ES.12
The student will investigate and understand the origin and evolution of the atmosphere and the
interrelationship of geologic processes, biologic processes, and human activities on its composition
and dynamics.
Most textbooks give atmospheric composition as a list of the most abundant gases and examine
their distribution among the four layers of the atmosphere. Please see your textbook for this
treatment. I prefer to discuss atmospheric composition from the standpoint of the most important
atmospheric components: nitrogen, oxygen, carbon dioxide, ozone, water vapor, dust, and
aerosols. Both the cycling of these components and their impact on life and the environment are
important considerations.
Most students will state that carbon dioxide is the most abundant gas in the atmosphere, because
it gets so much attention in the media – they will persist in this view, even when you present them
with the real data. Others will claim oxygen is most abundant. Very few will know that nitrogen is
the most abundant or will realize its importance to life. The nitrogen cycle is really outside the
scope of Earth Science, narrowly interpreted. It is examined thoroughly in biology. Oxygen is
important for respiration, carbon dioxide for photosynthesis and warming the atmosphere. The
point should be made regarding oxygen and carbon dioxide that processes adding one of these
remove the other. Photosynthesis removes carbon dioxide and adds oxygen. Respiration,
combustion, and decomposition remove oxygen and add carbon dioxide. The exception is volcanic
activity which adds carbon dioxide, but does not remove oxygen.
Ozone (O3) is created from lightning in the stratosphere, where it absorbs incoming ultraviolet
radiation. It is also a byproduct of human activity in the troposphere, where it is considered a
pollutant, thus illustrating the definition of a pollutant as “a substance out of place.” Water vapor is
added to the atmosphere by volcanic activity, evaporation, transpiration (byproduct of cellular
respiration), and combustion. It is removed by condensation. In addition to being part of the water
cycle, it is the most important greenhouse gas and is a major moderator of climate variability. Dust
enters the atmosphere by volcanic eruptions and wind erosion (increased by human activity
exposing soil). It is removed by precipitation. Dust is important as condensation nuclei, a cold
surface on which water can condense. If there were no dust in the atmosphere, we would have
few clouds and precipitation would be rare. Aerosols, both from volcanic activity or pollution,
reduce incoming solar radiation and produce smog and acid rain.
Key concepts include
a) scientific evidence for atmospheric changes over geologic time
The atmosphere formed very early in the history of Earth from outgassing of the interior through
volcanic activity. There is debate on how much of the ocean and atmosphere was contributed by
comet impact. Most noteworthy is that the original atmosphere contained no free oxygen. All free
oxygen (O2) has been produced from photosynthesis. The earliest fossil blue green alga in the
form of stromatolites dates from 3.8 billion years ago. Geologic evidence for the lack of significant
amounts of free oxygen prior to approximately 2 billion years ago is the lack of oxygenated iron in
sediments prior to that time – no red beds (rust covered grains) and lots of sedimentary pyrite.
After 2 billion years ago, the situation was reversed – lots of red beds and no sedimentary pyrite.
At 2 billion years ago, it is estimated that atmospheric levels of oxygen reached 1% of present
atmospheric levels – enough to support cellular respiration.
b) current theories related to the effects of early life on the chemical makeup of the atmosphere
Not only did photosynthesis add free oxygen to the atmosphere, it also resulted in the
sequestration of carbon dioxide in rock in the formation of limestone and fossil fuels through
geologic time. Thus, it is apparent that the amount of carbon dioxide has been reduced over time.
Ancient Earth may have been a much warmer planet than it is today as a result of a much greater
greenhouse effect.
c) comparison of Earth’s atmosphere to that of other planets
Outer gas giants primarily have atmospheres of hydrogen and methane. Mercury has no
atmosphere (original lost due to solar wind from a young sun). Venus has a dense atmosphere
many times Earth’s surface pressure with sulfuric acid clouds and 95% carbon dioxide. Because of
this high amount of carbon dioxide, the ambient temperature of Venus is extremely hot due to an
enormous greenhouse effect. Mars’ atmosphere also contains 95% carbon dioxide, but the density
is much less than Earth’s. Because of the low density, the Martian atmosphere does not support a
large greenhouse effect.
d) atmospheric regulation mechanisms including the effects of density differences and energy
transfer
Energy transfer is accomplished through four means. Most textbooks list only 3: conduction,
convection, and radiation. Technically this is correct, but downplays the importance of evaporation
and condensation in energy transfer. ~70% of Earth’s surface is ocean. The top one meter of
seawater is evaporated annually, removing latent heat from the ocean, which is released in the
upper troposphere when condensation occurs in the formation of clouds.
If one traces the actual transfer of energy from a budgetary point of view, one finds that the Earth’s
surface has a surplus of energy and the atmosphere a deficit of energy received from the sun. So
energy must be transferred from the earth to the atmosphere. The lowest layer of the atmosphere
(troposphere which contains all weather) is heated primarily by infrared radiation from Earth’s
surface and from evaporation and condensation. This is known as the greenhouse effect. Since
the atmosphere is heated from the bottom like a pot of soup, it is inherently unstable. Warm air
rises because it is less dense. This is responsible for the creation of all weather (see ES.13 d for
further details).
In addition, when the energy budget is considered from a geographic point of view, equatorial
regions have a surplus of energy and Polar Regions have a deficit of energy (see ES.4 b) due to
the angle of incidence of solar radiation. Thus energy must be transferred from the Equator to the
Poles. This transfer, through convection, is responsible for global wind patterns and storm tracks
(see ES.13 for details).
e) potential atmospheric compositional changes due to human, biologic, and geologic activity
Periods of intense volcanic activity may add excess carbon dioxide (warmer), aerosols, and dust
(cooler), thus affecting the climate over relatively short periods of time. The most noteworthy
impact of humans on atmospheric composition results from the release of carbon dioxide from the
combustion of fossil fuels, producing a warmer atmosphere. A warmer atmosphere can result in
more evaporation & potentially greater cloudiness. More low clouds will have a cooling effect;
more high clouds can have a warming effect. More aerosols from increasing industrial activity and
automobile traffic in the developing world can scatter incoming radiation, thus potentially cooling
the atmosphere. Exhaust from combustion can also lead to acid rain. A warmer ocean and tundra
(due to global warming) can result in the release of methane to the atmosphere, now sequestered
in methane hydrate. Methane can absorb three times the heat of carbon dioxide leading to even
warmer temperatures. It was thought that melting of sea ice in the Arctic would result in a warmer
atmosphere, as less radiation would be reflected back to space. Recent studies have shown that it
is resulting in increased evaporation and greater cloudiness in the Arctic. CFC’s
(chlorofluorocarbons such as Freon) are human produced pollutants that are very long lived in the
atmosphere and are responsible for the thinning of the ozone layer.
Please note that web pages in support of atmospheric sciences will be presented at the end of
ES.13.
After reviewing material in your textbook and exploring the websites below, go to the file labeled
ES.12 Review Responses and open it in Word. Type your answers below each question and make
them a distinctive readable color or font. E-mail this file as an attachment word document to the
address provided in the introduction and please keep your answers succinct.