Download 3 - Environmental Intermediate

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3. THE ATMOSPHERE
The atmosphere is the name for a layer of gases that surround a body of sufficiently large mass.
The gases are attracted by the gravity of the body, and held fast if gravity is sufficient
(thaerefore mass must be large) and the atmosphere's temperature is low. Some planets (called
gas giants) consist mainly of various gases, and thus have very deep atmospheres. Earth,
Venus, Mars, and Pluto have atmospheres that surround their surfaces. Other bodies in the
solar system such as the moon possess extremely thin atmospheres.
This topic will focus on the structure of the Earth’s atmosphere.
3.1 Structure and Composition
Earth's atmosphere is a layer of gases surrounding
the planet Earth and retained by the Earth's gravity. It
contains roughly 78% nitrogen and 21% oxygen, trace
amounts of other gases, and water vapor. The mixture
of gases is commonly known as air.
The atmosphere protects life on Earth by absorbing
ultraviolet solar radiation and reducing temperature
extremes between day and night.
The atmosphere has no abrupt cut-off. It slowly
becomes thinner and fades away into space. There is
no definite boundary between the atmosphere and
outer space. Three-quarters of the atmosphere's mass
is within 11 km of the planetary surface. The Karman
line, at 100 km, is frequently used as the boundary
between atmosphere and space.
The Earth’s atmosphere is made up of several layers,
that is, in order from the earth surface: Troposphere,
startosphere,
mesosphere,
thermosphere
and
exosphere. The boundaries between these regions
are named the tropopause, stratopause, and
mesopause. In addition, the ozone layer occurs, which
is a layer of air which is rich in ozone (O3). The ozone
layert runs from the stratosphere to the thermosphere
(10-50km). Each of the mentioned strata has a
different composition, temperature and pressure.
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J. Henwood
Intermediate Environmental Science- Atmosphere V1.0
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3.1.1 Temperature and the atmospheric layers
The average temperature of the atmosphere at the surface of earth is 14 °C.
The temperature of the Earth's atmosphere varies with altitude and between the different
atmospheric layers:
- troposphere: From the Greek word "tropos" meaning to turn or mix. The troposphere is
-
the lowest layer of the atmosphere starting at the surface going up to between 7 km at
the poles and 17 km at the equator with some variation due to weather factors. The
troposphere has a great deal of vertical mixing due to solar heating at the surface.
Weather occurs within the trophosphere;
stratosphere: from that 7–17 km range to about 50 km, temperature increasing with
-
height;
mesosphere: from about 50 km to the range of 80 km to 85 km, temperature decreasing
-
with height; and
thermosphere: from 80–85 km to 640+ km, temperature increasing with height.
The temperatures present within the layers
is presented below:
In some cases, the ionosphere is also
distinguished. This is the part of the
atmosphere that is ionized by solar radiation
and includes the exosphere and the
thermosphere.
The processes occurring within the atmosphere are rather complex. Within the troposphere,
weather occurs and thus the interactions occurring are rather complex, as given in the diagram
on the following page.
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J. Henwood
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3.2 Solar Radiation and the Atmosphere
Solar radiation is radiant energy emitted by the sun, particularly electromagnetic energy. On
Earth, solar radiation is obvious as daylight when the sun is above the horizon. This occurs
during daytime, and at night in summer near the poles, but not at all in winter near the poles.
When the direct radiation is not blocked by clouds, it is experienced as sunshine, a combination
of bright yellow light (sunlight in the strict sense) and heat.
3.2.1 Interaction of Solar Radiation with the Atmosphere
The average energy density (the average energy per metre square) of solar radiation just
above the Earth's atmosphere , in a plane perpendicular to the rays, is about 1367 Wm-2, a
value called the solar constant (although it fluctuates by a few parts per thousand from day to
day, and is more at the equator than at the poles). The Earth allows 1/4th the solar constant or
~342 Wm-2 (a value known as insolation) to pass through the surface of the atmosphere since
radiation amounting to this value fall at 90o to the surface of the atmosphere. Of this 342m-2 the
amount received at the surface varies spatiotemporally (through space and time) and depends
on the state of the atmosphere and the latitude.
3.2.2 The Global Energy Budget
The energy that reaches the earths atmosphere comes from the sun and the absorption and
loss of radiation from the earth and its atmosphere determine our climate. If the earth had no
atmosphere the mean surface temperature would be 255K (-18oC), well below the freezing point
of water. The atmosphere serves to retain heat near the surface and the earth is thereby made
habitable. This accounting for incoming and outgoing energy is called the global energy
balance and could potentially be upset by any significant changes to the earth’s atmosphere.
