The big picture

The atmospheric system comprises of the gases that surround the earth’s surface and are retained there by the earth’s magnetic field.

Figure 1. Planet earth surrounded by its atmosphere.

We usually take the atmosphere for granted, although it is all around us and vital for life to exist including our own survival. For example:

• It provides a shield from meteorites.

• It protects us from harmful radiation from the sun.

• It moderates and stablises our climate including temperature.

• It is from where we obtain the oxygen we breathe and from where plants acquire the carbon dioxide they require for photosynthesis.

The atmosphere does not occur in isolation but is linked to the lithosphere, hydrosphere and biosphere through biogeochemical cycles (e.g. carbon cycle). Since the creation of the earth the composition of the atmosphere has changed dramatically. Living organisms have contributed to this change which has led to an atmosphere which supports a wide variety of organisms today. However human activity can cause adverse changes to the composition of the atmosphere e.g. through combustion of fossil fuels that increases the level of carbon dioxide in the air. This change in atmospheric composition can lead to climate change.

The atmosphere is made of a number of layers that differ in pressure and temperature. Within the inner most layer, heat from the sun warms the tropics more than the polar regions resulting in a temperature difference which drives the movement of air and hence circulation around the planet. These atmospheric circulation system can transfer pollutants over large distances causing global problems.

Watch the following video Aerosols: Airborne particles in Earth’s atmosphere by Bad Astronomer which illustrates the amount and type of material that is moved around the plant. List the different materials that are transported within the atmospheric circulation system.

A changing atmosphere

The earth’s atmosphere is a dynamic system that has changed over time.

History of the earth’s atmosphere

The early volcanic earth

The earth was formed about 4.6 billion years ago. This early earth was showered with debris from outer space which together with the decay of radioactive elements and volcanic activity lead to extreme temperatures and an environment unsuitable for life. This period from the formation of the earth to 4 billion years ago is referred to as the Hadean era named after the Greek god ‘Hades’ of the underworld due to the conditions that resembled ‘hell’.

Figure 1. Volcanic earth.

The earth's early atmosphere predominately consisted of hydrogen and helium gases. These low density gases escaped into space as a result of solar winds and the presence of only a weak magnetic field within the earth. Overtime volcanic emissions led to an atmosphere comprised of water vapour, carbon dioxide, methane, ammonia and hydrogen sulphide. Free oxygen was absent during this earlier period of the earth.

Formation of oceans and early forms of life

As heat radiated into space, the earth began to cool. Water condensed forming great oceans about 4 billion years ago. This signaled the end of the Hadean era and the start of the Archean era which spanned from 4 billion to 2.5 billion years ago.

Life first appeared on the earth about 3.8 billion years ago in the form of simple celled bacteria (prokaryotes). These early bacteria were anaerobic and included methane producing bacteria that used carbon dioxide and hydrogen. Bacteria evolved and developed the ability to capture and use light energy. Overtime this lead to the development of photosynthetic bacteria which produced oxygen as a by-product and included the group of cyanobacteria (also referred to as blue-green algae) responsible for increasing the levels of atmospheric oxygen from less than 1% to almost 21% today. Fossil evidence found in Western Australia dates the existence of cyanobacteria to 3.5 billion years ago.

Figure 2. Mounds of bacteria called stromatolites covered in layers of cyanobacteria in Western Australia.

Any oxygen formed during Archean era quickly reacted with other gases and with iron sulphide in the oceans to form red iron oxide which then precipitated out onto the seabed. Over time this led to the formation of sedimentary rocks containing red bands of oxidised iron.

Figure 3. Sedimentary rocks with bands of iron oxide

During the Proterozoic era (2.5 billion to 542 million years ago) that followed the Archean era, levels of atmospheric oxygen continued to increase and the following occurred:

• The levels of carbon dioxide decreased.

• Higher up in the atmosphere oxygen molecules were split by sunlight energy into atomic oxygen which lead to the formation of ozone.

• More complex single celled organisms (eukaryotes) appeared about two billion years ago followed by multicellular organisms about one billion years ago.

Life was restricted to the oceans until the ozone layer had developed enough to shield the earth’s surface from the harmful effects of UV light. The first green plants appeared on land about 500 million years ago during the Phanerozoic era (542 million years ago to the present day).

Figure 4. Relationship between oxygen levels in the atmosphere over time and key events.

The following video ‘Earth and early atmosphere’ by Big History Project provides a summary of the changes that have occurred to the Earth’s atmosphere and also includes the phenomenon called ‘snowball earth’

Composition of the earth’s atmosphere today

Nitrogen and oxygen are the main components of our atmosphere with smaller amounts of argon, neon, carbon dioxide, water vapour and other trace elements. The atmosphere is a dynamic system with continual inputs and outputs. For example the hydrological cycle and nutrient cycles influence levels of gases in the atmosphere. Some components of the atmosphere such as carbon dioxide, water vapour and ozone vary significantly from one location to another and overtime and are effected by human activities such as combustion of fossil fuels.

Figure 5. Main gases present in the atmosphere by volume.

Theory of Knowledge

The atmosphere is a shared resource. Do more economically developed countries have a duty to take greater responsibility of its management?

Structure and function of the atmosphere I

As discussed in the previous section the atmosphere comprises of a variety of transparent gases dominated by inert nitrogen and oxygen. The atmosphere is maintained by the earth’s gravitational forces which creates air pressure. A barometer can be used to measure air pressure and at sea level is about 760mm Hg (mercury) and declines with increasing altitude. The lowest pressure on land is found at the highest peak of Mount Everest.

Figure 1. Pressure decreases with altitude.

