knowing the arrangement of these major climatic zones makes it easier to understand the distribution of living organisms in biomes.
A biome is a set of ecosystems characteristic of a biogeographic area, named for its predominant plant and animal species. For simplification purposes 11 major biomes can be defined.
The polar cap regions of Greenland and the Antarctic are a sterile desert in which no life can be sustained.
The tundra only exists in the circumpolar regions. It hosts a modest flora of moors, grasses, mosses and lichens, and a fauna that, albeit not highly diversified, is well adapted to the extreme climatic conditions.
In the taiga, or subarctic boreal forest, the flora consists primarily of conifers, which can withstand the frigid climate. This is the northernmost wooded zone on the planet.
The temperate forest, or mixed forest, is made up mostly of deciduous trees. The specific tree species are determined by the climate, which can be oceanic, continental or mixed. Found in Europe, Asia and North America, this type of biome accommodates an abundant and highly varied fauna.
The prairie, which includes the steppes of Asia, the pampas of Argentina and the South African Veld, is characterized by the profuse growth of annual herbaceous plants.
The chaparral biome is found around the Mediterranean and in regions with comparable climates in California, Chile, South Africa and southern Australia. The vegetation is adapted to a climate that is warm and dry in the summer and mild in winter.
The desert is an arid region with almost no flora or fauna. Depending on its altitude and latitude, a desert can be very hot or very cold. The savanna is found in tropical climates that alternate between a rainy season and a shorter dry season. This biome consists of tall grasses, interspersed more or less densely with trees or shrubs.
The tropical rainforest, or equatorial forest, is typical of the intertropical Torrid Zone. It is characterized by tall trees and a profusion of plant and animal species. Today, all of the planet's equatorial rainforests are threatened by deforestation.
The temperate rainforest is characteristic of temperate regions with heavy precipitation. Its flora consists largely of conifers or deciduous trees. This biome can be found in the North West of the United States as well as in southwestern Canada, southern Chile, Tasmania and New Zealand.
The mountain biome is actually a set of biomes that differ according to altitude rather than latitude. Deciduous trees in the lower zones give way to conifers and then to alpine tundra, while the peaks are snow-capped or rocky deserts.
These different environments influence the presence, activities and lifestyles of a large proportion of humankind. The deserts, high plateaux and subarctic regions where living conditions are particularly harsh have very low densities of population. In contrast, the temperate zones are very hospitable to human populations. The regions near the oceans are also increasingly populated, even though they are exposed to the significant climatic risk of repeated natural disasters.


CNRS    sagascience