The 1/4th of the solar constant or ~342 Wm-2 allowed through is mainly made up of the visible
region of the spectrum, with some in the UV region. Notably, the stratosphere absorbs UV due
to the ozone present and results in warming of the stratosphere. The lower atmosphere is
however transparent to UV and visible light, which thus pass through. Therefore it gains
relatively little energy from incoming radiation.
The following diagrams describe what happens to the incoming insolation. These diagrams need
to be understood and known. However, the radiation energy values need not be memorised.
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J. Henwood
Intermediate Environmental Science- Atmosphere V1.0
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Light passes through the atmosphere and meets clouds and a fraction is relected back to outer
space. However, a proportion passess through and reaches the ground. At ground level light is
also reflected unchanged especially if snow, ice or water (such as the ocean) are present. The
fraction of reflected light is termed the albedo and is over 0,5 for clouds but below 0.1 for
oceans. The global average for albedo is 0.3.
Whereas some light is reflected unchanged from the clouds or from the ground, a proportion of
the transmitted radiant energy in the visible region penetrates to the ground and is absorbed.
The radiation absorbed by the ground is transformed into other forms of energy namely heat
energy. This will be emitted once again from the ground in one of three forms: latent heat,
evapotranspiration and surface radiation. Latent heat is the energy emitted or absorbed
during a change of state, such as when liquid water forms a gas (water absorbed) or the
reverse. On the other hand, evapotranspiration is the process by which plants loose water
through transpiration, coupled with the loss through evaporation of from seas, pools etc.
Radiation, of course, refers to EMR. Whereas the radiation absorbed is in the UV/VIS region,
the radiation emitted from the ground is in the IR region (long wavelength: 100-150nm). This is
since, the ground retains a certain proportion of the absorbed radiation as heat and emits
radiation of a longer wavelength (thus less energy). Conclusively, the earth releases energy in
three forms i.e. latent heat, evapotranspiration and IR radiation, which in essence are three
forms of heat energy.
Whereas latent heat and heat due to evapotranspiration is absorbed by gases in the
atmosphere (and is not lost to space), the emitted radiation undergoes a different process.
Several atmospheric constituents absorb the IR radiation emitted, such as carbon dioxide, water
vapour and ozone. Methane, nitrous oxide and CFC’s are also important absorbers. Therefore,
some of the energy emitted will be radiated back to space, a part is absorbed by the
atmosphere, and a part will be returned to the ground (back radiation).
The back radiation is once again absorbed by the surface of the earth and the process of
absorbtion and radiation by the ground is again repeated until the wavelength is further
increased and the radiation looses energy. The result is transfer of energy from the upper
atmosphere to the ground and direct warming of the air nearest the ground together with
evaporation/ condensation. Overall more energy is retained next to the surface thus giving a
higher mean temperature.
To note is that the energy entering the atmosphere (342 Wm-2) is equal to the sum of the energy
lost from the atmosphere after reflection from clouds and radiation by the ground. In essence
this balance must be kept. Increase in the input over the output of energy will result in global
warming whereas a decrease in input results in global cooling. Alternatively, if less energy is
released from the atmosphere for example, since the atmosphere itself absorbs more IR (due to
more absorbing substances like CO2) global warming occurs since the atmosphere next to the
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surface retains more energy. Therefore, for constant earth temperature, the energy stored in the
atmosphere and ocean does not change in time, so energy equal to the incident solar radiation
must be radiated back to space. The Earth reflects about 30% of the incident solar flux; the
remaining 70% is absorbed, warms the land, atmosphere and oceans, and powers life on this
planet. Eventually this energy is reradiated to space.
This balance is very important. If disrupted the potential effects can be catastrophic.
3.2.3 The Greenhouse Effect
This is the process by which energy is recycled in the atmosphere to warm the Earth's surface
and is an essential piece of Earth's climate. The term greenhouse effect may be used to refer
either to the natural greenhouse effect, due to naturally occuring greenhouse gases, or to the
enhanced (man made: anthropogenic) greenhouse effect, which results from gases emitted
as a result of human activities
The greenhouse effect the process by which energy is recycled in the atmosphere to warm the
Earth's surface and is an essential piece of Earth's climate. Under stable conditions, the total
amount of energy entering the system from solar radiation will exactly balance the amount being
radiated into space, thus allowing the Earth maintain a constant average temperature over time.