Unlike pressure, temperature does not uniformly change with altitude but allows us to demarcate the earth’s atmosphere into four distinct layers called the troposphere (covered in this section), stratosphere, mesosphere and thermosphere (covered in the next section).

Troposphere

This is the layer closest to the earth’s surface and includes where we live. It extends up to about 10km above sea level and is where:

• The earth surface absorbs heat from the sun. The warm earth then heats the atmosphere through conduction. The troposphere is warmest near the earth surface with temperature declining by around 6.5°C per km.

• Wind speeds increase with height. The jet stream which blows powerful winds towards the east occurs at the top of the troposphere.

• Most of the atmospheric mass is found. This includes nearly all the water vapour, clouds and pollutants.

• Most of our weather occurs.

• Humans and other organisms have most interaction e.g. through exchange of gases or through introduction of pollutants.

• The greenhouse effect occurs and helps to regulate the temperature of the Earth.

Figure 2. The troposphere is where rain forming stratocumulus clouds are formed.

What is the greenhouse effect?

When energy from the sun enters the earth’s atmosphere as short wave radiation, some of it is absorbed by the earth’s surface. As the ground warms, heat energy is radiated back into the atmosphere in the form of long wave radiation. What happens next depends on the presence of gases referred to as greenhouse gases (GHGs) which absorb long wave radiation.

• In the absence of GHGs, the heat would be radiated back into space potentially resulting in an average global temperature of about -18°C and a very different environment to the one we currently live in.

• In the presence of GHGs, the long wave radiation is absorbed by the gases resulting in warming of the atmosphere to an average global temperature of around 15°C. This is commonly referred to as the natural greenhouse effect.

Hence human activities that alter the concentration of GHGs in the atmosphere can impact on global temperatures.

Figure 3. The greenhouse effect.

Not all energy from the sun entering the earth’s atmosphere reaches the ground. Some of the solar energy is reflected back into space by clouds, particles in the area and surfaces such as ice and snow. This reflection is known as the albedo effect.

Main GHGs and their sources

• Water vapour is the most abundant GHG. A rise in temperature results in more water vapour leading to further warming, which allows more water to evaporate and this positive feedback cycle continues.

• Carbon dioxide concentrations are increased by burning of fossil fuels, respiration, volcanic activity and deforestation. Plants and trees act as carbon sinks removing carbon dioxide from the atmosphere and effectively storing it in the form of biomass, hence deforestation also reduces available carbon sinks, thereby further exacerbating the situation.

• Methane arises from emissions from livestock, anaerobic decomposition of waste, rice cultivation and fossil fuels.

Figure 4. Cattle are a major source of methane gas.

• Nitrous oxide sources include fertilizers, combustion and industrial processes.

• Chlorofluorocarbons (CFCs) and hydrochloroflurocarbons (HCFC) have been used as liquid coolants (in refrigerators and air conditioning systems), in the production of plastic foam and as industrial solvents.

• Perfluorocarbon is used in production of aluminium.

• Sulphur hexa-fluoride is used in production of magnesium.

Global warming is discussed more fully in subtopics 7.2 and 7.3 which examine climate change.

The following video ‘The greenhouse effect’ by NASA EarthObservatory provides an overview of the process:

The tropopause occurs at the top of the troposphere and marks the upper limit of where our weather occurs. The temperature in the tropopause remains constant despite increasing height.

Structure and function II

The earth's atmosphere consists of four distinct layers. The troposphere was discussed in the previous section. In this section we consider the stratosphere, mesosphere and the thermosphere.

Stratosphere

This layer extends from 10 to 50 km above sea level and is where:

• Stratospheric ozone absorbs ultra violet radiation from the sun. Temperature is constant at about -60°C in the lower part of the stratosphere which is shielded by the ozone layer but then increases with altitude.

• The air is dry.

• Winds increase with height.

The stratopause marks the end of the stratosphere and is where the temperature remains constants with altitude.

Figure 1. Ozone layer in the stratosphere filters the ultraviolet rays from the sun.

Mesosphere

This layer ranges from about 50 to 80 km and is where:

• Without the presence of ozone or other particulates to absorb UV radiation, the temperature declines with height. It is the coldest part of the atmosphere with temperatures falling to -100°C.

• There are strong winds with speeds up to around 3,000 km/h.

The mesopause occurs at the end of the mesosphere and is where the temperature does not change.

Figure 2. Both the mesosphere and stratosphere provide some protection against meteorites.

Thermosphere

This layer extends beyond about 80 km to between 500 km and 1,000 km. Within the thermosphere:

• UV and X-radiation from the sun is absorbed which breaks apart molecules into atoms (oxygen, nitrogen and helium atoms are the main components in the upper thermosphere).

• The temperature increases with height and can reach beyond 2,000°C. This heat can cause the layer to expand causing variation in depth overtime from 500 to 1,000 km.

The ionosphere is also located within the thermosphere and comprises of an area in which the particles are electrically charged. The ability of shortwave radio waves to bounce off these ions back to earth is used by amateur radio enthusiasts to communicate over large distances. This is also where aurorae polaris (polar lights consisting of both the Northern lights in the northern hemisphere and Southern lights in the southern hemisphere) occur as a result of electrically charged particles from the sun colliding with ions in the ionosphere.

Figure 3. Northern lights (aurora borealis) in Iceland.

There is debate about where the earth’s atmosphere ends and space begins. The Karman Line at 100 km above sea level has been accepted by the International Aeronautic Federation. The international space station orbits within the thermosphere.

Figure 4. Layers of the earth's atmosphere.

Examiner Tip

Ensure you can construct and label a sketch diagram of height against temperature for the different layers within the earth's atmosphere.

International-mindedness

The atmosphere does not occur in isolation but is connected to the lithosphere, hydrosphere and biosphere all around the world.