However, recent measurements indicate that the Earth is presently absorbing 0.85 ± 0.15 W/m 2
more than it emits into space. This increase, associated with global warming, is believed to have
been caused by the recent increase in greenhouse gas concentrations.
3.2.3.1 Mechanism
The key to the greenhouse effect is the fact that the atmosphere is relatively transparent to
incoming solar radiation as staed further abovem but strongly absorbes the infrared radiation
that is emitted from the ground. In fact, most of the solar radiation heats the surface, but the
atmosphere is heated from radiation emitted from the ground.
The mechanism of the greenhouse effect is given in the diagram below.
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This figure is a simplified, schematic representation of the flows of energy between space, the atmosphere,
and the Earth's surface, and shows how these flows combine to trap heat near the surface and create the
greenhouse effect. Energy exchanges are expressed in watts per square meter (W/m2).
In the Global energy budget, UV/VIS radiation reaches the surface and is absorbed. This warms
the land which in turn releases heat (through latent heat and evapotranspiration) and radiation
(in the form of IR radiation, that is heat). Whereas a part of the radiation heat/ radiation is lost
from the atmosphere to space (as described further above) the greenhouse gases close to the
surface of the earth absorb the IR radiation and heat and increase in temperature. Therefore,
the atmosphere close to the surface is warmed.
The gases in the atmosphere themselves reflect (back radiation) and release IR radiation and
heat, which are absorbed by the ground. The ground releases the radiation, once again as latent
heat, evapotranspiration and IR radiation, and the process is repeated. This cycling of heat is
the greenhouse effect and is much what occurs in a normal greenhouse, yet on a much smaller
scale.
3.2.3.2 The greenhouse gases
The molecules/atoms that constitute the bulk of the atmosphere; oxygen (O2), nitrogen (N2) and
argon; do not interact with infrared radiation significantly. In the Earth’s atmosphere, the
dominant infrared absorbing gases are water vapor, carbon dioxide, and ozone, Other
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absorbers of significance include methane, nitrous oxide and the chlorofluorocarbons. The latter
molecules are called the greenhouse gases since they constitute the molecules which absorb IR
radiation, and thus contribute to the warming of the atmosphere. The most important gas is
water varpour, accounting for 36% of the greenhouse effect, second comes CO2 (12%) and third
O3 (3%).
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3.3 The Ozonosphere
3.3.1 Ozone
Ozone (O3) is a triatomic molecule, consisting of three oxygen atoms joined together. It is an
allotrope of oxygen that is much less stable than the diatomic species, O2. Ozone is a pale blue
gas at standard temperature and pressure, and one of the most toxic compounds known. It is
also unstable, decaying to ordinary oxygen:
2 O 3  3 O2
This reaction proceeds more rapidly with increasing temperature and decreasing pressure.
O3 is present in low concentrations throughout the Earth's atmosphere: ground level ozone is an
air pollutant with harmful effects on lung function, found due to photochemical smog. However in
the upper atmosphere it prevents damaging ultraviolet light from reaching the Earth's surface.
It is formed in many ways from O2, namely by electrical discharges such as lightning, and by
action of high energy electromagnetic radiation. Certain electrical equipment generates
significant levels of ozone, especially devices using high voltages, such as laser printers,
photocopiers, and arc welders. Electric motors using brushes can generate ozone from repeated
sparking inside the unit. Large motors, such as those used by elevators or hydraulic pumps, will
generate more ozone than smaller motors
3.3.2 The Ozone Layer
The highest concentrations of ozone in the atmosphere are in the stratosphere, in a region also
known as the ozone layer. Here it filters out the shorter wavelengths (less than 320 nm) of
ultraviolet light (270 to 400 nm) from the Sun that would be harmful to most forms of life in large
doses.
3.3.1 Formation and destruction
The ozone-oxygen cycle is the process by which ozone is continually regenerated in Earth's
stratosphere, all the while converting ultraviolet radiation into heat energy.
3.3.1.1 How ozone is made
In the first step, an ozone molecule's life begins when intense ultraviolet solar radiation (less
than 240 nm in wavelength) breaks apart an oxygen molecule (O2) into two oxygen atoms,
called oxygen radicals. These atoms react with other oxygen molecules to form 2 ozone
molecules.
O2 + (radiation < 240nm) → 2O.
O2 + O. + M → O 3 + M
Here "M" is a so-called "third body collision partner", a molecule (usually nitrogen or oxygen)
which carries off the excess energy of the reaction. Without it the reaction cannot occur.
Ozone forms slowly since there isn't a lot of solar energy at wavelengths less than 240 nm. This
production process would require about one year to replace the amount of ozone that exists at
around 20 km above Earth's surface.
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3.3.1.2 How ozone works
When ozone in the upper atmosphere is hit by ultraviolet solar radiation, it quickly undergoes a
chemical reaction. The triatomic ozone molecule becomes diatomic molecular oxygen plus a
free oxygen atom:
O3 + radiation → O2 + O.
Free atomic oxygen then quickly reacts with other oxygen molecules and forms ozone again:
O2 + O. + M → O 3 + M
The overall effect is to convert penetrating UV light into harmless heat.
4.3.1.3 How ozone is removed
When an oxygen atom and an ozone molecule meet, they recombine to form two oxygen
molecules:
O3 + O. → 2O2
The overall amount of ozone in the stratosphere is determined by a balance between production
by solar radiation, and removal by recombination. The removal rate is much slower than the
period of the ozone-oxygen cycle.
Certain free radicals (very active molecules), the most important being hydroxyl (OH.), nitric
oxide (NO.), and atoms of chlorine (Cl.) and bromine (Br.), catalyze the recombination reaction,
leading to an ozone layer that is thinner than it would be if the catalysts were not present.
Most of the OH. and NO. are naturally present in the stratosphere, but human activity, especially
the by-product of chlorofluorocarbons (CFCs) and halons, greatly increased the Cl. and Br.
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concentrations, leading to ozone depletion. Each Cl. or Br. atom can catalyze tens of thousands
of decomposition reactions before it is removed from the stratosphere as follows:
O3 + Cl. → O2 + ClO
ClO → O. + Cl.
As noted, Cl. In the above reaction is recycled, thus leading to further destruction.
Similarly for OH. and NO.:
O3 + OH. → O2 + O2H
O3 + NO. → O2 + NO2
However, these two are not recycled as mush as Cl. and Br..
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3.4 Weather and Climate
Weather is an all-encompassing term used to describe all phenomena that can occur in the
atmosphere of a planet. The term is normally taken to mean the activity of these phenomena
over short periods of time, usually no more than a few days in length. Average atmospheric
conditions over significantly longer periods are known as climate. Usage of the two terms
often overlaps and the concepts are obviously very closely related.
3.4.1 Variation in Insolation
Weather phenomena result from temperature differences around the globe, which arise mainly
due to variation in insolation. Insolation varies on a spatial (over distance) and temporal (through
time) basis. Temporal variation is in turn seasonal (varies over the seasons) and diurnal (varies
through the day). Thus areas closer to the tropics, around the equator, receive more energy
from the Sun than more northern and southern regions, nearer to the Earth's poles. In addition,
insolation is hiegher deuring the day, particularly when the sun occupies a central position in the
sky, and is stronger during summer. Finally, different surface areas (such as ocean waters,
forested lands, and ice sheets) have differing reflectivity (albedo), and therefore absorb and
radiate different amounts of the solar energy they receive.
Surface temperature differences cause vertical wind currents. A hot surface heats the air above
it, and the air expands and rises, lowering the air pressure and drawing colder air into its place.
Rising and expanding air gives up its heat and so cools, which causes it to shrink and sink,
increasing air pressure and displacing the air already below it.
Horizontal wind currents are formed at the boundaries between differentially heated areas and
can be intensified by the presence of sloped surfaces. The simple systems thus formed can then
display emergent behaviour to produce more complex systems and thus all other weather
phenomena. A large scale example of this process can be seen in the Hadley cell and other
forms of atmospheric circulation. A smaller scale example would be coastal breezes.
The fundamental causes of weather are thus surface temperature, and to a lesser extent,
elevation.
Because the Earth's axis is tilted (not perpendicular to its orbital plane), sunlight is incident at
different angles at different times of the year. In June the Northern Hemisphere is tilted towards
the sun, so at any given Northern Hemisphere latitude sunlight falls more directly on that spot
than in December. This effect causes seasons. Any precession in a planet's orbit will affect the
amount of energy received at a particular spot throughout the year and may influence long-term
weather patterns..
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3.4.2 Global circulation
Atmospheric circulation is the large-scale
movement of air, by which heat is distributed
on the surface of the Earth. The large-scale
structure of the atmospheric circulation varies
from year to year, but the basic structure
remains fairly constant. Three cells are said to
account for this circulation that is the Hadley
cell, the Ferrell cell and the polar cell, as
shown in the diagram below.
3.4.2.1 Hadley cells
The Hadley cell mechanism is well understood and provides an explanation for wind
observations very well. It is a closed circulation loop, which begins at the equator with warm,
moist air lifted aloft in equatorial low pressure areas to the tropopause and carried poleward. At
about 30°N/S latitude, it descends in a cooler high pressure area. Some of the descending air
travels equatorially along the surface, closing the loop of the Hadley cell and creating the Trade
Winds.
3.4.3 Coriolis Force
The Coriolis effect is caused by the presence of the coriolis force, which is in turn caused by
rotation of the earth. Once air has been set in motion by the pressure gradient force, it
undergoes an apparent deflection from its path, as seen by an observer on the earth. This
apparent deflection is called the "Coriolis force" and is a result of the earth's rotation.
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As air moves from high to low pressure in the northern hemisphere, it is deflected to the right by
the Coriolis force. In the southern hemisphere, air moving from high to low pressure is deflected
to the left by the Coriolis force.
The amount of deflection the air makes is directly related to both the speed at which the air is
moving and its latitude. Therefore, slowly blowing winds will be deflected only a small amount,
while stronger winds will be deflected more. Likewise, winds blowing closer to the poles will be
deflected more than winds at the same speed closer to the equator. The Coriolis force is zero
right at the equator.
The Coriolis effect is responsible for the direction of rotation of cyclones. Due to the effect,
cyclones rotate counterclockwise in the Northern hemisphere and clockwise in the Southern
hemisphere.
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3.5 Natural Climate Change
Climate change refers to the variation in the Earth's global climate or in regional climates over
time. It describes changes in the variability or average state of the atmosphere (or average
weather) over time scales ranging from decades to millions of years. These changes may come
from processes internal to the Earth, be driven by external forces (e.g. variations in sunlight
intensity) or, most recently, be caused by human activities.
Weather is a chaotic dynamical system, but in many cases, it is observed that the climate (i.e.,
the average state of weather) is fairly stable and predictable. This includes the average
temperature, amount of precipitation, days of sunlight, and many other variables that might be
measured at any given site. However, there are also changes within the Earth's environment
that can affect the climate.
3.5.1 Glaciation
Glaciers are recognized as one of the most sensitive indicators of climate change, advancing
substantially during climate cooling (e.g., the Little Ice Age of the last century) and retreating
during climate warming. Glaciers grow and collapse, both contributing to natural variability and
greatly amplifying external forces. For the last century, however, glaciers have been unable to
regenerate enough ice during the winters to make up for the ice lost during the summer months.
The most important climate processes of the last several million years are the glacial and
interglacial cycles of the present ice age. Though shaped by orbital variations, the internal
responses involving continental ice sheets and 130 m sea-level change certainly played a key
role in deciding what climate response would be observed in most regions.
3.5.2 Factors driving climate change
3.5.2.1 Solar variation
The sun, as the ultimate source of nearly all energy in the climate system, is an integral part of
shaping Earth's climate. On the longest time scales, the sun itself is getting brighter as it
continues its evolution. Early in Earth's history, it is thought to have been too cold to support
liquid water at the Earth's surface.
On more modern time scales, there are also a variety of forms of solar variation, including the
11–year solar cycle and longer-term modulations. However, the 11–year sunspot cycle does not
manifest itself clearly in the climatological data. These variations are considered to be influential
in triggering the Little Ice Age and for some of the warming observed from 1900 to 1950.
3.5.2.2 Orbital variations
In their impact on climate, orbital variations are in some sense an extension of solar variability,
because slight variations in the Earth's orbit lead to changes in the distribution and abundance
of sunlight reaching the Earth's surface. Such orbital variations, known as Milankovitch cycles,
are a highly predictable consequence of basic physics due to the mutual interactions of the
Earth, its moon, and the other planets. These variations are considered the driving factors
underlying the glacial and interglacial cycles of the present ice age. Subtler variations are also
present, such as the repeated advance and retreat of the Sahara desert in response to orbital
precession.
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Variations in the Earth's eccentricity, axial tilt, and precession comprise the three dominant
cycles, collectively known as the Milankovitch Cycles for Milutin Milankovitch, the Serbian
astronomer who is generally credited with calculating their magnitude. Taken in unison,
variations in these three cycles creates alterations in the seasonality of solar radiation reaching
the Earth's surface. These times of increased or decreased solar radiation directly influence the
Earth's climate system, thus impacting the advance and retreat of Earth's glaciers.
Eccentricity: The first of the three
Milankovitch Cycles is the Earth's
eccentricity. Eccentricity is, simply,
the shape of the Earth's orbit around
the Sun. This constantly fluctuating,
orbital shape ranges between more
and less elliptical (0 to 5% ellipticity)
on a cycle of about 100,000 years.
These oscillations, from more elliptic
to less elliptic, are of prime importance
to glaciation in that it alters the
distance from the Earth to the Sun,
thus changing the distance the Sun's
short wave radiation must travel to reach Earth, subsequently reducing or increasing the amount
of radiation received at the Earth's surface in different seasons.
Axial tilt: The second of the three Milankovitch Cycles, is the inclination of the Earth's axis in
relation to its plane of orbit around the Sun. Oscillations in the degree of Earth's axial tilt occur
on a periodicity of 41,000
years from 21.5 to 24.5
degrees.
Today the Earth's axial tilt
is about 23.5 degrees,
which largely accounts for
our seasons. Because of
the periodic variations of
this angle the severity of
the
Earth's
seasons
changes. With less axial tilt the
Sun's solar radiation is more
evenly
distributed
between
winter and summer. However,
less tilt also increases the
difference in radiation receipts
between the equatorial and polar
regions.
Precession: The third and final
of the Milankovitch Cycles is
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Earth's precession. Precession is the Earth's slow wobble as it spins on axis. This wobbling of
the Earth on its axis can be likened to a top running down, and beginning to wobble back and
forth on its axis. The precession of Earth wobbles from pointing at Polaris (North Star) to
pointing at the star Vega. When this shift to the axis pointing at Vega occurs, Vega would then
be considered the North Star. This top-like wobble, or precession, has a periodicity of 23,000
years.
Due to this wobble a climatically significant alteration must take place. When the axis is tilted
towards Vega the Northern Hemisphere will experience winter when the Earth is furthest from
the Sun and summer when the Earth is closest to the Sun. This coincidence will result in greater
seasonal contrasts.
3.5.2.3 Volcanism
A single eruption of the kind that occurs several times per century can impact climate, causing
cooling for a period of a few years. For example, the eruption of Mount Pinatubo in 1991 is
barely visible on the global temperature profile. Huge eruptions, known as large igneous
provinces, occur only a few times every hundred million years, but can reshape climate for
millions of years and cause mass extinctions.
3.5.2.4 Greenhouse gases
Current studies indicate that radiative forcing by greenhouse gases is the primary cause of
global warming. Greenhouse gases are also important in understanding Earth's climate history.
According to these studies, the greenhouse effect, which is the warming produced as
greenhouse gases trap heat, plays a key role in regulating Earth's temperature.
Over the last 600 million years, carbon dioxide concentrations have varied from perhaps >5000
ppm to less than 200 ppm, due primarily to the impact of geological processes and biological
innovations. Curiously, it has been argued (Veizer et al. 1999) that variations in greenhouse gas
concentrations over tens of millions of years have not been well correlated to climate change,
with plate tectonics perhaps playing a more dominant role. However there are several examples
of rapid changes in the concentrations of greenhouse gases in the Earth's atmosphere that do
appear to correlate to strong warming, including the Paleocene–Eocene thermal maximum, the
Permian–Triassic extinction event, and the end of the Varangian snowball earth event.
During the modern era, rising carbon dioxide levels are implicated as the primary cause to
global warming since 1950.
3.5.2.4 Plate tectonics
On the longest time scales, plate tectonics will reposition continents, shape oceans, build and
tear down mountains and generally serve to define the stage upon which climate exists. More
recently, plate motions have been implicated in the intensification of the present ice age when,
approximately 3 million years ago, the North and South American plates collided to form the
Isthmus of Panama and shut off direct mixing between the Atlantic and Pacific Oceans.
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3.5.2.5 Human influences
Anthropogenic factors are acts by humans that change the environment and influence climate.
The biggest factor of present concern is the increase in CO 2 levels due to emissions from fossil
fuel combustion, followed by aerosols (particulate matter in the atmosphere) which exerts a
cooling effect. Other factors, including land use, ozone depletion, and deforestation also impact
climate.
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